Re: [PATCH] gdb: Don't skip prologue for explicit line breakpoints in assembler

[Andrew Burgess <andrew.burgess@embecosm.com>]

* Kevin Buettner <kevinb@redhat.com> [2019-06-19 18:11:47 -0700]:

> On Wed, 12 Jun 2019 13:34:03 +0100
> Andrew Burgess <andrew.burgess@embecosm.com> wrote:
> 
> > It was observed that in some cases, placing a breakpoint in an
> > assembler file using filename:line-number syntax would result in the
> > breakpoint being placed at a different line within the file.
> > 
> > For example, consider this x86-64 assembler:
> > 
> >     test:
> >             push   %rbp		/* Break here.  */
> >             mov    %rsp, %rbp
> >             nop			/* Stops here.  */
> > 
> > The user places the breakpoint using file:line notation targeting the
> > line marked 'Break here', GDB actually stops at the line marked 'Stops
> > here'.
> > 
> > The reason is that the label 'test' is identified as the likely start
> > of a function, and the call to symtab.c:skip_prologue_sal causes GDB
> > to skip forward over the instructions that GDB believes to be part of
> > the prologue.
> > 
> > I believe however, that when debugging assembler code, where the user
> > has instruction-by-instruction visibility, if they ask for a specific
> > line, GDB should (as far as possible) stop on that line, and not
> > perform any prologue skipping.  I don't believe that the behaviour of
> > higher level languages should change, in these cases skipping the
> > prologue seems like the correct thing to do.
> 
> I agree with all of this.
> 
> > In order to implement this change I needed to extend our current
> > tracking of when the user has requested an explicit line number.  We
> > already tracked this in some cases, but not in others (see the changes
> > in linespec.c).  However, once I did this I started to see some
> > additional failures (in tests gdb.base/break-include.exp
> > gdb.base/ending-run.exp gdb.mi/mi-break.exp gdb.mi/mi-reverse.exp
> > gdb.mi/mi-simplerun.exp) where we currently expected a breakpoint
> > placed at one file and line number to be updated to reference a
> > different line number, this was fixed by removing some code in
> > symtab.c:skip_prologue_sal.  My concern here is that removing this
> > check didn't cause anything else to fail.
> 
> Did you investigate the reason for the failures with this hunk
> left in place?...
> 
> > @@ -3812,12 +3821,6 @@ skip_prologue_sal (struct symtab_and_line *sal)
> >  
> >    sal->pc = pc;
> >    sal->section = section;
> > -
> > -  /* Unless the explicit_line flag was set, update the SAL line
> > -     and symtab to correspond to the modified PC location.  */
> > -  if (sal->explicit_line)
> > -    return;
> > -
> >    sal->symt> FAIL: gdb.base/break-include.exp: break break-include.c:53
> ab = start_sal.symtab;
> >    sal->line = start_sal.line;
> >    sal->end = start_sal.end;
> 
> The rest of the patch looks fine to me.  Deleting those lines might
> be okay also, but I'd like to understand why adding additional
> explicit line number tracking caused these failures:
> 
> FAIL: gdb.base/break-include.exp: break break-include.c:53
> FAIL: gdb.base/ending-run.exp: Cleared 2 by line
> FAIL: gdb.base/ending-run.exp: clear 2 by default

Thanks for taking a look at this.

Just to be clear, this is the hunk that is in question (in symtab.c):

  @@ -3812,12 +3821,6 @@ skip_prologue_sal (struct symtab_and_line *sal)

     sal->pc = pc;
     sal->section = section;
  -
  -  /* Unless the explicit_line flag was set, update the SAL line
  -     and symtab to correspond to the modified PC location.  */
  -  if (sal->explicit_line)
  -    return;
  -
     sal->symtab = start_sal.symtab;
     sal->line = start_sal.line;
     sal->end = start_sal.end;


If we take the first of these 'gdb.base/break-include.exp: break
break-include.c:53', then in a pre-patched test run the log looks like
this:

  (gdb) break break-include.c:53
  Breakpoint 1 at 0x4004af: file /path/to/gdb/src/gdb/testsuite/gdb.base/break-include.inc, line 18.
  (gdb) PASS: gdb.base/break-include.exp: break break-include.c:53

And in a post-patch world, but with the hunk above removed here's the
same part of the log file:

  (gdb) break break-include.c:53
  Breakpoint 1 at 0x4004af: file /path/to/gdb/src/gdb/testsuite/gdb.base/break-include.c, line 54.
  (gdb) FAIL: gdb.base/break-include.exp: break break-include.c:53

What we see is that in the failing case the line number has failed to
update from break-include.c:53 to break-include.inc:18, instead it has
updated to break-include.c:54.

To understand what's going on we need to consider two steps, first in
convert_linespec_to_sals the file and line is used to index into the
line table, this is where the break-include.c:54 comes from, there is
no entry for line 53, and 54 is the next available line.

Then skip_prologue_sal is called, this is where we move forward to
break-include.inc line 18, and this is where the difference is.

In the pre-patch world, the sal that is passe into skip_prologue_sal
is not marked as explicit_line, and so we successfully update the file
and line.

In the post patch world, the sal now is marked as explicit_line, so
the above check triggers and GDB doesn't update the file/line in the
sal, this leaves us stuck on break-include.c:54.

The other two failures all stem from the exact same problem, in
gdb.base/ending-run.exp many breakpoints are placed, and then cleared
using the 'clear' command.  One of the breakpoints (breakpoint 4) is
placed at the wrong file/line as a result of not updating, this then
causes the 'clear' tests to not clear the expected breakpoints.

What worries more about the above hunk is that it never triggers
during testing (in a pre-patched world).  I applied this patch to
master:

  diff --git a/gdb/symtab.c b/gdb/symtab.c
  index 4920d94a247..5cd5bb69147 100644
  --- a/gdb/symtab.c
  +++ b/gdb/symtab.c
  @@ -3816,7 +3816,11 @@ skip_prologue_sal (struct symtab_and_line *sal)
     /* Unless the explicit_line flag was set, update the SAL line
        and symtab to correspond to the modified PC location.  */
     if (sal->explicit_line)
  -    return;
  +    {
  +      fprintf ("Got here!\n");
  +      abort ();
  +      return;
  +    }
   
     sal->symtab = start_sal.symtab;
     sal->line = start_sal.line;

And all the tests still pass.  This code has been in place for ~9
years and unfortunately didn't have any tests associated when it was
added.

I've spent some time trying to figure out what conditions might need
to be true in order to trigger this code, but so far I've not managed
to figure it out - any suggestions would be appreciated.

Thanks,
Andrew


> 
> Kevin