linux-kernel - Re: [PATCH] percpu: Fix self-assignment of __old in raw_cpu_generic_try

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Message-ID: <20230608155426.GA188192@dev-arch.thelio-3990X>
Date:   Thu, 8 Jun 2023 08:54:26 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mark.rutland@....com, arnd@...db.de, ndesaulniers@...gle.com,
        trix@...hat.com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev
Subject: Re: [PATCH] percpu: Fix self-assignment of __old in
 raw_cpu_generic_try_cmpxchg()

On Thu, Jun 08, 2023 at 10:29:40AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 07, 2023 at 02:20:59PM -0700, Nathan Chancellor wrote:
> > After commit c5c0ba953b8c ("percpu: Add {raw,this}_cpu_try_cmpxchg()"),
> > clang built ARCH=arm and ARCH=arm64 kernels with CONFIG_INIT_STACK_NONE
> > started panicking on boot in alloc_vmap_area():
> > 
> >   [    0.000000] kernel BUG at mm/vmalloc.c:1638!
> >   [    0.000000] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
> >   [    0.000000] Modules linked in:
> >   [    0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.4.0-rc2-ARCH+ #1
> >   [    0.000000] Hardware name: linux,dummy-virt (DT)
> >   [    0.000000] pstate: 200000c9 (nzCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >   [    0.000000] pc : alloc_vmap_area+0x7ec/0x7f8
> >   [    0.000000] lr : alloc_vmap_area+0x7e8/0x7f8
> > 
> > Compiling mm/vmalloc.c with W=2 reveals an instance of -Wshadow, which
> > helps uncover that through macro expansion, '__old = *(ovalp)' in
> > raw_cpu_generic_try_cmpxchg() can become '__old = *(&__old)' through
> > raw_cpu_generic_cmpxchg(), which results in garbage being assigned to
> > the inner __old and the cmpxchg not working properly.
> > 
> > Add an extra underscore to __old in raw_cpu_generic_try_cmpxchg() so
> > that there is no more self-assignment, which resolves the panics.
> > 
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/1868
> 
> First time I see Closes; is this an 'official' tag?

It is, but only recently:

https://git.kernel.org/linus/0d828200ad56505a827610af876ca0b138b943a6

checkpatch.pl wants Closes: after Reported-by: so I have just started
getting into the habit of using it even when I am the reporter :)

> > Debugged-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Fixes: c5c0ba953b8c ("percpu: Add {raw,this}_cpu_try_cmpxchg()")
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> 
> Durr, C is such a lovely language :-)

:)

> I'll go stick it in locking/core to go with the bunch that wrecked it.
> Thanks!

Thanks for the quick response!

Cheers,
Nathan