[llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection

[Hal Finkel via llvm-dev]

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
> 
> 
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory