llvm dev - Jun 2016 - [RFC] Embedded bitcode and related upstream (Part II)

Thanks for the feedback! Replies inline. 
> On Jun 12, 2016, at 11:44 PM, Eric Christopher <echristo at
gmail.com> wrote:
> 
> Hi Steven,
> 
> Great to see the commentary and updates here. I've got a few questions
about some of this work. It might be nice to see some separate RFCs for a couple
of things, but we'll figure that out after you send out patches probably :)
> 
> What needs to be improved:
> 1. Whitelist for command line options that can be used with bitcode:
> Current trunk implementation embeds all the cc1 command line options (that
includes header include paths, warning flags and other front-end options) in the
command line section. That is lot of redundant information. To re-create the
object file from the embedded optimized bitcode, most of these options are
useless. On the other hand, they can leak information of the source code. One
solution will be keeping a list of all the options that can affect code
generation but not encoded in the bitcode. I have internally prototyped with
disallowing these options explicitly and allowed only the reminder of the 
options to be embedded (http://reviews.llvm.org/D17394
<http://reviews.llvm.org/D17394>). A better solution might be encoding
that information in "Options.td" as specific group.
> 
> This is really interesting. I'm not a particularly security minded
person so I don't have a lot of commentary there. An explicit whitelist
sounds a bit painful to keep maintained, explicitly having a group in Options.td
sounds pretty nice. You'll need to add them to multiple groups, but it seems
pretty nice.
I have already implemented the new approach in http://reviews.llvm.org/D21230
<http://reviews.llvm.org/D21230>. It creates a new group for all the cc1
options that can affect codegen but not having a corresponding attribute in the
bitcode. When I wrote up this patch, I think it is also a good idea to extend
the group to driver flags so clang driver can issue warnings when using these
flags with LTO because they are likely to be dropped in the process. That is my
next thing to do if someone reviews my patch and agrees that is right thing to
do.
>  
> 2. Assembly input handling:
> This is a workaround to allow source code written in assembly to work with
"-fembed-bitcode" options. When compiling assembly source code with
"-fembed-bitcode", clang-as creates an empty section "__LLVM,
__asm" in the object file. That is just a way to distinguish object files
compiled from assembly source from those compiled from higher level source code
but forgot to use "-fembed-bitcode" options. Linker can use this
section to diagnose if "-fembed-bitcode" is consistently used on all
the object files participated in the linking.
> 
> I'm surprised you want a separate and empty section and not a header
flag as those are easier to keep around and won't take up a precious mach-o
section. There are probably other options here as well. There are probably other
options or concerns that someone shipping bitcode might have here as well, but
I'm sure those are being talked about - doesn't have too much affect on
the community though.
I suppose you mean the alternative is to burn a macho command for that. Well,
that is a limited resource and we don't have much left. Plus, using empty
section will make this accessible to other binary format, not only macho files.
I also have an interesting thought about handle the assembly, that is to wrap it
in module assembly in a bitcode file. I am not sure it would preserve the all
semantics of the original assembly and that would mean I need to somehow teach
the assembler about bitcode (which might make this not very attractive). Yes,
you might be right this doesn't affect the community, if no one else is
interesting in a solution for the problem we have, then this might not be
suitable for contributing. I am happy to keep it downstream.
> 
> 3. Bitcode symbol hiding:
> There was some concerns for leaking source code information when using
bitcode feature. One approach to avoid the leak is to add a pass which renames
all the globals and metadata strings. The also keeps a reverse map in case the
original name needs to be recovered. The final bitcode should contain no more
symbols or debug info than a stripped binary. To make sure modified bitcode can
still be linked correctly, the renaming need to be consistent across all bitcode
participated in the linking and everything that is external of the linkage unit
need to be preserved. This means the pass can only be run during the linking and
requires some LTO api.
> 
> How are you planning to ensure the safety of the reverse map? Seems that
requiring linking is a bit icky, but might work. Are you mostly worried about
function names that could be stripped out? What LTO api are you envisioning
here?
The reverse map is emitted as a separate file from the output binary/bitcode. It
should not be shipped together with the binary output, just like dSYM bundle.
The reason it needs to be done after linking is a limitation of the symbol
hiding technique. It requires that the symbols must be resolved. Think about the
following case:
a.o:
T export_symbol
T global_symbol
t local_symbol
b.o:
U global_symbol

To make sure the bitcode after symbol hiding pass can still link and produce the
same output, the pass need to rename them:
a.o:
T export_symbol    --> export_symbol (preserve)
T global_symbol    --> hidden_symbol_1 (rename, but need to have the same
name as the one in b.o)
t local_symbol       --> hidden_symbol_2 (rename, but don't care what it
becomes)
b.o:
U global_symbol   --> hidden_symbol_1
The pass need to know what symbols to keep and a global renaming table so the
names after renaming are consistent across all the modules.
 
>  
> 4. Debug info strip to line-tables pass:
> As the name suggested, this pass strip down the full debug info to
line-tables only. This is also one of the steps we took to prevent the leak of
source code information in bitcode.
> 
> I'm very curious about what's going on here. Could you elaborate?
:)
Cc Adrian
He would know more about it. I would only know that it can reconstruct
-gline-tables-only debug info from full debug info. We use it as a part of the
bitcode pipeline because we don't want the bitcode file to be exceedingly
large but I can see this pass to be useful in other circumstances.

Steven
> 
> Thanks a ton for the update - glad to see this being worked on!
> 
> -eric
>  
> 
> Please let me know what do you think about the pieces above or if you have
any concerns about the methodology. I will put up patches for review soon.
> 
> Thanks
> 
> Steven
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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On Mon, Jun 13, 2016 at 9:37 AM Steven Wu <stevenwu at apple.com> wrote:
> Thanks for the feedback! Replies inline.
>
> On Jun 12, 2016, at 11:44 PM, Eric Christopher <echristo at
gmail.com> wrote:
>
> Hi Steven,
>
> Great to see the commentary and updates here. I've got a few questions
> about some of this work. It might be nice to see some separate RFCs for a
> couple of things, but we'll figure that out after you send out patches
> probably :)
>
> What needs to be improved:
>> 1. Whitelist for command line options that can be used with bitcode:
>> Current trunk implementation embeds all the cc1 command line options
>> (that includes header include paths, warning flags and other front-end
>> options) in the command line section. That is lot of redundant
information.
>> To re-create the object file from the embedded optimized bitcode, most
of
>> these options are useless. On the other hand, they can leak information
of
>> the source code. One solution will be keeping a list of all the options
>> that can affect code generation but not encoded in the bitcode. I have
>> internally prototyped with disallowing these options explicitly and
allowed
>> only the reminder of the  options to be embedded (
>> http://reviews.llvm.org/D17394). A better solution might be encoding
>> that information in "Options.td" as specific group.
>>
>
> This is really interesting. I'm not a particularly security minded
person
> so I don't have a lot of commentary there. An explicit whitelist sounds
a
> bit painful to keep maintained, explicitly having a group in Options.td
> sounds pretty nice. You'll need to add them to multiple groups, but it
> seems pretty nice.
>
>
> I have already implemented the new approach in
> http://reviews.llvm.org/D21230. It creates a new group for all the cc1
> options that can affect codegen but not having a corresponding attribute in
> the bitcode. When I wrote up this patch, I think it is also a good idea to
> extend the group to driver flags so clang driver can issue warnings when
> using these flags with LTO because they are likely to be dropped in the
> process. That is my next thing to do if someone reviews my patch and agrees
> that is right thing to do.
>
>
Honestly I think it's better if we implement almost everything in the
bitcode that doesn't involve a pass manager configuration - and then make
that more explicit as far as library calls. Same with TargetMachine
configuration etc. What do you think?

