[Bug breakpoints/27151] Step will skip subsequent statements for malloc functions

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27151

--- Comment #6 from Tom de Vries <vries at gcc dot gnu.org> ---
This is slightly more convoluted.

I tried to reproduce the problem on openSUSE Factory.  Using
-fcf-protection=full, there I managed to get a .plt.sec section.  But gdb
handled it ok.

It did not take the "stepped into dynsym resolve code" path, but handled things
fine along another path.

So I debugged once more the ubuntu exec on leap.  I found that at some point we
do:
...
      /* If we are in a function call trampoline (a stub between the            
         calling routine and the real function), locate the real                
         function.  That's what tells us (a) whether we want to step            
         into it at all, and (b) what prologue we want to run to the            
         end of, if we do step into it.  */
      real_stop_pc = skip_language_trampoline (frame, stop_pc);
...
and end up in objc_language::skip_trampoline, and then in
gdbarch_skip_trampoline_code, and then in find_solib_trampoline_target:
...
/* If PC is in a shared library trampoline code stub, return the                
   address of the `real' function belonging to the stub.                        
   Return 0 if PC is not in a trampoline code stub or if the real               
   function is not found in the minimal symbol table.                           

   We may fail to find the right function if a function with the                
   same name is defined in more than one shared library, but this               
   is considered bad programming style.  We could return 0 if we find           
   a duplicate function in case this matters someday.  */

CORE_ADDR
find_solib_trampoline_target (struct frame_info *frame, CORE_ADDR pc)
{
  struct minimal_symbol *tsymbol = lookup_solib_trampoline_symbol_by_pc (pc);
  if (tsymbol != NULL)
    {
      for (objfile *objfile : current_program_space->objfiles ())
        {
          for (minimal_symbol *msymbol : objfile->msymbols ())
            {

...

So, we find that the pc is a trampoline for malloc, and start iterating over
the minsyms in the objfiles.

With openSUSE Leap (glibc 2.26), we find this as first match:
...
$ nm /lib64/ld-linux-x86-64.so.2  | grep malloc
0000000000019710 W malloc
...

With openSUSE Factory (glibc 2.32), we have instead rtld_malloc so
skip_language_trampoline returns 0.

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