Re: [RFC] elf: fine-grained output LD_DEBUG log when symbol lookup error

[Lv Ying <lvying.system.thoughts@gmail.com>]

Hi Florian:

Thanks again for your reply, I should express clearly the reason for this modification.


> I see.  If we want to make the change, we would have to tweak
> _dl_signal_cexception, not remove the call to it.  But I'm not sure if
> this kind of change would be appropriate for "libs".  Maybe we can add a
> new LD_DEBUG variant that prints more useful information.
This patch does not remove the call to it, just call it when LD_DEBUG=symbols or LD_DEBUG=all,
because the exception message belongs to the symbol lookup process. This patch is inspired by
commit ff9f1c5f324(Fix DL_DEBUG_UNUSED to elide the VDSO and handle PLT references properly.),
As this commit msg said:

elf/dl-lookup (_dl_lookup_symbol_x): If DL_DEBUG_UNUSED, ignore undefined symbol errors.
Before this commit, the undefined symbol error will output if LD_DEBUG is set any value（except statistics）.
This commit believes that when LD_DEBUG is set to DL_DEBUG_UNUSED, there is no need to report
undefined symbol errors. Similarly, LD_DEBUG environment variable may support more settings in the future.
These settings may have the same considerations as DL_DEBUG_UNUSED, therefore, based on scalability considerations,
refined output of debug info, that is, LD_DEBUG=symbols/all output these debug info.
> In my experience, ld.so trace output isn't that helpful today for
> discovering root causes of real issues, so I'd welcome concrete
> proposals for improvements.
Florian, thank you for the experience you shared. As a newbie, this trace output is very helpful for me to understand the ld.so processing.
Please consider my proposal.


Thanks again!
Lv Ying