Re: [rfc/rfa] Use ARM exception tables as GDB unwinder

Daniel Jacobowitz
2010-10-21T16:02:20Z

On Thu, Oct 21, 2010 at 05:50:51PM +0200, Ulrich Weigand wrote:
> Hmm, I'm seeing many functions marked as can't-unwind in the Ubuntu
> copy of glibc, so I was assuming this just means that for some reason
> the unwind data couldn't be generated reliably enough to allow run-time
> unwinding. However, for debugging purposes we still want to attempt to
> show a backtrace, so I'm falling back to prologue parsing ...

Oh, right - can't unwind can be generated by the compiler, or by the
linker. It should cover all non-fexceptions code with can't-unwind
markers.

> Note the large gap between 0x15890 and 0x25a50, which contains many
> functions, including "raise", for which the "pop {r7, r14}" instruction
> certainly isn't correct. This caused test suite failures before I
> added the check above.
>
> I understood the ARM standard to say that this was expected, because
> for C or assembly routines, presence of an unwind entry is optional.
> Is this not the case?

No, this is not the case. The linker is supposed to fix it up:

/* Scan .ARM.exidx tables, and create a list describing edits which should be
made to those tables, such that:

1. Regions without unwind data are marked with EXIDX_CANTUNWIND entries.
2. Duplicate entries are merged together (EXIDX_CANTUNWIND, or unwind
codes which have been inlined into the index).

If MERGE_EXIDX_ENTRIES is false, duplicate entries are not merged.

The edits are applied when the tables are written
(in elf32_arm_write_section).
*/

If it's not doing that, we should figure out why - it can lead to
crashes in libgcc, if the unwinder is invoked, rather than the correct
failure to unwind.

I think 2.19 didn't do this but 2.20 did.

> Ah, I didn't see the readelf code (because I was looking at an old
> checkout, I would appear). I can add those personality routines as well
> (however, these are nowhere used in the glibc I'm looking at).

Correct, they're only used with C cleanups or C++ exception throw/catch.

--
Daniel Jacobowitz
CodeSourcery