Re: [PATCH] perf unwind-libdw: Fix reading the stack of a 32-bit task

[Arnaldo Carvalho de Melo]

On Wed, Sep 23, 2026 at 01:33:48PM -0700, Ian Rogers wrote:
> On Wed, Sep 23, 2026 at 12:34 PM Stian Halseth <stian@xxxxxx> wrote:
> >
> > memory_read() hands libdw 8 bytes whatever the sampled task's ABI. For a
> > 32-bit task libdw keeps the low 32 bits (__libdwfl_frame_reg_set()), so
> > on a big-endian machine each word read from the stack or a DSO is the
> > one at addr + 4, and unwinding stops at the first frame restored from
> > the stack. On sparc the 8-byte load from a stack address that is only
> > 4-byte aligned even traps, and perf dies with SIGBUS. Read 4 bytes for a
> > PERF_SAMPLE_REGS_ABI_32 sample.
> >
> > A recording of the other byte order has each sample swapped in 8-byte
> > units when it is read (perf_event__all64_swap()), which leaves the
> > 4-byte words of a 32-bit stack dump reversed and paired up wrongly.
> > Undo the swap of the unit holding the word and swap the word itself, as
> > __evsel__parse_sample() does for PERF_SAMPLE_CPU.
> >
> > Suggested-by: Ian Rogers <irogers@xxxxxxxxxx>
> > Signed-off-by: Stian Halseth <stian@xxxxxx>
> 
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks, applied to perf-tools-next, for v7.4.

- Arnaldo