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SubjectRe: [PATCH] perf tools: Fix compilation on powerpc
Hi,

On Tue, Sep 28, 2021 at 09:52:53PM +0200, Jiri Olsa wrote:
> Got following build fail on powerpc:
>
> CC arch/powerpc/util/skip-callchain-idx.o
> In function ‘check_return_reg’,
> inlined from ‘check_return_addr’ at arch/powerpc/util/skip-callchain-idx.c:213:7,
> inlined from ‘arch_skip_callchain_idx’ at arch/powerpc/util/skip-callchain-idx.c:265:7:
> arch/powerpc/util/skip-callchain-idx.c:54:18: error: ‘dwarf_frame_register’ accessing 96 bytes \
> in a region of size 64 [-Werror=stringop-overflow=]
> 54 | result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/powerpc/util/skip-callchain-idx.c: In function ‘arch_skip_callchain_idx’:
> arch/powerpc/util/skip-callchain-idx.c:54:18: note: referencing argument 3 of type ‘Dwarf_Op *’
> In file included from /usr/include/elfutils/libdwfl.h:32,
> from arch/powerpc/util/skip-callchain-idx.c:10:
> /usr/include/elfutils/libdw.h:1069:12: note: in a call to function ‘dwarf_frame_register’
> 1069 | extern int dwarf_frame_register (Dwarf_Frame *frame, int regno,
> | ^~~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
>
> The dwarf_frame_register args changed with [1],
> Updating ops_mem accordingly.
>
> [1] https://sourceware.org/git/?p=elfutils.git;a=commit;h=5621fe5443da23112170235dd5cac161e5c75e65

The warning (new with GCC11) and the fix are correct.

The code probably worked fine before because the next var on the stack
was the dummy Dwarf_Op which would have been as if the ops_mem[2] was
actually ops_mem[3] already. You don't need the dummy var now and
Dwarf_Op *ops doesn't need to be initialized (the dwarf_frame_register
will initialize it).

Note that [1] didn't change the args of the dwarf_frame_register, but
did fix a similar bug in one of the elfutils own tests (and updated
the comment on the function to hopefully be more clear).

Cheers,

Mark

> Cc: Mark Wielaard <mjw@redhat.com>
> Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> Signed-off-by: Jiri Olsa <jolsa@redhat.com>
> ---
> tools/perf/arch/powerpc/util/skip-callchain-idx.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/arch/powerpc/util/skip-callchain-idx.c b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
> index 3018a054526a..20cd6244863b 100644
> --- a/tools/perf/arch/powerpc/util/skip-callchain-idx.c
> +++ b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
> @@ -45,7 +45,7 @@ static const Dwfl_Callbacks offline_callbacks = {
> */
> static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
> {
> - Dwarf_Op ops_mem[2];
> + Dwarf_Op ops_mem[3];
> Dwarf_Op dummy;
> Dwarf_Op *ops = &dummy;
> size_t nops;
> --
> 2.31.1
>

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