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

[Andrew Cagney <cagney at gnu dot 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