>
>
>> 2. Assembly input handling:
>> This is a workaround to allow source code written in assembly to work
>> with "-fembed-bitcode" options. When compiling assembly
source code with
>> "-fembed-bitcode", clang-as creates an empty section
"__LLVM, __asm" in the
>> object file. That is just a way to distinguish object files compiled
from
>> assembly source from those compiled from higher level source code but
>> forgot to use "-fembed-bitcode" options. Linker can use this
section to
>> diagnose if "-fembed-bitcode" is consistently used on all the
object files
>> participated in the linking.
>>
>
> I'm surprised you want a separate and empty section and not a header
flag
> as those are easier to keep around and won't take up a precious mach-o
> section. There are probably other options here as well. There are probably
> other options or concerns that someone shipping bitcode might have here as
> well, but I'm sure those are being talked about - doesn't have too
much
> affect on the community though.
>
>
> I suppose you mean the alternative is to burn a macho command for that.
> Well, that is a limited resource and we don't have much left. Plus,
using
> empty section will make this accessible to other binary format, not only
> macho files. I also have an interesting thought about handle the assembly,
> that is to wrap it in module assembly in a bitcode file. I am not sure it
> would preserve the all semantics of the original assembly and that would
> mean I need to somehow teach the assembler about bitcode (which might make
> this not very attractive). Yes, you might be right this doesn't affect
the
> community, if no one else is interesting in a solution for the problem we
> have, then this might not be suitable for contributing. I am happy to keep
> it downstream.
>
>
You could use a section for ELF. I'm not sure what the right thing to do
for PE would be, but it'll either be a section or a command like thing. The
wrapping asm in module assembly might work, though you'll need to make some
(hopefully all programmatic) textual changes to keep it working. You might
be able to teach clang how to do that.

>
> 3. Bitcode symbol hiding:
>> There was some concerns for leaking source code information when using
>> bitcode feature. One approach to avoid the leak is to add a pass which
>> renames all the globals and metadata strings. The also keeps a reverse
map
>> in case the original name needs to be recovered. The final bitcode
should
>> contain no more symbols or debug info than a stripped binary. To make
sure
>> modified bitcode can still be linked correctly, the renaming need to be
>> consistent across all bitcode participated in the linking and
everything
>> that is external of the linkage unit need to be preserved. This means
the
>> pass can only be run during the linking and requires some LTO api.
>>
>
> How are you planning to ensure the safety of the reverse map? Seems that
> requiring linking is a bit icky, but might work. Are you mostly worried
> about function names that could be stripped out? What LTO api are you
> envisioning here?
>
>
> The reverse map is emitted as a separate file from the output
> binary/bitcode.
>
That makes some sense.

> It should not be shipped together with the binary output, just like dSYM
> bundle.
> The reason it needs to be done after linking is a limitation of the symbol
> hiding technique. It requires that the symbols must be resolved. Think
> about the following case:
> a.o:
> T export_symbol
> T global_symbol
> t local_symbol
> b.o:
> U global_symbol
>
> To make sure the bitcode after symbol hiding pass can still link and
> produce the same output, the pass need to rename them:
> a.o:
> T export_symbol    --> export_symbol (preserve)
> T global_symbol    --> hidden_symbol_1 (rename, but need to have the
same
> name as the one in b.o)
> t local_symbol       --> hidden_symbol_2 (rename, but don't care
what it
> becomes)
> b.o:
> U global_symbol   --> hidden_symbol_1
> The pass need to know what symbols to keep and a global renaming table so
> the names after renaming are consistent across all the modules.
>
>
Right. That's where I thought you were going. Would this instead be better
implemented as a tool/library that could "anonymize" a bitcode file? I
realize it wouldn't "work" for shipping to the store if you ran it
on
bitcode before it's been linked, but it might be a good way for people to
submit bitcode files to us as well. Just throwing ideas out here :)
>
>
>> 4. Debug info strip to line-tables pass:
>> As the name suggested, this pass strip down the full debug info to
>> line-tables only. This is also one of the steps we took to prevent the
leak
>> of source code information in bitcode.
>>
>
> I'm very curious about what's going on here. Could you elaborate?
:)
>
>
> Cc Adrian
> He would know more about it. I would only know that it can reconstruct
> -gline-tables-only debug info from full debug info. We use it as a part of
> the bitcode pipeline because we don't want the bitcode file to be
> exceedingly large but I can see this pass to be useful in other
> circumstances.
>
:)

-eric

>
> Steven
>
>
> Thanks a ton for the update - glad to see this being worked on!
>
> -eric
>
>
>>
>> Please let me know what do you think about the pieces above or if you
>> have any concerns about the methodology. I will put up patches for
review
>> soon.
>>
>> Thanks
>>
>> Steven
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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>>> 3. Bitcode symbol hiding:
[...] 
>>> 4. Debug info strip to line-tables pass:
>>> As the name suggested, this pass strip down the full debug info to
line-tables only. This is also one of the steps we took to prevent the leak of
source code information in bitcode.
>>> 
>>> I'm very curious about what's going on here. Could you
elaborate? :)
>> 
>> Cc Adrian
>> He would know more about it. I would only know that it can reconstruct
-gline-tables-only debug info from full debug info. We use it as a part of the
bitcode pipeline because we don't want the bitcode file to be exceedingly
large but I can see this pass to be useful in other circumstances.
> 
> :)
IIRC, removing the type information and flattening the scope tree also helps to
prevent leaking information about the source file, as part of (3) above.

> On Jun 13, 2016, at 9:37 AM, Steven Wu <stevenwu at apple.com> wrote:
> 
> Thanks for the feedback! Replies inline. 
> 
>> On Jun 12, 2016, at 11:44 PM, Eric Christopher <echristo at
gmail.com <mailto:echristo at gmail.com>> wrote:
>> 
>> Hi Steven,
>> 
>> Great to see the commentary and updates here. I've got a few
questions about some of this work. It might be nice to see some separate RFCs
for a couple of things, but we'll figure that out after you send out patches
probably :)
>> 
>> What needs to be improved:
>> 1. Whitelist for command line options that can be used with bitcode:
>> Current trunk implementation embeds all the cc1 command line options
(that includes header include paths, warning flags and other front-end options)
in the command line section. That is lot of redundant information. To re-create
the object file from the embedded optimized bitcode, most of these options are
useless. On the other hand, they can leak information of the source code. One
solution will be keeping a list of all the options that can affect code
generation but not encoded in the bitcode. I have internally prototyped with
disallowing these options explicitly and allowed only the reminder of the 
options to be embedded (http://reviews.llvm.org/D17394
<http://reviews.llvm.org/D17394>). A better solution might be encoding
that information in "Options.td" as specific group.
>> 
>> This is really interesting. I'm not a particularly security minded
person so I don't have a lot of commentary there. An explicit whitelist
sounds a bit painful to keep maintained, explicitly having a group in Options.td
sounds pretty nice. You'll need to add them to multiple groups, but it seems
pretty nice.
> 
> I have already implemented the new approach in
http://reviews.llvm.org/D21230 <http://reviews.llvm.org/D21230>. It
creates a new group for all the cc1 options that can affect codegen but not
having a corresponding attribute in the bitcode. When I wrote up this patch, I
think it is also a good idea to extend the group to driver flags so clang driver
can issue warnings when using these flags with LTO because they are likely to be
dropped in the process. That is my next thing to do if someone reviews my patch
and agrees that is right thing to do.
> 
>>  
>> 2. Assembly input handling:
>> This is a workaround to allow source code written in assembly to work
with "-fembed-bitcode" options. When compiling assembly source code
with "-fembed-bitcode", clang-as creates an empty section
"__LLVM, __asm" in the object file. That is just a way to distinguish
object files compiled from assembly source from those compiled from higher level
source code but forgot to use "-fembed-bitcode" options. Linker can
use this section to diagnose if "-fembed-bitcode" is consistently used
on all the object files participated in the linking.
>> 
>> I'm surprised you want a separate and empty section and not a
header flag as those are easier to keep around and won't take up a precious
mach-o section. There are probably other options here as well. There are
probably other options or concerns that someone shipping bitcode might have here
as well, but I'm sure those are being talked about - doesn't have too
much affect on the community though.
> 
> I suppose you mean the alternative is to burn a macho command for that.
Well, that is a limited resource and we don't have much left. Plus, using
empty section will make this accessible to other binary format, not only macho
files. I also have an interesting thought about handle the assembly, that is to
wrap it in module assembly in a bitcode file. I am not sure it would preserve
the all semantics of the original assembly and that would mean I need to somehow
teach the assembler about bitcode (which might make this not very attractive).
Yes, you might be right this doesn't affect the community, if no one else is
interesting in a solution for the problem we have, then this might not be
suitable for contributing. I am happy to keep it downstream.
> 
>> 
>> 3. Bitcode symbol hiding:
>> There was some concerns for leaking source code information when using
bitcode feature. One approach to avoid the leak is to add a pass which renames
all the globals and metadata strings. The also keeps a reverse map in case the
original name needs to be recovered. The final bitcode should contain no more
symbols or debug info than a stripped binary. To make sure modified bitcode can
still be linked correctly, the renaming need to be consistent across all bitcode
participated in the linking and everything that is external of the linkage unit
need to be preserved. This means the pass can only be run during the linking and
requires some LTO api.
>> 
>> How are you planning to ensure the safety of the reverse map? Seems
that requiring linking is a bit icky, but might work. Are you mostly worried
about function names that could be stripped out? What LTO api are you
envisioning here?
> 
> The reverse map is emitted as a separate file from the output
binary/bitcode. It should not be shipped together with the binary output, just
like dSYM bundle.
> The reason it needs to be done after linking is a limitation of the symbol
hiding technique. It requires that the symbols must be resolved. Think about the
following case:
> a.o:
> T export_symbol
> T global_symbol
> t local_symbol
> b.o:
> U global_symbol
> 
> To make sure the bitcode after symbol hiding pass can still link and
produce the same output, the pass need to rename them:
> a.o:
> T export_symbol    --> export_symbol (preserve)
> T global_symbol    --> hidden_symbol_1 (rename, but need to have the
same name as the one in b.o)
> t local_symbol       --> hidden_symbol_2 (rename, but don't care
what it becomes)
> b.o:
> U global_symbol   --> hidden_symbol_1
> The pass need to know what symbols to keep and a global renaming table so
the names after renaming are consistent across all the modules.
>  
>>  
>> 4. Debug info strip to line-tables pass:
>> As the name suggested, this pass strip down the full debug info to
line-tables only. This is also one of the steps we took to prevent the leak of
source code information in bitcode.
>> 
>> I'm very curious about what's going on here. Could you
elaborate? :)
> 
> Cc Adrian
> He would know more about it. I would only know that it can reconstruct
-gline-tables-only debug info from full debug info. We use it as a part of the
bitcode pipeline because we don't want the bitcode file to be exceedingly
large but I can see this pass to be useful in other circumstances.
It’s a pass that aims at downgrading full debug info to -gline-tables-only debug
info. It performs a deep copy of the debug metadata graph, removing all types
and variables while leaving all locations, subprograms, and inline information
intact. It then makes the IR point to the newly created stripped subprograms,
CU, and locations.

-- adrian
> 
> Steven
> 
>> 
>> Thanks a ton for the update - glad to see this being worked on!
>> 
>> -eric
>>  
>> 
>> Please let me know what do you think about the pieces above or if you
have any concerns about the methodology. I will put up patches for review soon.
>> 
>> Thanks
>> 
>> Steven
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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On Tue, Jun 14, 2016 at 6:09 PM Adrian Prantl <aprantl at apple.com>
wrote:
> On Jun 13, 2016, at 9:37 AM, Steven Wu <stevenwu at apple.com> wrote:
>
> Thanks for the feedback! Replies inline.
>
> On Jun 12, 2016, at 11:44 PM, Eric Christopher <echristo at
gmail.com> wrote:
>
> Hi Steven,
>
> Great to see the commentary and updates here. I've got a few questions
> about some of this work. It might be nice to see some separate RFCs for a
> couple of things, but we'll figure that out after you send out patches
> probably :)
>
> What needs to be improved:
>> 1. Whitelist for command line options that can be used with bitcode:
>> Current trunk implementation embeds all the cc1 command line options
>> (that includes header include paths, warning flags and other front-end
>> options) in the command line section. That is lot of redundant
information.
>> To re-create the object file from the embedded optimized bitcode, most
of
>> these options are useless. On the other hand, they can leak information
of
>> the source code. One solution will be keeping a list of all the options
>> that can affect code generation but not encoded in the bitcode. I have
>> internally prototyped with disallowing these options explicitly and
allowed
>> only the reminder of the  options to be embedded (
>> http://reviews.llvm.org/D17394). A better solution might be encoding
>> that information in "Options.td" as specific group.
>>
>
> This is really interesting. I'm not a particularly security minded
person
> so I don't have a lot of commentary there. An explicit whitelist sounds
a
> bit painful to keep maintained, explicitly having a group in Options.td
> sounds pretty nice. You'll need to add them to multiple groups, but it
> seems pretty nice.
>
>
> I have already implemented the new approach in
> http://reviews.llvm.org/D21230. It creates a new group for all the cc1
> options that can affect codegen but not having a corresponding attribute in
> the bitcode. When I wrote up this patch, I think it is also a good idea to
> extend the group to driver flags so clang driver can issue warnings when
> using these flags with LTO because they are likely to be dropped in the
> process. That is my next thing to do if someone reviews my patch and agrees
> that is right thing to do.
>
>
>
>> 2. Assembly input handling:
>> This is a workaround to allow source code written in assembly to work
>> with "-fembed-bitcode" options. When compiling assembly
source code with
>> "-fembed-bitcode", clang-as creates an empty section
"__LLVM, __asm" in the
>> object file. That is just a way to distinguish object files compiled
from
>> assembly source from those compiled from higher level source code but
>> forgot to use "-fembed-bitcode" options. Linker can use this
section to
>> diagnose if "-fembed-bitcode" is consistently used on all the
object files
>> participated in the linking.
>>
>
> I'm surprised you want a separate and empty section and not a header
flag
> as those are easier to keep around and won't take up a precious mach-o
> section. There are probably other options here as well. There are probably
> other options or concerns that someone shipping bitcode might have here as
> well, but I'm sure those are being talked about - doesn't have too
much
> affect on the community though.
>
>
> I suppose you mean the alternative is to burn a macho command for that.
> Well, that is a limited resource and we don't have much left. Plus,
using
> empty section will make this accessible to other binary format, not only
> macho files. I also have an interesting thought about handle the assembly,
> that is to wrap it in module assembly in a bitcode file. I am not sure it
> would preserve the all semantics of the original assembly and that would
> mean I need to somehow teach the assembler about bitcode (which might make
> this not very attractive). Yes, you might be right this doesn't affect
the
> community, if no one else is interesting in a solution for the problem we
> have, then this might not be suitable for contributing. I am happy to keep
> it downstream.
>
>
> 3. Bitcode symbol hiding:
>> There was some concerns for leaking source code information when using
>> bitcode feature. One approach to avoid the leak is to add a pass which
>> renames all the globals and metadata strings. The also keeps a reverse
map
>> in case the original name needs to be recovered. The final bitcode
should
>> contain no more symbols or debug info than a stripped binary. To make
sure
>> modified bitcode can still be linked correctly, the renaming need to be
>> consistent across all bitcode participated in the linking and
everything
>> that is external of the linkage unit need to be preserved. This means
the
>> pass can only be run during the linking and requires some LTO api.
>>
>
> How are you planning to ensure the safety of the reverse map? Seems that
> requiring linking is a bit icky, but might work. Are you mostly worried
> about function names that could be stripped out? What LTO api are you
> envisioning here?
>
>
> The reverse map is emitted as a separate file from the output
> binary/bitcode. It should not be shipped together with the binary output,
> just like dSYM bundle.
> The reason it needs to be done after linking is a limitation of the symbol
> hiding technique. It requires that the symbols must be resolved. Think
> about the following case:
> a.o:
> T export_symbol
> T global_symbol
> t local_symbol
> b.o:
> U global_symbol
>
> To make sure the bitcode after symbol hiding pass can still link and
> produce the same output, the pass need to rename them:
> a.o:
> T export_symbol    --> export_symbol (preserve)
> T global_symbol    --> hidden_symbol_1 (rename, but need to have the
same
> name as the one in b.o)
> t local_symbol       --> hidden_symbol_2 (rename, but don't care
what it
> becomes)
> b.o:
> U global_symbol   --> hidden_symbol_1
> The pass need to know what symbols to keep and a global renaming table so
> the names after renaming are consistent across all the modules.
>
>
>
>
>> 4. Debug info strip to line-tables pass:
>> As the name suggested, this pass strip down the full debug info to
>> line-tables only. This is also one of the steps we took to prevent the
leak
>> of source code information in bitcode.
>>
>
> I'm very curious about what's going on here. Could you elaborate?
:)
>
>
> Cc Adrian
> He would know more about it. I would only know that it can reconstruct
> -gline-tables-only debug info from full debug info. We use it as a part of
> the bitcode pipeline because we don't want the bitcode file to be
> exceedingly large but I can see this pass to be useful in other
> circumstances.
>
>
> It’s a pass that aims at downgrading full debug info to -gline-tables-only
> debug info. It performs a deep copy of the debug metadata graph, removing
> all types and variables while leaving all locations, subprograms, and
> inline information intact. It then makes the IR point to the newly created
> stripped subprograms, CU, and locations.
>
>
Sounds like what I'd expect.

