linux-kernel - Re: outside array bounds error on ppc64

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Message-ID: <20220607150406.GF25951@gate.crashing.org>
Date:   Tue, 7 Jun 2022 10:04:06 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Bagas Sanjaya <bagasdotme@...il.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Yang Li <yang.lee@...ux.alibaba.com>
Subject: Re: outside array bounds error on ppc64_defconfig, GCC 12.1.0

On Tue, Jun 07, 2022 at 12:05:18PM +1000, Michael Ellerman wrote:
> > arch/powerpc/lib/sstep.c:287:23: error: array subscript [3, 4] is outside array bounds of 'union <anonymous>[1]' [-Werror=array-bounds]
> >   287 |                 up[3] = tmp;
> >       |                 ~~~~~~^~~~~
> 
> This happens because we have a generic byte reverse function
> (do_byte_reverse()), that takes a size as a parameter. So it will
> reverse 8, 16, 32 bytes etc.
> 
> In some cases the compiler can see that we're passing a pointer to
> storage that is smaller than 32 bytes, but it isn't convinced that the
> size parameter is also smaller than 32 bytes.
> 
> Which I think is reasonable, the code that sets the size is separate
> from this code, so the compiler can't really deduce that it's safe.
> 
> I don't see a really simple fix. I tried clamping the size parameter to
> do_byte_reverse() with max(), but that didn't work :/

-Wno-error or at least -Wno-error=array-bounds is a good, simple fix.


Segher