linux-kernel - Re: [PATCH bpf] bpf/btf: Accept function names that contain dots

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Date:   Mon, 19 Jun 2023 18:24:49 +0300
From:   Eduard Zingerman <eddyz87@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, martin.lau@...ux.dev, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        nathan@...nel.org, ndesaulniers@...gle.com, trix@...hat.com,
        stable@...r.kernel.org
Subject: Re: [PATCH bpf] bpf/btf: Accept function names that contain dots

On Mon, 2023-06-19 at 15:55 +0200, Florent Revest wrote:
> On Mon, Jun 19, 2023 at 1:20 PM Florent Revest <revest@...omium.org> wrote:
> > 
> > On Thu, Jun 15, 2023 at 7:05 PM Eduard Zingerman <eddyz87@...il.com> wrote:
> > > 
> > > On Thu, 2023-06-15 at 17:44 +0200, Florent Revest wrote:
> > > > An easy reproducer is:
> > > > 
> > > > $ touch pwet.c
> > > > 
> > > > $ clang -g -fsanitize=kernel-address -c -o pwet.o pwet.c
> > > > $ llvm-dwarfdump pwet.o | grep module_ctor
> > > > 
> > > > $ clang -fno-integrated-as -g -fsanitize=kernel-address -c -o pwet.o pwet.c
> > > > $ llvm-dwarfdump pwet.o | grep module_ctor
> > > >                 DW_AT_name      ("asan.module_ctor")
> > > 
> > > Interestingly, I am unable to reproduce it using either
> > > clang version 14.0.0-1ubuntu1 or clang main (bd66f4b1da30).
> > 
> > Somehow, I didn't think of trying other clang versions! Thanks, that's
> > a good point Eduard. :)
> > 
> > I also can't reproduce it on a 14x build.
> > 
> > However, I seem to be able to reproduce it on main:
> > 
> >   git clone https://github.com/llvm/llvm-project.git
> >   mkdir llvm-project/build
> >   cd llvm-project/build
> >   git checkout bd66f4b1da30
> >   cmake -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_BUILD_TYPE=Release -G
> > "Unix Makefiles" ../llvm
> >   make -j $(nproc)
> > 
> >   bin/clang -fno-integrated-as -g -fsanitize=kernel-address -c -o
> > ~/pwet.o ~/pwet.c
> >   bin/llvm-dwarfdump ~/pwet.o | grep module_ctor
> >   # Shows module_ctor
> > 
> > I started a bisection, hopefully that will point to something interesting
> 
> The bisection pointed to a LLVM patch from Nick in October 2022:
> e3bb359aacdd ("[clang][Toolchains][Gnu] pass -g through to assembler")
> 
> Based on the context I have, that commit sounds fair enough. I don't
> think LLVM does anything wrong here, it seems like BPF should be the
> one dealing with dots in function debug info.

That explains why I could not reproduce the issue: I tried with gas 2.38.
Using gas 2.40 I see the same behavior as you.

If one tries to generate assembly file with '-fsanitize':

  $ clang -fno-integrated-as -g -fsanitize=kernel-address -S -o pwet.s pwet.c
  $ cat pwet.s
  	.text
  	.file	"pwet.c"
  	.p2align	4, 0x90                         # -- Begin function asan.module_ctor
  	.type	asan.module_ctor,@function
  asan.module_ctor:                       # @asan.module_ctor
  .Lfunc_begin0:
      ...

And then compile it using Gnu assembler:

  $ as --64 -o pwet.o pwet.s -g -gdwarf-5

The behavior differs between 2.38 and 2.40, the older version does not
produce debug entry for 'asan.module_ctor', while newer does.