How are you planning on hooking all of these up?

-eric

> -- adrian
>
>
> Steven
>
>
> Thanks a ton for the update - glad to see this being worked on!
>
> -eric
>
>
>>
>> Please let me know what do you think about the pieces above or if you
>> have any concerns about the methodology. I will put up patches for
review
>> soon.
>>
>> Thanks
>>
>> Steven
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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> On Jun 14, 2016, at 3:27 PM, Eric Christopher <echristo at gmail.com>
wrote:
> 
> 
> 
> On Mon, Jun 13, 2016 at 9:37 AM Steven Wu <stevenwu at apple.com
<mailto:stevenwu at apple.com>> wrote:
> Thanks for the feedback! Replies inline. 
> 
>> On Jun 12, 2016, at 11:44 PM, Eric Christopher <echristo at
gmail.com <mailto:echristo at gmail.com>> wrote:
>> 
>> Hi Steven,
>> 
>> Great to see the commentary and updates here. I've got a few
questions about some of this work. It might be nice to see some separate RFCs
for a couple of things, but we'll figure that out after you send out patches
probably :)
>> 
>> What needs to be improved:
>> 1. Whitelist for command line options that can be used with bitcode:
>> Current trunk implementation embeds all the cc1 command line options
(that includes header include paths, warning flags and other front-end options)
in the command line section. That is lot of redundant information. To re-create
the object file from the embedded optimized bitcode, most of these options are
useless. On the other hand, they can leak information of the source code. One
solution will be keeping a list of all the options that can affect code
generation but not encoded in the bitcode. I have internally prototyped with
disallowing these options explicitly and allowed only the reminder of the 
options to be embedded (http://reviews.llvm.org/D17394
<http://reviews.llvm.org/D17394>). A better solution might be encoding
that information in "Options.td" as specific group.
>> 
>> This is really interesting. I'm not a particularly security minded
person so I don't have a lot of commentary there. An explicit whitelist
sounds a bit painful to keep maintained, explicitly having a group in Options.td
sounds pretty nice. You'll need to add them to multiple groups, but it seems
pretty nice.
> 
> I have already implemented the new approach in
http://reviews.llvm.org/D21230 <http://reviews.llvm.org/D21230>. It
creates a new group for all the cc1 options that can affect codegen but not
having a corresponding attribute in the bitcode. When I wrote up this patch, I
think it is also a good idea to extend the group to driver flags so clang driver
can issue warnings when using these flags with LTO because they are likely to be
dropped in the process. That is my next thing to do if someone reviews my patch
and agrees that is right thing to do.
> 
> 
> Honestly I think it's better if we implement almost everything in the
bitcode that doesn't involve a pass manager configuration - and then make
that more explicit as far as library calls. Same with TargetMachine
configuration etc. What do you think?
I believe that is always the ultimate goal but it is not an short-term task and
every time I go through the lists of TargetMachine configuration, there are new
flags added.
The easiest solution might be add all of them to the module flags but we
probably need to go through them one by one to see if that is appropriate and
will not conflict with function multiversioning. We also need tests to make sure
flags are properly encoded in the bitcode.
>  
>>  
>> 2. Assembly input handling:
>> This is a workaround to allow source code written in assembly to work
with "-fembed-bitcode" options. When compiling assembly source code
with "-fembed-bitcode", clang-as creates an empty section
"__LLVM, __asm" in the object file. That is just a way to distinguish
object files compiled from assembly source from those compiled from higher level
source code but forgot to use "-fembed-bitcode" options. Linker can
use this section to diagnose if "-fembed-bitcode" is consistently used
on all the object files participated in the linking.
>> 
>> I'm surprised you want a separate and empty section and not a
header flag as those are easier to keep around and won't take up a precious
mach-o section. There are probably other options here as well. There are
probably other options or concerns that someone shipping bitcode might have here
as well, but I'm sure those are being talked about - doesn't have too
much affect on the community though.
> 
> I suppose you mean the alternative is to burn a macho command for that.
Well, that is a limited resource and we don't have much left. Plus, using
empty section will make this accessible to other binary format, not only macho
files. I also have an interesting thought about handle the assembly, that is to
wrap it in module assembly in a bitcode file. I am not sure it would preserve
