Re: [PATCH] powerpc: Remove backtrace implementation

[Florian Weimer <fweimer@redhat.com>]

* Tulio Magno Quites Machado Filho:

> Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
>
>> On 12/02/2021 14:21, Florian Weimer wrote:
>>> * Adhemerval Zanella:
>>> 
>>> Does this really work well (enough) on systems which do not build
>>> everything with asynchronous unwind tables?  If I recall correctly, GCC
>>> enabled them by default only quite recently.
>>
>> This should work as well any other architecture since it uses the libgcc_s
>> routines.  Afaik the powerpc implementation was done as optimization, not
>> for correctness.
>
> I think Florian is correct.
>
> GCC enabled -fasynchronous-unwind-tables by default on ppc* ELF targets on
> version 8 [1].
> So if you take a Debian 10 ppc64le, build the backtrace example from
> linux-manpages [2] with gcc-7 or with a newer GCC using
> 'gcc-9 -g0 -fno-asynchronous-unwind-tables', this executable won't be able to
> print the correct backtrace, e.g.:
>
> $ ./testrun.sh ./tst-no-asynchronous-unwind-tables 3
> backtrace() returned 1 addresses
> ./tst-no-asynchronous-unwind-tables(+0xb48) [0x7fff99a00b48]
>
> 8 addresses were expected in the previous test.
>
> With that said, I don't think the powerpc files are just an optimization and
> removing them may cause old binaries to stop getting the right backtrace.
>
> [1] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=0359465c703a
> [2] https://www.man7.org/linux/man-pages/man3/backtrace.3.html

Meh, I feel like I have an egg on my face.  I was pretty sure yesterday
that the change was safe after all.  Thanks for checking.

Is there are reason why the libgcc_s unwinder does not use the backchain
if no DWARF data is available?

Florian