Re: Regression loading a tracefile in 7_3

[Pedro Alves <pedro@codesourcery.com>]

On Friday 20 May 2011 03:03:20, Marc Khouzam wrote:
> Hi,
> 
> I believe I'm seeing a regression loading a tracefile in GDB 7.3
> I create a simple tracefile using GDB/gdbserver 7.3 but when I try to load
> that tracefile, I get an error about "PC register is not available", which
> causes my Eclipse session to abort.  This is not happening in 7.2 or 7.2.1.
> 
> > gdb.7.3 a.out
> GNU gdb (GDB) 7.2.90.20110519-cvs
> (gdb) interpreter-exec mi "-target-select tfile trace.7.3"
> ~"Created tracepoint 1 for target's tracepoint 1 at 0x804851f.\n"
> ^error,msg="PC register is not available"
> 

Hmm.

#0  throw_error (error=NOT_AVAILABLE_ERROR, fmt=0x7f7908 "PC register is not available") at ../../src/gdb/exceptions.c:423
#1  0x000000000053de3a in regcache_read_pc (regcache=0xd4b280) at ../../src/gdb/regcache.c:994
#2  0x0000000000497053 in enable_break (info=0xd0df60, from_tty=1) at ../../src/gdb/solib-svr4.c:1536
#3  0x00000000004980a8 in svr4_solib_create_inferior_hook (from_tty=1) at ../../src/gdb/solib-svr4.c:2194
#4  0x000000000046faa7 in solib_create_inferior_hook (from_tty=1) at ../../src/gdb/solib.c:1240
#5  0x0000000000586de2 in post_create_inferior (target=0xbc7120, from_tty=1) at ../../src/gdb/infcmd.c:427
#6  0x00000000004d2ca8 in tfile_open (filename=0xd0d790 "/home/pedro/gdb/marck_tfile/build/gdb/./testsuite/basic.tf", from_tty=1)
    at ../../src/gdb/tracepoint.c:3410

Here, in solib-svr4.c:

      /* Otherwise we find the dynamic linker's base address by examining
	 the current pc (which should point at the entry point for the
	 dynamic linker) and subtracting the offset of the entry point.

         This is more fragile than the previous approaches, but is a good
         fallback method because it has actually been working well in
         most cases.  */
      if (!load_addr_found)
	{
	  struct regcache *regcache
	    = get_thread_arch_regcache (inferior_ptid, target_gdbarch);

	  load_addr = (regcache_read_pc (regcache)
		       - exec_entry_point (tmp_bfd, tmp_bfd_target));
	}

We could skip this if the PC is not available, but:

 - why do we even try to set a shared library breakpoint
   if the target has no execution (e.g., when debugging a core
   file) in the first place?  I'll skipping enable_break in that
   case and see if anything breaks.  By inspection, it looks like
   nothing will.  svr4_in_dynsym_resolve_code will always
   return false if we skip enable_break, but then again that function
   is only used for run control.

   (Hmm, that fallback assumes the program is stopped at the dynamic
   loader's entry point, which isn't a safe assumption when we attach
   to an inferior rather than spawning it.  Might be better to try
   anyway than not do anything at all though.)

-- 
Pedro Alves