Re: [PATCH] gdb: skip objfiles with no BFD in DWARF unwinder

[Jan Vrany <jan.vrany@labware.com>]

On Tue, 2022-04-05 at 10:46 -0400, Simon Marchi wrote:
> On 2022-04-05 06:04, Jan Vrany via Gdb-patches wrote:
> > While playing with JIT reader I experienced GDB to crash on null-pointer
> > dereference when stepping through non-jitted code.
> > 
> > The problem was that dwarf2_frame_find_fde () assumed that all objfiles
> > have BFD but that's not always true. To address this problem, this
> > commit skips such objfiles.
> > 
> > As for the test, I initially tried to use temporary breakpoint and then
> > 'continue' to get out of the jitted code but this for some reason did
> > not trigger the crash. Using 'finish' to get out of jitted code followed
> > by 'next' triggered it.
> > ---
> >  gdb/dwarf2/frame.c                    |  3 +++
> >  gdb/objfiles.h                        |  4 +++-
> >  gdb/testsuite/gdb.base/jit-reader.exp | 10 ++++++++++
> >  3 files changed, 16 insertions(+), 1 deletion(-)
> > 
> > diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
> > index 5878d72f922..514ae8c694f 100644
> > --- a/gdb/dwarf2/frame.c
> > +++ b/gdb/dwarf2/frame.c
> > @@ -1565,6 +1565,9 @@ dwarf2_frame_find_fde (CORE_ADDR *pc, dwarf2_per_objfile **out_per_objfile)
> >        CORE_ADDR offset;
> >        CORE_ADDR seek_pc;
> > 
> > +      if (objfile->obfd == nullptr)
> > +	continue;
> 
> Ok, I presume that this is the case only for JIT-ed objfiles whose
> debuginfo was created using the JIT debug info reader API.  If the JIT
> engine produced an executable file in memory with DWARF in it and we
> consumed it, then I suppose that objfile will have a bfd, since it was
> opened and parsed using bfd.

Exactly. jit_bfd_try_read_symtab() calls

  objfile = symbol_file_add_from_bfd (nbfd.get (), ...);

> 
> > +
> >        comp_unit *unit = find_comp_unit (objfile);
> >        if (unit == NULL)
> >  	{
> > diff --git a/gdb/objfiles.h b/gdb/objfiles.h
> > index 8bd76705688..429dea1da4c 100644
> > --- a/gdb/objfiles.h
> > +++ b/gdb/objfiles.h
> > @@ -636,7 +636,9 @@ struct objfile
> >    struct compunit_symtab *compunit_symtabs = nullptr;
> > 
> >    /* The object file's BFD.  Can be null if the objfile contains only
> > -     minimal symbols, e.g. the run time common symbols for SunOS4.  */
> > +     minimal symbols (e.g. the run time common symbols for SunOS4) or
> > +     if the objfile is a dynamic objfile (e.g. created by JIT reader
> > +     API).  */
> > 
> >    bfd *obfd;
> > 
> > diff --git a/gdb/testsuite/gdb.base/jit-reader.exp b/gdb/testsuite/gdb.base/jit-reader.exp
> > index d94360cd7d9..0de552b1ce5 100644
> > --- a/gdb/testsuite/gdb.base/jit-reader.exp
> > +++ b/gdb/testsuite/gdb.base/jit-reader.exp
> > @@ -271,6 +271,16 @@ proc jit_reader_test {} {
> >  		 "#1 ${any} in main ${any}" \
> >  		]
> >      }
> > +
> > +    # check that the DWARF unwinder does not crash in presence of
> 
> Capital C to "check".
> 
> > +    # JIT objfiles.
> > +    gdb_test "fin" \
> > +	[multi_line \
> > +		 "Run till exit from ${any} jit_function_stack_mangle ${any}" \
> > +		 "main ${any} at ${any}.*" \
> > +		]
> > +    gdb_test "bt" "#0  main ${any} at ${any}"
> > +    gdb_test "next"
> 
> Please take in consideration Lancelot's comment, the patch LGTM with
> that feedback addressed.

Sure! Thanks Lancelot!

Jan