build failure with gcc 4.6.0 "array subscript is above array bounds"

[Andreas Schwab]

Ian Campbell <Ian.Campbell at citrix.com> writes:

> I noticed this with a defconfig build:
>       CC      arch/powerpc/kernel/ptrace.o
>     arch/powerpc/kernel/ptrace.c: In function 'arch_ptrace':
>     arch/powerpc/kernel/ptrace.c:1502:5: error: array subscript is above array bounds [-Werror=array-bounds]
>     arch/powerpc/kernel/ptrace.c:1530:5: error: array subscript is above array bounds [-Werror=array-bounds]
>
> That corresponds to:
> 	tmp = ((unsigned long *)child->thread.fpr)
> 		[TS_FPRWIDTH * (index - PT_FPR0)];
>
> child->thread.fpr is "double fpr[32][TS_FPRWIDTH]".
>
> index has already been bounds checked so we know it is <= PT_FPSCR.
>
> I tried to fix but I don't really know enough about PPC to figure out
> the correct fix is. PT_FPSCR is "PT_FPR0 + 32" on ppc64, which seems
> consistent with the fpr definition.

Perhaps there should be a union that overlays fpr with an array of
longs.

> On ppc32 PT_FPSCR is "PT_FPR0 + 2*32 + 1", I tried replacing the 32 with
> "PT_FPSCR - PT_FPR0" (+ 1) but that got me into the BUILD_BUG_ONs at
> line 346 and 374. At this point I'm afraid gave up trying to fix things,
> I hope the report is useful anyway...

On ppc32 a single ptrace call can only read/write half of an fpr, so
each fpr occupies two slots.

Andreas.

-- 
Andreas Schwab, schwab at redhat.com
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