linux-kernel - Re: [PATCH v2 0/2] perf trace: Fix support for the new BPF feature in clang 12

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Message-ID: <Zw_MFwkejeWC2qbv@x1>
Date: Wed, 16 Oct 2024 11:22:15 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Song Liu <songliubraving@...com>, Howard Chu <howardchu95@...il.com>,
	Andrea Righi <andrea.righi@...ux.dev>, peterz@...radead.org,
	mingo@...hat.com, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, james.clark@...aro.org,
	alan.maguire@...cle.com
Subject: Re: [PATCH v2 0/2] perf trace: Fix support for the new BPF feature
 in clang 12

On Tue, Oct 15, 2024 at 07:06:35PM -0700, Namhyung Kim wrote:
> Hi Arnaldo,
> 
> On Tue, Oct 15, 2024 at 05:37:38PM -0300, Arnaldo Carvalho de Melo wrote:
> > On Tue, Oct 15, 2024 at 04:58:56PM -0300, Arnaldo Carvalho de Melo wrote:
> > > So I'm trying adding extra bounds checking, marking the index as
> > > volatile, adding compiler barriers, etc, all the fun with the verifier,
> > > but got distracted with other stuff, coming back to this now.
> >  
> > > Ok, the following seems to do the trick:
> >  
> > > [acme@...l-per740-01 perf-tools]$ git diff
> > > diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> > > index 3b30aa74a3ae..ef87a04ff8d0 100644
> > > --- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> > > +++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> > > @@ -486,6 +486,7 @@ static int augment_sys_enter(void *ctx, struct syscall_enter_args *args)
> > >                                 augmented = true;
> > >                 } else if (size < 0 && size >= -6) { /* buffer */
> > >                         index = -(size + 1);
> > > +                       index &= 7; // To satisfy the bounds checking with the verifier in some kernels
> > >                         aug_size = args->args[index];
> > >  
> > >                         if (aug_size > TRACE_AUG_MAX_BUF)
> > > 
> > > I'll now test it without Howard's patch to see if it fixes the RHEL8 +
> > > clang 17 case.
> > 
> > It works with this one-liner + the simplified patch from Howard and also
> > on this other system (RHEL9), as well as with Fedora 40, it would be
> > nice if someone could test with clang 16 and report back the version of
> > the kernel tested as well as the distro name/release, that way I can try
> > to get my hands on such as system and test there as well.
> > 
> > Its all at:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools.git tmp.perf-tools
> > 
> > This is the current set of patches that when further tested will go to
> > Linus for v6.12:
> > 
> > ⬢[acme@...lbox perf-tools]$ git log --oneline torvalds/master..
> > ff14baa7a290bf42 (HEAD -> perf-tools, x1/perf-tools, perf-tools/tmp.perf-tools) perf trace augmented_raw_syscalls: Add more checks to pass the verifier
> > 46180bec048aad85 perf trace augmented_raw_syscalls: Add extra array index bounds checking to satisfy some BPF verifiers
> > 45d1aadac64869a2 perf build: Change the clang check back to 12.0.1
> 
> Wouldn't it be better to have this change after fixing the verifier
> issues in the later commits?

I'm still testing it, this is a one-liner, so I think that the order in
which the patches are applied isn't important. Also Howard's patch (the
simplified one) doesn't clash with it.
 
> > 4e21679eb81b5f0d perf trace: The return from 'write' isn't a pid
> > 2d2314d4b09b5ed9 tools headers UAPI: Sync linux/const.h with the kernel headers
> > ⬢[acme@...lbox perf-tools]$
> 
> I guess you also need the syscalltbl fix from Jiri Slaby.
> 
> https://lore.kernel.org/linux-perf-users/3a592835-a14f-40be-8961-c0cee7720a94@kernel.org/

Right, he provided a report about the patch I sent solving the case, I
have to check if he replied to my question about perf trace actually
_working_ on a 32-bit arch system.

I also want to test it, I'm trying to get hold of such a system.

- Arnaldo