Hi,
I would like to propose that we introduce a mechanism in IR to allow
arbitrary data to be stashed before a function body. The purpose of
this would be to allow additional data about a function to be looked
up via a function pointer. Two use cases come to mind:
1) We'd like to be able to use UBSan to check that the type of the
function pointer of an indirect function call matches the type of
the function being called. This can't really be done efficiently
without storing type information near the function.
2) Allowing GHC's tables-next-to-code ABI [1] to be implemented.
In general, I imagine this feature could be useful for the
implementation of languages which require runtime metadata for
each function.
The proposal is that an IR function definition acquires a constant
operand which contains the data to be emitted immediately before
the function body (known as the prefix data). To access the data
for a given function, a program may bitcast the function pointer to
a pointer to the constant's type. This implies that the IR symbol
points to the start of the prefix data.
To maintain the semantics of ordinary function calls, the prefix data
must have a particular format. Specifically, it must begin with a
sequence of bytes which decode to a sequence of machine instructions,
valid for the module's target, which transfer control to the point
immediately succeeding the prefix data, without performing any other
visible action. This allows the inliner and other passes to reason
about the semantics of the function definition without needing to
reason about the prefix data. Obviously this makes the format of the
prefix data highly target dependent.
This requirement could be relaxed when combined with my earlier symbol
offset proposal [2] as applied to functions. However, this is outside
the scope of the current proposal.
Example:
%0 = type <{ i32, i8* }>
define void @f() prefix %0 <{ i32 1413876459, i8* bitcast ({ i8*, i8* }*
@_ZTIFvvE to i8*) }> {
ret void
}
This is an example of something that UBSan might generate on an
x86_64 machine. It consists of a signature of 4 bytes followed by a
pointer to the RTTI data for the type 'void ()'. The signature when
laid out as a little endian 32-bit integer decodes to the instruction
'jmp .+0x0c' (which jumps to the instruction immediately succeeding
the 12-byte prefix) followed by the bytes 'F' and 'T' which
identify
the prefix as a UBSan function type prefix.
A caller might check that a given function pointer has a valid signature
like this:
%4 = bitcast void ()* @f to %0*
%5 = getelementptr %0* %4, i32 0, i32 0
%6 = load i32* %5
%7 = icmp eq i32 %6, 1413876459
In the specific case above, where the function pointer is a constant,
optimisation passes such as globalopt could potentially be adapted
to recognise prefix data and hence replace %6 etc with a constant.
(This is one reason why I decided to represent prefix data in IR
rather than, say, using inline asm as proposed in the GHC thread [1].)
Thoughts?
Thanks,
--
Peter
[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-February/047550.html
[2] http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-April/061511.html
On Wed, Jul 17, 2013 at 6:06 PM, Peter Collingbourne <peter at pcc.me.uk>wrote:> Hi, > > I would like to propose that we introduce a mechanism in IR to allow > arbitrary data to be stashed before a function body. The purpose of > this would be to allow additional data about a function to be looked > up via a function pointer. Two use cases come to mind: > > 1) We'd like to be able to use UBSan to check that the type of the > function pointer of an indirect function call matches the type of > the function being called. This can't really be done efficiently > without storing type information near the function. >How efficient does it have to be? Have some alternatives already proven to be "too slow"? (e.g. a binary search into a sorted table)> > 2) Allowing GHC's tables-next-to-code ABI [1] to be implemented. > In general, I imagine this feature could be useful for the > implementation of languages which require runtime metadata for > each function. > > The proposal is that an IR function definition acquires a constant > operand which contains the data to be emitted immediately before > the function body (known as the prefix data). To access the data > for a given function, a program may bitcast the function pointer to > a pointer to the constant's type. This implies that the IR symbol > points to the start of the prefix data. > > To maintain the semantics of ordinary function calls, the prefix data > must have a particular format. Specifically, it must begin with a > sequence of bytes which decode to a sequence of machine instructions, > valid for the module's target, which transfer control to the point > immediately succeeding the prefix data, without performing any other > visible action. This allows the inliner and other passes to reason > about the semantics of the function definition without needing to > reason about the prefix data. Obviously this makes the format of the > prefix data highly target dependent. >I'm not sure that something this target dependent is the right choice. Your example below suggests that the frontend would then need to know magic to put "raw" in the instruction stream. Have you considered having the feature expose just the intent "store this data attached to the function, to be accessed very quickly", and then have an intrinsic ("llvm.getfuncdata.i{8,16,32,64}"?) which extracts the data in a target-dependent way? Forcing clients to embed deep target-specific-machine-code knowledge in their frontends seems like a step in the wrong direction for LLVM.