Don't crash if parse_definition_macro's 'body' is NULL

  Hi,

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1708192
      https://bugzilla.redhat.com/show_bug.cgi?id=1708786

Fedora's rpm-build's "debugedit" program will silently corrupt
.debug_macro strings when a binary is compiled with -g3.  Later in the
build phase, gdb-add-index is invoked to extract the DWARF index from
the binary, and GDB will segfault because
dwarf2read.c:parse_definition_macro's 'body' variable is NULL.

This very simple patch is just a safeguard against this scenario; it
is not a fix for the problem (which actually happens on "debugedit",
and which Mark Wielaard is already working on), but at least it makes
GDB not crash on invalid DWARF, which is a plus IMO.

OK for master?

gdb/ChangeLog:
2019-05-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1708192
	* dwarf2read.c (parse_macro_definition): Check whether 'body' is
	NULL, and complain/return if that's the case.
---
 gdb/ChangeLog    | 6 ++++++
 gdb/dwarf2read.c | 7 +++++++
 2 files changed, 13 insertions(+)
  

>>>>> "Sergio" == Sergio Durigan Junior <sergiodj@redhat.com> writes:

Sergio> This very simple patch is just a safeguard against this scenario; it
Sergio> is not a fix for the problem (which actually happens on "debugedit",
Sergio> and which Mark Wielaard is already working on), but at least it makes
Sergio> GDB not crash on invalid DWARF, which is a plus IMO.

I don't really get why read_direct_string (and the other functions like
it) returns NULL when it sees an empty string.  How is something like
"#define X" represented such that this doesn't return NULL?

Other complaints in the caller of parse_macro_definition provide a bit
more context, like the macro name, source file, and line number:

	      complaint (_("debug info gives %s macro %s with %s line %d: %s"),
			 at_commandline ? _("command-line") : _("in-file"),
			 is_define ? _("definition") : _("undefinition"),
			 line == 0 ? _("zero") : _("non-zero"), line, body);

... so maybe the new complaint could as well.

Tom
  

On Monday, May 13 2019, Tom Tromey wrote:

>>>>>> "Sergio" == Sergio Durigan Junior <sergiodj@redhat.com> writes:
>
> Sergio> This very simple patch is just a safeguard against this scenario; it
> Sergio> is not a fix for the problem (which actually happens on "debugedit",
> Sergio> and which Mark Wielaard is already working on), but at least it makes
> Sergio> GDB not crash on invalid DWARF, which is a plus IMO.
>
> I don't really get why read_direct_string (and the other functions like
> it) returns NULL when it sees an empty string.  How is something like
> "#define X" represented such that this doesn't return NULL?

Thanks for the review.

We've already talked on IRC, but I'll just mention it here as well:

When read_indirect_string_at_offset is called (from
dwarf_decode_macro_bytes), you see this code:

  ...
  if (sect->buffer[str_offset] == '\0')
    return NULL;
  ...

You were wondering why a case like "#define X" didn't trigger this bug,
because NULL should be returned.  However, as I found, in this scenario
sect->buffer will contain "X ".  I.e., it will always contain the
macro's name + its (optional) value.  What we're actually dealing with
here, in debugedit's case, is the corruption of the .debug_macro
section, which renders the define useless.

A more in-depth analysis by Keith can be found here:

  https://bugzilla.redhat.com/show_bug.cgi?id=1684303#c2

> Other complaints in the caller of parse_macro_definition provide a bit
> more context, like the macro name, source file, and line number:
>
> 	      complaint (_("debug info gives %s macro %s with %s line %d: %s"),
> 			 at_commandline ? _("command-line") : _("in-file"),
> 			 is_define ? _("definition") : _("undefinition"),
> 			 line == 0 ? _("zero") : _("non-zero"), line, body);
>
> ... so maybe the new complaint could as well.

I did my best and updated the complaint to contain more info.  I'm
afraid the only extra bit I was able to add was the line number, which,
by my tests, will not be entirely correct (perhaps due to the corruption
itself).  I'll send the patch soon.

Thanks,