the all semantics of the original assembly and that would mean I need to somehow
teach the assembler about bitcode (which might make this not very attractive).
Yes, you might be right this doesn't affect the community, if no one else is
interesting in a solution for the problem we have, then this might not be
suitable for contributing. I am happy to keep it downstream.
> 
> 
> You could use a section for ELF. I'm not sure what the right thing to
do for PE would be, but it'll either be a section or a command like thing.
The wrapping asm in module assembly might work, though you'll need to make
some (hopefully all programmatic) textual changes to keep it working. You might
be able to teach clang how to do that.
I will look into this.
>  
>> 
>> 3. Bitcode symbol hiding:
>> There was some concerns for leaking source code information when using
bitcode feature. One approach to avoid the leak is to add a pass which renames
all the globals and metadata strings. The also keeps a reverse map in case the
original name needs to be recovered. The final bitcode should contain no more
symbols or debug info than a stripped binary. To make sure modified bitcode can
still be linked correctly, the renaming need to be consistent across all bitcode
participated in the linking and everything that is external of the linkage unit
need to be preserved. This means the pass can only be run during the linking and
requires some LTO api.
>> 
>> How are you planning to ensure the safety of the reverse map? Seems
that requiring linking is a bit icky, but might work. Are you mostly worried
about function names that could be stripped out? What LTO api are you
envisioning here?
> 
> The reverse map is emitted as a separate file from the output
binary/bitcode.
> 
> That makes some sense.
>  
> It should not be shipped together with the binary output, just like dSYM
bundle.
> The reason it needs to be done after linking is a limitation of the symbol
hiding technique. It requires that the symbols must be resolved. Think about the
following case:
> a.o:
> T export_symbol
> T global_symbol
> t local_symbol
> b.o:
> U global_symbol
> 
> To make sure the bitcode after symbol hiding pass can still link and
produce the same output, the pass need to rename them:
> a.o:
> T export_symbol    --> export_symbol (preserve)
> T global_symbol    --> hidden_symbol_1 (rename, but need to have the
same name as the one in b.o)
> t local_symbol       --> hidden_symbol_2 (rename, but don't care
what it becomes)
> b.o:
> U global_symbol   --> hidden_symbol_1
> The pass need to know what symbols to keep and a global renaming table so
the names after renaming are consistent across all the modules.
>  
> 
> Right. That's where I thought you were going. Would this instead be
better implemented as a tool/library that could "anonymize" a bitcode
file? I realize it wouldn't "work" for shipping to the store if
you ran it on bitcode before it's been linked, but it might be a good way
for people to submit bitcode files to us as well. Just throwing ideas out here
:)
Agree. It is a good tool to have for submitting bitcode bug report and we have a
MetaRenamer to achieve part of that goal. The hard part of that goal is to
rename without changing the semantics of the code. Even the renamer is running
on 'llvm-linked' bitcode, you still need extra information to do it
perfectly (info like compiler_rt symbols, system library interface). Some linker
support will help to fix that problem. For the libLTO solution I proposed, it
will be similar to thinlto interface. It requires the equivalent of:
 thinlto_create_codegen
 thinlto_codegen_dispose
 thinlto_codegen_add_module
 thinlto_codegen_process
 thinlto_codegen_add_must_preserve_symbol
And 2 more to lookup the new name and to write the mapping out to the disk.

>>  
>> 4. Debug info strip to line-tables pass:
>> As the name suggested, this pass strip down the full debug info to
line-tables only. This is also one of the steps we took to prevent the leak of
source code information in bitcode.
>> 
>> I'm very curious about what's going on here. Could you
elaborate? :)
> 
> Cc Adrian
> He would know more about it. I would only know that it can reconstruct
-gline-tables-only debug info from full debug info. We use it as a part of the
bitcode pipeline because we don't want the bitcode file to be exceedingly
large but I can see this pass to be useful in other circumstances.
Adrian, do you want me to post the patch for you?

Steven
> 
> :)
> 
> -eric
>  
> 
> Steven
> 
>> 
>> Thanks a ton for the update - glad to see this being worked on!
>> 
>> -eric
>>  
>> 
>> Please let me know what do you think about the pieces above or if you
have any concerns about the methodology. I will put up patches for review soon.
>> 
>> Thanks
>> 
>> Steven
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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