Re: [PATCH] frv-tdep.c: Stop backtraces in entry func, not entry file

[Andrew Cagney <cagney@gnu.org>]

> On Oct 14,  5:55pm, Andrew Cagney wrote:
> 
> 
>> > An FR-V user reported being unable to see useful backtraces when
>> > debugging functions inside the entry file.  This patch fixes that
>> > problem.  I think there's a problem with inside_entry_func() itself,
>> > but that's a separate issue.  (If you are in the entry file,
>> > backtraces don't stop at the entry func - they attempt to continue
>> > beyond, but they do stop shortly thereafter.)
> 
>> 
>> What happens if that test is removed?
> 
> 
> At the moment, the behavior is the same whether the test is there or
> not.  That is, things work as expected so long as your're in main()
> or above.  If you go below main(), then things break down.  The
> inside_entry_func() test ought to stop backtraces from going too
> far, but for FR-V at least, it seems to be broken.
> 
> Here's an example of where things break down:
> 
> 0x00010118 in _start ()
> 1: x/i $pc  0x10118 <_start+280>:       call 0x11dc0 <exit>
> (gdb) bt
> #0  0x00010118 in _start ()
> #1  0x00018ed4 in _write_r (ptr=0x0, fd=0, buf=0x0, cnt=5)
>     at /ocotillo2/devo-frv/frv-elf/bld/../../devo/newlib/libc/reent/writer.c:58
> Previous frame inner to this frame (corrupt stack?)

In that case, the test should be removed.  It definitly doesn't belong 
there.  Instead your ABI should specify a robust way of detecting a 
terminated stack.  For instance, the PPC specifies that the stack chain 
is ended with a ZERO entry.

Here's the relevant comments in "frame.c":

   /* If we're already inside the entry function for the main objfile,
      then it isn't valid.  Don't apply this test to a dummy frame -
      dummy frame PC's typically land in the entry func.  Don't apply
      this test to the sentinel frame.  Sentinel frames should always
      be allowed to unwind.  */
   /* NOTE: cagney/2003-02-25: Don't enable until someone has found
      hard evidence that this is needed.  */
   /* NOTE: cagney/2003-07-07: Fixed a bug in inside_main_func - wasn't
      checking for "main" in the minimal symbols.  With that fixed
      asm-source tests now stop in "main" instead of halting the
      backtrace in wierd and wonderful ways somewhere inside the entry
      file.  Suspect that deprecated_inside_entry_file and
      inside_entry_func tests were added to work around that (now
      fixed) case.  */
   /* NOTE: cagney/2003-07-15: danielj (if I'm reading it right)
      suggested having the inside_entry_func test use the
      inside_main_func msymbol trick (along with entry_point_address I
      guess) to determine the address range of the start function.
      That should provide a far better stopper than the current
      heuristics.  */
   /* NOTE: cagney/2003-07-15: Need to add a "set backtrace
      beyond-entry-func" command so that this can be selectively
      disabled.  */

I'd like to avoid re-introducing a dependency on inside_entry_func() as 
that places garish requirements on the object file readers :-(

I also suspect that just removing the test fixes the bug.  The other 
relevant comment is:

   /* If we're inside the entry file, it isn't valid.  Don't apply this
      test to a dummy frame - dummy frame PC's typically land in the
      entry file.  Don't apply this test to the sentinel frame.
      Sentinel frames should always be allowed to unwind.  */
   /* NOTE: drow/2002-12-25: should there be a way to disable this
      check?  It assumes a single small entry file, and the way some
      debug readers (e.g.  dbxread) figure out which object is the
      entry file is somewhat hokey.  */
   /* NOTE: cagney/2003-01-10: If there is a way of disabling this test
      then it should probably be moved to before the ->prev_p test,
      above.  */
   /* NOTE: vinschen/2003-04-01: Disabled.  It turns out that the call
      to deprecated_inside_entry_file destroys a meaningful backtrace
      under some conditions.  E. g. the backtrace tests in the
      asm-source testcase are broken for some targets.  In this test
      the functions are all implemented as part of one file and the
      testcase is not necessarily linked with a start file (depending
      on the target).  What happens is, that the first frame is printed
      normaly and following frames are treated as being inside the
      enttry file then.  This way, only the #0 frame is printed in the
      backtrace output.  */

Corinna disabled this for just the reason you describe.

Andrew