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Date: Thu, 8 Jun 2023 10:29:40 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: mark.rutland@arm.com, arnd@arndb.de, ndesaulniers@google.com,
	trix@redhat.com, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	patches@lists.linux.dev
Subject: Re: [PATCH] percpu: Fix self-assignment of __old in
 raw_cpu_generic_try_cmpxchg()
Message-ID: <20230608082940.GF998233@hirez.programming.kicks-ass.net>
References: <20230607-fix-shadowing-in-raw_cpu_generic_try_cmpxchg-v1-1-8f0a3d930d43@kernel.org>
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In-Reply-To: <20230607-fix-shadowing-in-raw_cpu_generic_try_cmpxchg-v1-1-8f0a3d930d43@kernel.org>

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?

> Debugged-by: Nick Desaulniers <ndesaulniers@google.com>
> Fixes: c5c0ba953b8c ("percpu: Add {raw,this}_cpu_try_cmpxchg()")
> Signed-off-by: Nathan Chancellor <nathan@kernel.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!