llvm dev - Nov 2014 - [LLVMdev] Using the unused "version" field in the bitcode wrapper (redux)

The fundamental problem is that IR has never been treated as "user
input" before.  It has always been an ephemeral format, and if some
component comes along and sees something it doesn't recognize, that is ipso
facto a compiler bug, not a user input error, and it's perfectly okay to
crash on a compiler bug.  Changing that fundamental assumption would be pretty
pervasive.

I will say that the bitcode reader itself is pretty good about identifying
bitcode it doesn't recognize; it's the components deeper inside LLVM
that are more worrisome.
--paulr

From: Reid Kleckner [mailto:rnk at google.com]
Sent: Tuesday, November 25, 2014 11:43 AM
To: Sean Silva
Cc: Robinson, Paul; Bruce Hoult; llvmdev at cs.uiuc.edu; Yung, Douglas
Subject: Re: [LLVMdev] Using the unused "version" field in the bitcode
wrapper (redux)

On Mon, Nov 24, 2014 at 9:41 PM, Sean Silva <chisophugis at
gmail.com<mailto:chisophugis at gmail.com>> wrote:
I fail to see why all that re-engineering effort IS "required" to
support the use-cases we've provided, and why future-proofing a large body
of software that quite honestly was never designed for it and has genuinely very
few reasons why it should be.

If we're going to support bitcode as a long-term storage format, everything
that pulls information out of bitcode MUST be future-proofed.  Doing it piece by
piece is a LOT of engineering effort to make LLVM user-friendly in this way; it
was never intended to have that degree of self-defense.

I don't understand what you mean by "future-proofed" in this
context. If you mean "never crash on bad user input", then your point
doesn't make sense to me. LLVM is engineered to never crash on bad user
input in the same sense that it is engineered to correctly compile code; neither
is allowed, and if either happens it is a bug.

This is true, but we can say from experience that there are a lot of these kinds
of bugs and it will take a lot of effort to fix them all. Personally, I think
this worth doing. We should be using the new diagnostic handler on the
LLVMContext, and most of LLVM should have a way of returning without exploding.
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On Nov 25, 2014 11:59 AM, "Robinson, Paul" <
Paul_Robinson at playstation.sony.com> wrote:
>
> The fundamental problem is that IR has never been treated as "user
input"
before.  It has always been an ephemeral format, and if some component
comes along and sees something it doesn't recognize, that is ipso facto a
compiler bug, not a user input error, and it's perfectly okay to crash on a
compiler bug.  Changing that fundamental assumption would be pretty
pervasive.

Sorry, but this is at variance to my experience. In my experience, the
fundamental assumption is that we should never crash on bad user input. And
LLVM as a library definitely considers bitcode as "user input".

Of course, -disable-verify and similar voids that warranty.
>
>
>
> I will say that the bitcode reader itself is pretty good about
identifying bitcode it doesn't recognize; it's the components deeper
inside
LLVM that are more worrisome.

Could you give a couple examples? It sounds like you are talking about a
systemic problem, but so far only one example has been presented.

-- Sean Silva
>
> --paulr
>
>
>
> From: Reid Kleckner [mailto:rnk at google.com]
> Sent: Tuesday, November 25, 2014 11:43 AM
> To: Sean Silva
> Cc: Robinson, Paul; Bruce Hoult; llvmdev at cs.uiuc.edu; Yung, Douglas
>
> Subject: Re: [LLVMdev] Using the unused "version" field in the
bitcode
wrapper (redux)
>
>
>
> On Mon, Nov 24, 2014 at 9:41 PM, Sean Silva <chisophugis at
gmail.com> wrote:
>>
>> I fail to see why all that re-engineering effort IS
"required" to
support the use-cases we've provided, and why future-proofing a large body
of software that quite honestly was never designed for it and has genuinely
very few reasons why it should be.
>>
>>
>>
>> If we're going to support bitcode as a long-term storage format,
everything that pulls information out of bitcode MUST be future-proofed.
Doing it piece by piece is a LOT of engineering effort to make LLVM
user-friendly in this way; it was never intended to have that degree of
self-defense.
>
>
>
> I don't understand what you mean by "future-proofed" in this
context. If
you mean "never crash on bad user input", then your point doesn't
make
sense to me. LLVM is engineered to never crash on bad user input in the
same sense that it is engineered to correctly compile code; neither is
allowed, and if either happens it is a bug.
>
>
>
> This is true, but we can say from experience that there are a lot of
these kinds of bugs and it will take a lot of effort to fix them all.
Personally, I think this worth doing. We should be using the new diagnostic
handler on the LLVMContext, and most of LLVM should have a way of returning
without exploding.
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Where is the testing that proves all this works?  Patches welcome…

