Re: tests/backtrace-dwarf.c failure due to -freorder-blocks-and-partition

[Mark Wielaard <mark@klomp.org>]

Hi Martin,

On Fri, 2018-08-03 at 09:41 +0200, Martin Liška wrote:
> As slightly discussed with Mark, there are tests that expect 'main'
> will be present in backtrace. That's not always true on x86_64
> because
> -freorder-blocks-and-partition option is on by default. Then one can
> see:
> 
> [   88s] FAIL: run-backtrace-dwarf.sh
> [   88s] ============================
> [   88s] 
> [   88s] 0x7f1fd49800cb	raise
> [   88s] 0x7f1fd49694e9	abort
> [   88s] 0x5627fddd0188	callme
> [   88s] 0x5627fddd0192	doit
> [   88s] 0x5627fddd01a3	main.cold.1
> [   88s] 0x7f1fd496afeb	__libc_start_main
> [   88s] 0x5627fddd04aa	_start
> [   88s] /home/abuild/rpmbuild/BUILD/elfutils-0.173/tests/backtrace-
> dwarf: dwfl_thread_getframes: no error
> [   88s] 0x5627fddd01a3	main.cold.1
> 
> Thus I'm suggesting to disable the option for tests?
> Thoughts?

So the problem is that some tests look for a 'main' symbol.
This is imho for C based programs a natural way to see if we can unwind
to the start of the program (everything before 'main' is infrastructure
that isn't really relevant to the user). But in some cases the 'main'
symbol is munged into something else. 'main.cold.1' in this case.

The first question is, does the program also contain a 'main' symbol?
If so, what does it cover?
Could you eu-readelf -s tests/backtrace-dwarf | grep main

Now if it does, the question is why didn't we see it?
Is main.cold.1 an alias? Then we probably should look harder/smarter.
Or does it now cover any of the backtrace addresses?

If there isn't, or it isn't actually called, then the question is, is
that actually legal? It seems, at least for C and C++ based programs
that they should start in 'main'. If not they are not, is that because
gcc did an illegal transformation? Or does it only look that way
because we cannot unwind correctly (did it do some tail call)?

We could just use -freorder-blocks-and-partition. But I would like to
first really understand why it is necessary.

If you could maybe post the binary somewhere for inspection that would
be great.

Thanks,

Mark