llvm dev - Oct 2016 - Using '__attribute__((section("name")))' for inline assembly injection

I recently examined a bug in a program, and it turned out that the customer
was using the section attribute as a form of inline-assembly mechanism, with
something like:

 

__attribute__((section("sectionName\nasm\nasm\nasm")))

 

this was really ugly and not at all obvious where the problem originated.
Is there any way of getting LLVM or CLang to validate the name used in the
section attribute?  The GCC definition says that this only permits
alpha-numeric characters, but I know that it is also very common for people
to use '_' and '.' in section names so alpha-numeric checking is
probably a
bit over-zealous, but embedded newlines definitely should be considered
suspicious.  Or perhaps there is an existing check in 'clang-tidy'?

 

The reason they were doing this in the first place, is that the 'naked'
attribute does not do what they want, which was to allow them to build
custom prologue and epilogue code for a function.

 

Thanks,

 

            MartinO

 

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On Wed, Oct 05, 2016 at 09:56:56AM +0100, Martin J. O'Riordan via llvm-dev
wrote:
> I recently examined a bug in a program, and it turned out that the customer
> was using the section attribute as a form of inline-assembly mechanism,
with
> something like:
> 
> __attribute__((section("sectionName\nasm\nasm\nasm")))
With clang, this does exactly what is requested. Note that GNU as will
give you a warning when using -save-temps or -S, but that's a separate
question.

Joerg

People also try to use
__attribute__((section(".mysection,\"rwx\", at progbits")))
to get executable
code sections for toy experiments in self-modifying code, but we don't
support that either. Basically, we take the section name and always quote
it, unlike GCC, and I don't think we want to change that. Normally LLVM
doesn't round trip through assembly, so it would be infeasible to attempt
to interpret strings in section names in all the ways that GCC allows
through to gas.

My advice for users in this situation would be to try the naked attribute
again, since we've fixed several bugs around it in the last two years. If
that doesn't work, adding a standalone assembly file to the project has
always been and will continue to be the most reliable way to implement a
function with a custom prologue.

On Wed, Oct 5, 2016 at 1:56 AM, Martin J. O'Riordan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I recently examined a bug in a program, and it turned out that the
> customer was using the section attribute as a form of inline-assembly
> mechanism, with something like:
>
>
>
> __attribute__((section(“sectionName\n*asm*\n*asm*\n*asm*”)))
>
>
>
> this was really ugly and not at all obvious where the problem originated.
> Is there any way of getting LLVM or CLang to validate the name used in the
> section attribute?  The GCC definition says that this only permits
> alpha-numeric characters, but I know that it is also very common for people
> to use ‘_’ and ‘.’ in section names so alpha-numeric checking is probably
> a bit over-zealous, but embedded newlines definitely should be considered
> suspicious.  Or perhaps there is an existing check in ‘clang-tidy’?
>
>
>
> The reason they were doing this in the first place, is that the ‘naked’
> attribute does not do what they want, which was to allow them to build
> custom prologue and epilogue code for a function.
>
>
>
> Thanks,
>
>
>
>             MartinO
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Would it be useful for Clang to warn about section names with unusual
characters?

 -Hal

----- Original Message -----
> From: "Reid Kleckner via llvm-dev" <llvm-dev at
lists.llvm.org>
> To: "Martin J. O'Riordan" <martin.oriordan at
movidius.com>
> Cc: "LLVM Developers" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, October 5, 2016 12:06:51 PM
> Subject: Re: [llvm-dev] Using
'__attribute__((section("name")))' for inline assembly
injection
> 
> 
> 
> People also try to use
> __attribute__((section(".mysection,\"rwx\", at
progbits"))) to get
> executable code sections for toy experiments in self-modifying code,
> but we don't support that either. Basically, we take the section
> name and always quote it, unlike GCC, and I don't think we want to
> change that. Normally LLVM doesn't round trip through assembly, so
> it would be infeasible to attempt to interpret strings in section
> names in all the ways that GCC allows through to gas.
> 
> 
> My advice for users in this situation would be to try the naked
> attribute again, since we've fixed several bugs around it in the
> last two years. If that doesn't work, adding a standalone assembly
> file to the project has always been and will continue to be the most
> reliable way to implement a function with a custom prologue.
> 
> 
> On Wed, Oct 5, 2016 at 1:56 AM, Martin J. O'Riordan via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> 
> 
> 
> 
> 
> 
> I recently examined a bug in a program, and it turned out that the
> customer was using the section attribute as a form of
> inline-assembly mechanism, with something like:
> 
> 
> 
> __attribute__((section(“sectionName\n asm \n asm \n asm ”)))
> 
> 
> 
> this was really ugly and not at all obvious where the problem
> originated. Is there any way of getting LLVM or CLang to validate
> the name used in the section attribute? The GCC definition says that
> this only permits alpha-numeric characters, but I know that it is
> also very common for people to use ‘ _ ’ and ‘ . ’ in section names
> so alpha-numeric checking is probably a bit over-zealous, but
> embedded newlines definitely should be considered suspicious. Or
> perhaps there is an existing check in ‘ clang-tidy ’?
> 
> 
> 
> The reason they were doing this in the first place, is that the ‘
> naked ’ attribute does not do what they want, which was to allow
> them to build custom prologue and epilogue code for a function.
> 
> 
> 
> Thanks,
> 
> 
> 
> MartinO
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory