Re: [PATCH] dwarf2read.c: Check type of linkage name attribute prior to decoding

[Doug Evans <dje@google.com>]

Kevin Buettner writes:
  > On Mon, 3 Aug 2015 16:31:08 -0700
  > Doug Evans <dje@google.com> wrote:
  >
  > > I wonder, though, if this is a good place for using the dwarf  
assembler.
  > > Seems so. We just need a MIPS_linkage_name attribute
  > > that isn't a string. The dwarf assembler test would be a lot
  > > smaller.
  >
  > My updated patch, below, does this.
  >
  > > What if there was a wrapper on dwarf2_attr, dwarf2_string_attr
  > > or some such, and it returned either the attribute (if the attribute
  > > is present *and* is a string) or NULL.
  > > And if the attribute is present but not a string it logs a
  > > complaint (standard bad debug info complaint) and returns NULL.
  >
  > I've introduced the wrapper that you recommend and have used it in
  > all places that made sense to me.  There were a few spots where using
  > it would have made things more complicated, so I left those alone.
  >
  > Here's the updated change / patch:
  >
  > dwarf2read.c: Check type of string valued attributes prior to decoding.
  >
  > This change introduces a new function, dwarf2_string_attr(), which is
  > a wrapper for dwarf2_attr().  dwarf2read.c has been updated to
  > call dwarf2_string_attr in most instances where a string-valued
  > attribute is decoded to produce a string value.  In most cases, it
  > simplifies the code; in some instances, the complexity of the code
  > remains unchanged.
  >
  > I performed this change by looking for instances where the
  > result of DW_STRING was used in an assignment.  Many of these
  > had a pattern which (roughly) looks something like this:
  >
  >   struct attribute *attr = NULL;
  >
  >   attr = dwarf2_attr (die, name, cu);
  >   if (attr != NULL && DW_STRING (attr))
  >     {
  >       const char *str;
  >       ...
  >       str = DW_STRING (attr);
  >       ... /* Use str in some fashion.  */
  >     }
  >
  > Code of this form is transformed to look like this instead:
  >
  >   const char *str;
  >
  >   str = dwarf2_string_attr (die, name, cu)
  >   if (str != NULL)
  >     {
  >        ...
  >        /* Use str in some fashion.  */
  >        ...
  >     }
  >
  > In addition to invoking dwarf2_attr() and DW_STRING(),
  > dwarf2_string_attr() checks to make sure that the attribute's
  > `form' field matches one of DW_FORM_strp, DW_FORM_string, or
  > DW_FORM_GNU_strp_alt.  If it does not match one of these forms,
  > it will return a NULL value in addition to calling complaint().
  >
  > An earlier version of this patch did this type checking for one
  > particular instance where a string attribute was being decoded.
  > The situation that I was attempting to handle in that earlier patch is
  > this:
  >
  > The Texas Instruments compiler uses the encoding for
  > DW_AT_MIPS_linkage_name for other purposes.  TI uses the encoding,
  > 0x2007, for TI_AT_TI_end_line which, unlike DW_AT_MIPS_linkage_name,
  > does not have a string-typed value.  In this instance, GDB was attempting
  > to use an integer value as a string pointer, with predictable results.
  > (GDB would die with a segmentation fault.)
  >
  > I've added a test which reproduces the problem that I was orignally
  > wanting to fix.  It uses DW_AT_MIPS_linkage name with an associate
  > value which is a string, and again, where the value is a small
  > integer.
  >
  > My test case causes GDB to segfault in an unpatched GDB.  There
  > will be two PASSes in a patched GDB.
  >
  > Unpatched GDB:
  >
  > (gdb) ptype f
  > ERROR: Process no longer exists
  > UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
  > ERROR: Couldn't send ptype g to GDB.
  > UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g
  >
  > Patched GDB:
  >
  > (gdb) ptype f
  > type = bool ()
  > (gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
  > ptype g
  > type = bool ()
  > (gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g
  >
  > I see no regressions on an x86_64 native target.
  >
  > gdb/ChangeLog:
  >
  > 	* dwarf2read.c (dwarf2_string_attr): New function.
  > 	(lookup_dwo_unit, process_psymtab_comp_unit_reader)
  > 	(dwarf2_compute_name, dwarf2_physname, find_file_and_directory)
  > 	(read_call_site_scope, namespace_name, guess_full_die_structure_name)
  > 	(anonymous_struct_prefix, prepare_one_comp_unit): Use
  > 	dwarf2_string_attr in place of dwarf2_attr and DW_STRING.
  >
  > gdb/testsuite/ChangeLog:
  >
  > 	* gdb.dwarf2/dw2-bad-mips-linkage-name.c: New file.
  > 	* gdb.dwarf2/dw2-bad-mips-linkage-name.exp: New file.

Thanks for the ping!

LGTM