> > This requirement could be relaxed when combined with my earlier symbol > offset proposal [2] as applied to functions. However, this is outside > the scope of the current proposal. > > Example: > > %0 = type <{ i32, i8* }> > > define void @f() prefix %0 <{ i32 1413876459, i8* bitcast ({ i8*, i8* }* > @_ZTIFvvE to i8*) }> { > ret void > } > > This is an example of something that UBSan might generate on an > x86_64 machine. It consists of a signature of 4 bytes followed by a > pointer to the RTTI data for the type 'void ()'. The signature when > laid out as a little endian 32-bit integer decodes to the instruction > 'jmp .+0x0c' (which jumps to the instruction immediately succeeding > the 12-byte prefix) followed by the bytes 'F' and 'T' which identify > the prefix as a UBSan function type prefix. >Do you know whether OoO CPU's will still attempt to decode the "garbage" in the instruction stream, even if there is a jump over it? (IIRC they will decode ahead of the PC and hiccup (but not fault) on garbage). Maybe it would be better to steganographically encode the value inside the instruction stream? On x86 you could use 48b8<imm64> which only has 2 bytes overhead for an i64 (putting a move like that, which moves into a caller-saved register on entry, would effectively be a noop). This is some pretty gnarly target-dependent stuff which seems like it would best be hidden in the backend (e.g. architectures that have "constant island"-like passes might want to stash the data in there instead). -- Sean Silva -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130717/48a27a7a/attachment.html>
On Wed, Jul 17, 2013 at 9:06 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:> > To maintain the semantics of ordinary function calls, the prefix data > must have a particular format. Specifically, it must begin with a > sequence of bytes which decode to a sequence of machine instructions, > valid for the module's target, which transfer control to the point > immediately succeeding the prefix data, without performing any other > visible action. This allows the inliner and other passes to reason > about the semantics of the function definition without needing to > reason about the prefix data. Obviously this makes the format of the > prefix data highly target dependent.What if the prefix data was stored before the start of the function code? The function's symbol will point to the code just as before, eliminating the need to have instructions that skip the prefix data. It would look something like: | Prefix Data ... (variable length) | Prefix Data Length (fixed length [32 bits?]) | Function code .... | ^ function symbol points here (function code) I hope the simple ASCII art makes it through my mail client. To access the data, you do prefix_data = function_ptr - sizeof(prefix_length) - prefix_length Cheers, Jevin
On Wed, Jul 17, 2013 at 07:50:58PM -0700, Sean Silva wrote:> On Wed, Jul 17, 2013 at 6:06 PM, Peter Collingbourne <peter at pcc.me.uk>wrote: > > > Hi, > > > > I would like to propose that we introduce a mechanism in IR to allow > > arbitrary data to be stashed before a function body. The purpose of > > this would be to allow additional data about a function to be looked > > up via a function pointer. Two use cases come to mind: > > > > 1) We'd like to be able to use UBSan to check that the type of the > > function pointer of an indirect function call matches the type of > > the function being called. This can't really be done efficiently > > without storing type information near the function. > > > > How efficient does it have to be? Have some alternatives already proven to > be "too slow"? (e.g. a binary search into a sorted table)This has admittedly not been measured. It depends on the rate at which the program performs indirect function calls. But given the other use cases for this feature we might as well use it in UBSan as opposed to something which is going to be strictly slower.> > 2) Allowing GHC's tables-next-to-code ABI [1] to be implemented. > > In general, I imagine this feature could be useful for the > > implementation of languages which require runtime metadata for > > each function. > > > > The proposal is that an IR function definition acquires a constant > > operand which contains the data to be emitted immediately before > > the function body (known as the prefix data). To access the data > > for a given function, a program may bitcast the function pointer to > > a pointer to the constant's type. This implies that the IR symbol > > points to the start of the prefix data. > > > > To maintain the semantics of ordinary function calls, the prefix data > > must have a particular format. Specifically, it must begin with a > > sequence of bytes which decode to a sequence of machine instructions, > > valid for the module's target, which transfer control to the point > > immediately succeeding the prefix data, without performing any other > > visible action. This allows the inliner and other passes to reason > > about the semantics of the function definition without needing to > > reason about the prefix data. Obviously this makes the format of the > > prefix data highly target dependent. > > > > I'm not sure that something this target dependent is the right choice. Your > example below suggests that the frontend would then need to know magic to > put "raw" in the instruction stream. Have you considered having the feature > expose just the intent "store this data attached to the function, to be > accessed very quickly", and then have an intrinsic > ("llvm.getfuncdata.i{8,16,32,64}"?) which extracts the data in a > target-dependent way?The problem is that things like UBSan need to be able to understand the instruction stream anyway (to a certain extent). In UBSan's case, determining at runtime whether a function has prefix data depends on a specific signature of instructions at the start of the program. There are a wide variety of signatures that can be used here and I believe we shouldn't try to constrain the frontend author with a signature (at least partly) of our own design. I think that if someone wants a target-independent way of embedding prefix data it should be done as a library on top of the target-dependent facilities provided in IR. One could imagine a set of routines like this: /// Given some constant data, attach valid prefix data. void attachPrefixData(Function *F, Constant *Data); /// Returns an i1 indicating whether prefix data is present for FP. Value *hasPrefixData(Value *FP); /// Returns a pointer to the prefix data for FP. Value *getPrefixDataPointer(Value *FP, Type *DataType);> Forcing clients to embed deep > target-specific-machine-code knowledge in their frontends seems like a step > in the wrong direction for LLVM.Given a set of routines such as the ones described above, I think we can give frontends a choice of whether to do this or not. Besides, LLVM already contains plenty of target-specific information in its IR. Varargs, inline asm, calling conventions, etc. I don't think making all aspects of the IR target-independent should be a worthwhile goal for LLVM.> > This requirement could be relaxed when combined with my earlier symbol > > offset proposal [2] as applied to functions. However, this is outside > > the scope of the current proposal. > > > > Example: > > > > %0 = type <{ i32, i8* }> > > > > define void @f() prefix %0 <{ i32 1413876459, i8* bitcast ({ i8*, i8* }* > > @_ZTIFvvE to i8*) }> { > > ret void > > } > > > > This is an example of something that UBSan might generate on an > > x86_64 machine. It consists of a signature of 4 bytes followed by a > > pointer to the RTTI data for the type 'void ()'. The signature when > > laid out as a little endian 32-bit integer decodes to the instruction > > 'jmp .+0x0c' (which jumps to the instruction immediately succeeding > > the 12-byte prefix) followed by the bytes 'F' and 'T' which identify > > the prefix as a UBSan function type prefix. > > > > Do you know whether OoO CPU's will still attempt to decode the "garbage" in > the instruction stream, even if there is a jump over it? (IIRC they will > decode ahead of the PC and hiccup (but not fault) on garbage). Maybe it > would be better to steganographically encode the value inside the > instruction stream? On x86 you could use 48b8<imm64> which only has 2 bytes > overhead for an i64 (putting a move like that, which moves into a > caller-saved register on entry, would effectively be a noop).On the contrary, I think this is a good argument for allowing (not forcing) frontends to encode the prefix data as they please, thus enabling this kind of creativity.> This is some > pretty gnarly target-dependent stuff which seems like it would best be > hidden in the backend (e.g. architectures that have "constant island"-like > passes might want to stash the data in there instead).I think that adding support for things like constant islands is something that can be added incrementally at a later stage. One could consider for example an additional llvm::Function field which specifies the number of bytes that the backend may use at the beginning of the function such that the prefix data may be of any format. (Once this is in place the aforementioned library routines could become relatively trivial.) The backend could use this space to, say, insert a relative branch that skips the prefix data and a first constant island. Thanks, -- Peter
