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]
From: Finn Thain
Date: Mon Mar 04 2019 - 21:56:04 EST
On Mon, 4 Mar 2019, Geert Uytterhoeven wrote:
> On Mon, Mar 4, 2019 at 1:44 PM kbuild test robot <lkp@xxxxxxxxx> wrote:
> > tree: https://git.kernel.org/pub/scm/linux/kernel/git/geert/linux-m68k.git master
> > head: e223cadc191661c67cb419b3a53c7854ecc39e8b
> > commit: e223cadc191661c67cb419b3a53c7854ecc39e8b [1174/1174] Merge tag 'v5.0'
> > config: m68k-allmodconfig (attached as .config)
> > compiler: m68k-linux-gnu-gcc (Debian 8.2.0-11) 8.2.0
> > reproduce:
> > wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > chmod +x ~/bin/make.cross
> > git checkout e223cadc191661c67cb419b3a53c7854ecc39e8b
> > # save the attached .config to linux build tree
> > GCC_VERSION=8.2.0 make.cross ARCH=m68k
> >
> > All warnings (new ones prefixed by >>):
> >
> > In file included from include/linux/string.h:20,
> > from include/linux/bitmap.h:9,
> > from include/linux/nodemask.h:95,
> > from include/linux/mmzone.h:17,
> > from include/linux/gfp.h:6,
> > from include/linux/umh.h:4,
> > from include/linux/kmod.h:22,
> > from include/linux/module.h:13,
> > from drivers/nvme/target/admin-cmd.c:15:
> > In function 'memcpy_and_pad',
> > inlined from 'nvmet_execute_identify_ctrl' at drivers/nvme/target/admin-cmd.c:309:2:
> > >> arch/m68k/include/asm/string.h:72:25: warning: '__builtin_memcpy' forming offset 8 is out of the bounds [0, 7] [-Warray-bounds]
> > #define memcpy(d, s, n) __builtin_memcpy(d, s, n)
> > ^~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/string.h:456:3: note: in expansion of macro 'memcpy'
> > memcpy(dest, src, dest_len);
> > ^~~~~~
> >
> > vim +/__builtin_memcpy +72 arch/m68k/include/asm/string.h
> >
> > ea61bc46 Greg Ungerer 2010-09-07 69
> > ea61bc46 Greg Ungerer 2010-09-07 70 #define __HAVE_ARCH_MEMCPY
> > ea61bc46 Greg Ungerer 2010-09-07 71 extern void *memcpy(void *, const void *, __kernel_size_t);
> > ea61bc46 Greg Ungerer 2010-09-07 @72 #define memcpy(d, s, n) __builtin_memcpy(d, s, n)
> > ea61bc46 Greg Ungerer 2010-09-07 73
> >
> > :::::: The code at line 72 was first introduced by commit
> > :::::: ea61bc461d09e8d331a307916530aaae808c72a2 m68k/m68knommu: merge MMU and non-MMU string.h
> >
> > :::::: TO: Greg Ungerer <gerg@xxxxxxxxxxxx>
> > :::::: CC: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
>
> Yep, that's a funny one, which I saw myself this morning, and decided
> to dive into.
> Apparently this is due 1) the recently added -ffreestanding and
> 2) the kernel version being shorter than 8 characters (indeed, it
> didn't happen with allmodconfig on v5.0.0-rcX).
>
> The offending code is:
>
> memcpy_and_pad(id->fr, sizeof(id->fr),
> UTS_RELEASE, strlen(UTS_RELEASE), ' ');
>
> with UTS_RELEASE being "5.0.0+", which calls into:
>
> static inline void memcpy_and_pad(void *dest, size_t dest_len,
> const void *src, size_t count, int pad)
> {
> if (dest_len > count) {
>
> Of course this branch is taken, right?
>
Apparently that's not known at compile time. The compiler knows dest_len
is 8 but apparently doesn't know what count is, because -ffreestanding
means it can't evaluate strlen(UTS_RELEASE). If you change that to
__builtin_strlen(UTS_RELEASE), the problem goes away.
> memcpy(dest, src, count);
> memset(dest + count, pad, dest_len - count);
> } else
> memcpy(dest, src, dest_len);
> }
>
> This assembles to:
>
> .LC1:
> .string "5.0.0+"
>
> | drivers/nvme/target/admin-cmd.c:311: memcpy_and_pad(id->fr,
> sizeof(id->fr),
> pea .LC1 |
> jsr strlen |
>
> Woops, gcc no longer optimizes this away, due to -ffreestanding.
>
> | drivers/nvme/target/admin-cmd.c:311: memcpy_and_pad(id->fr,
> sizeof(id->fr),
> lea (64,%a3),%a1 |, ret, _7
> | include/linux/string.h:452: if (dest_len > count) {
> lea (16,%sp),%sp |,
> moveq #7,%d1 |,
> cmp.l %d0,%d1 | _6,
> jcs .L53 |
>
> And we end up with retaining both branches:
>
> | include/linux/string.h:453: memcpy(dest, src, count);
> move.l %d0,-(%sp) | _6,
> pea .LC1 |
> move.l %a1,-(%sp) | _7,
> move.l %d0,-12(%fp) |,
> move.l %a1,-16(%fp) |,
> jsr memcpy |
> | include/linux/string.h:454: memset(dest + count,
> pad, dest_len - count);
> move.l -12(%fp),%d0 |,
> moveq #8,%d1 |,
> sub.l %d0,%d1 | _6,
> move.l %d1,-(%sp) |,
> pea 32.w |
> move.l -16(%fp),%a1 |,
> pea (%a1,%d0.l) |
> jsr memset |
> lea (24,%sp),%sp |,
> jra .L54 |
> .L53:
> | include/linux/string.h:456: memcpy(dest, src, dest_len);
> move.l .LC1,(%a1) | MEM[(void *)"5.0.0+"], MEM[(void *)_7]
> move.l .LC1+4,4(%a1) | MEM[(void *)"5.0.0+"], MEM[(void *)_7]
>
> But given the warning, the compiler must have devised that taking the
> second branch would read beyond the source buffer???
>
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.
--
> .L54:
>
>
> Gr{oetje,eeting}s,
>
> Geert
>
>