Saving errno around signal handlers

[Carlos O'Donell]

On 03/03/2014 03:57 AM, Florian Weimer wrote:
> On 02/28/2014 11:16 PM, Joseph S. Myers wrote:
>> On Fri, 28 Feb 2014, Richard Henderson wrote:
>>
>>> I'll note that this *may* affect other tools, that have previously looked
>>> through trampolines via code inspection.
>>
>> I don't know if the comment in sysdeps/unix/sysv/linux/mips/sigaction.c is
>> still accurate:
>>
>> /* NOTE: Please think twice before making any changes to the bits of
>>     code below.  GDB needs some intimate knowledge about it to
>>     recognize them as signal trampolines, and make backtraces through
>>     signal handlers work right.  Important are both the names
>>     (__restore_rt) and the exact instruction sequence.
>>     If you ever feel the need to make any changes, please notify the
>>     appropriate GDB maintainer.  */
>>
>> (<https://sourceware.org/glibc/wiki/Development_Todo/Master#MIPS> points
>> to a summary of known MIPS cases of CFI information missing in assembly
>> sources, which includes this one.)
> 
> If we're fine with showing another stack frame in the backtrace of
> signal handlers, we could just avoid the fancy tail call and not
> change the restorer function at all. Then, from GDB's point of view,
> the signal handler is the glibc-provided wrapper (which obviously
> needs proper unwinding information etc.), and the user-installed
> handler is just some function called from a signal handler.

I'm not all that happy about that since that will certainly break
other tools that are only expecting the signal handler frame.

Cheers,
Carlos.