Now, I've known QA people who break software in two minutes as naturally as
breathing; I do not have that skill.  Given that I came up with *anything* in
two minutes, I am not filled with confidence.

Right now I'm in the middle of un-borking my bots in the wake of the
upstream "improvements" to GetSVN.cmake, but I will see what else I
can do.
--paulr

From: Sean Silva [mailto:chisophugis at gmail.com]
Sent: Tuesday, November 25, 2014 2:38 PM
To: Robinson, Paul
Cc: Yung, Douglas; llvmdev at cs.uiuc.edu; Reid Kleckner; Bruce Hoult
Subject: RE: [LLVMdev] Using the unused "version" field in the bitcode
wrapper (redux)


On Nov 25, 2014 11:59 AM, "Robinson, Paul" <Paul_Robinson at
playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>
wrote:
>
> The fundamental problem is that IR has never been treated as "user
input" before.  It has always been an ephemeral format, and if some
component comes along and sees something it doesn't recognize, that is ipso
facto a compiler bug, not a user input error, and it's perfectly okay to
crash on a compiler bug.  Changing that fundamental assumption would be pretty
pervasive.
Sorry, but this is at variance to my experience. In my experience, the
fundamental assumption is that we should never crash on bad user input. And LLVM
as a library definitely considers bitcode as "user input".

Of course, -disable-verify and similar voids that warranty.
>
>
>
> I will say that the bitcode reader itself is pretty good about identifying
bitcode it doesn't recognize; it's the components deeper inside LLVM
that are more worrisome.
Could you give a couple examples? It sounds like you are talking about a
systemic problem, but so far only one example has been presented.

-- Sean Silva
>
> --paulr
>
>
>
> From: Reid Kleckner [mailto:rnk at google.com<mailto:rnk at
google.com>]
> Sent: Tuesday, November 25, 2014 11:43 AM
> To: Sean Silva
> Cc: Robinson, Paul; Bruce Hoult; llvmdev at cs.uiuc.edu<mailto:llvmdev
at cs.uiuc.edu>; Yung, Douglas
>
> Subject: Re: [LLVMdev] Using the unused "version" field in the
bitcode wrapper (redux)
>
>
>
> On Mon, Nov 24, 2014 at 9:41 PM, Sean Silva <chisophugis at
gmail.com<mailto:chisophugis at gmail.com>> wrote:
>>
>> I fail to see why all that re-engineering effort IS
"required" to support the use-cases we've provided, and why
future-proofing a large body of software that quite honestly was never designed
for it and has genuinely very few reasons why it should be.
>>
>>
>>
>> If we're going to support bitcode as a long-term storage format,
everything that pulls information out of bitcode MUST be future-proofed.  Doing
it piece by piece is a LOT of engineering effort to make LLVM user-friendly in
this way; it was never intended to have that degree of self-defense.
>
>
>
> I don't understand what you mean by "future-proofed" in this
context. If you mean "never crash on bad user input", then your point
doesn't make sense to me. LLVM is engineered to never crash on bad user
input in the same sense that it is engineered to correctly compile code; neither
is allowed, and if either happens it is a bug.
>
>
>
> This is true, but we can say from experience that there are a lot of these
kinds of bugs and it will take a lot of effort to fix them all. Personally, I
think this worth doing. We should be using the new diagnostic handler on the
LLVMContext, and most of LLVM should have a way of returning without exploding.
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