On Thu, Jul 18, 2013 at 12:45:32PM -0400, Jevin Sweval wrote:> On Wed, Jul 17, 2013 at 9:06 PM, Peter Collingbourne <peter at pcc.me.uk> wrote: > > > > To maintain the semantics of ordinary function calls, the prefix data > > must have a particular format. Specifically, it must begin with a > > sequence of bytes which decode to a sequence of machine instructions, > > valid for the module's target, which transfer control to the point > > immediately succeeding the prefix data, without performing any other > > visible action. This allows the inliner and other passes to reason > > about the semantics of the function definition without needing to > > reason about the prefix data. Obviously this makes the format of the > > prefix data highly target dependent. > > > What if the prefix data was stored before the start of the function > code? The function's symbol will point to the code just as before, > eliminating the need to have instructions that skip the prefix data. > > It would look something like: > | Prefix Data ... (variable length) | Prefix Data Length (fixed length > [32 bits?]) | Function code .... | > > ^ function symbol points here (function code) > > I hope the simple ASCII art makes it through my mail client. > > To access the data, you do > > prefix_data = function_ptr - sizeof(prefix_length) - prefix_lengthA similar scheme is described in the next paragraph of my email:> > This requirement could be relaxed when combined with my earlier symbol > > offset proposal [2] as applied to functions. However, this is outside > > the scope of the current proposal.Unfortunately, this won't work for UBSan, as it needs to be able to take an arbitrary function pointer and determine whether the prefix data is present. If the function lives at the beginning of a segment boundary and does not have prefix data a segfault may occur when attempting to access the prefix data. It should definitely work for GHC though (and is how I understand the tables-next-to-code ABI to be implemented in its non-LLVM backend). Thanks, -- Peter
As much as I like this idea for it's use in languages with type systems
like Haskell and Scheme, this proposal would limit LLVM to non-Harvard
architectures. That's generally a really small minority of all processors,
but it would mean there could never be a clang-avr.
An alternative you could use is, instead of using the function pointer as
the variable where you are referring to a function, you could have the
variable be a pointer to a static struct with the data and the actual
function pointer. Basically, it's like how static class variables as
handled in C++.
I don't know LLVM IR, so I'll use C to explain.
Instead of this:
void func(void){}
int main(){
func();
return 0;
}
You could do this:
void func(void){}
/* You have to initialize this at compile time. */
struct {
char* data;
int len;
void (*ptr)(void) = func;
} func_data;
int main(){
func_data.ptr();
return 0;
}
On Jul 18, 2013 12:47 PM, "Jevin Sweval" <jevinsweval at
gmail.com> wrote:
> On Wed, Jul 17, 2013 at 9:06 PM, Peter Collingbourne <peter at
pcc.me.uk>
> wrote:
> >
> > To maintain the semantics of ordinary function calls, the prefix data
> > must have a particular format. Specifically, it must begin with a
> > sequence of bytes which decode to a sequence of machine instructions,
> > valid for the module's target, which transfer control to the point
> > immediately succeeding the prefix data, without performing any other
> > visible action. This allows the inliner and other passes to reason
> > about the semantics of the function definition without needing to
> > reason about the prefix data. Obviously this makes the format of the
> > prefix data highly target dependent.
>
>
> What if the prefix data was stored before the start of the function
> code? The function's symbol will point to the code just as before,
> eliminating the need to have instructions that skip the prefix data.
>
> It would look something like:
> | Prefix Data ... (variable length) | Prefix Data Length (fixed length
> [32 bits?]) | Function code .... |
>
> ^ function symbol points here (function code)
>
> I hope the simple ASCII art makes it through my mail client.
>
> To access the data, you do
>
> prefix_data = function_ptr - sizeof(prefix_length) - prefix_length
>
> Cheers,
> Jevin
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Hi,> What if the prefix data was stored before the start of the function > code? The function's symbol will point to the code just as before, > eliminating the need to have instructions that skip the prefix data.how many platforms would this work on? Last time I tried something analogous to this it fell through because the Darwin object code format didn't support it. I'm not saying that what you are suggesting wouldn't work on Darwin - I don't know. But given my past experience it would be wise to do a platform survey before going too far. Ciao, Duncan.