linux-kernel - Re: [m68k:master 1174/1174] arch/m68k/include/asm/string.h:72:25: warning: '__builtin_memcpy' forming offset 8 is out of the bounds [0, 7]

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Date:   Tue, 5 Mar 2019 19:58:54 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [m68k:master 1174/1174] arch/m68k/include/asm/string.h:72:25:
 warning: '__builtin_memcpy' forming offset 8 is out of the bounds [0, 7]

On Tue, 5 Mar 2019, Geert Uytterhoeven wrote:

> On Tue, Mar 5, 2019 at 3:58 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> > On Tue, 5 Mar 2019, Finn Thain wrote:
> > > Looks bogus to me.
> > >
> > > If you change memcpy to __builtin_memcpy, then we avoid the macro and the
> > > warning changes to,
> > >
> > > ./include/linux/string.h:456:3: warning: '__builtin_memcpy' forming offset [7, 8] is out of the bounds [0, 6] [-Warray-bounds]
> > >    __builtin_memcpy(dest, src, dest_len);
> > >
> > > The compiler has nothing to complain about here. dest is known to be
> > > id->fr and dest_len is known to be sizeof(id->fr).
> > >
> > > The error message indicates that gcc has applied the bounds [0, 6] to dest
> > > when in fact those are the bounds for src.
> > >
> >
> > My mistake. GCC is right, it seems memcpy will read past the end of
> > "5.0.0+".
> 
> But only if the else branch is taken, which is not the case.
> 

You and I know that, because we can see what values get passed to 
memcpy_and_pad(). But how is gcc to know that?

If we replace strlen with __builtin_strlen, this problem goes away. It's 
interesting that the kernel's strlen implementation in 
include/linux/string.h can't achieve this.

-- 

> Gr{oetje,eeting}s,
> 
>                         Geert
> 
>