Breakpad DWARF: Add support for DWARF 4 attribute forms.

This patch allows Breakpad's DWARF reader to at least read or skip
attributes using the new forms defined in version 4 of the DWARF
specification, instead of crashing.

Attributes encoded using DW_FORM_flag_present, DW_FORM_sec_offset, and
DW_FORM_exprloc should work fine now. However, compilation units using
DW_FORM_ref_sig8 to refer to types in .debug_types will need further work
to support. (GCC 4.6.2 does not emit .debug_types sections.)

[Ted Mielczarek]

LGTM, as best as I can tell.

[jimb]

In getting ready to land this, I had a crisis of conscience: it's really not
nice to report type signature attributes using ProcessAttributeUnsigned, because
they're really not constant values; they're a different kind of reference. The
whole point of distinguishing (say) references from unsigned values is to make
it trivial to avoid misinterpreting values, even without checking the forms
explicitly in the handler.

So this revision of the patch adds a new method to Dwarf2Handler and DIEHandler,
ProcessAttributeSignature, with all the attendant tests.

The patch is quite different. Could you review this one, too? Apologies for not
just doing it this way the first time.

[Lei Zhang (chromium)]

Will this make dump_syms work on newer distros like Fedora 16?

Are you using git/hg with other changes in your local tree? The uploaded patch
doesn't apply for me against svn r907.

http://breakpad.appspot.com/343003/diff/3001/src/common/dwarf/dwarf2reader.cc
File src/common/dwarf/dwarf2reader.cc (right):

http://breakpad.appspot.com/343003/diff/3001/src/common/dwarf/dwarf2reader.cc...
src/common/dwarf/dwarf2reader.cc:207: fprintf(stderr,"Unhandled form type");
remove the default case?

[jimb]

On Thu, Jan 26, 2012 at 3:41 PM,  <thestig@chromium.org> wrote:
> Will this make dump_syms work on newer distros like Fedora 16?

I don't know, but it's possible --- in what way does it fail on those distros?

> Are you using git/hg with other changes in your local tree? The uploaded
> patch doesn't apply for me against svn r907.

There is a series of three patches you must apply, listed in this issue:
https://code.google.com/p/google-breakpad/issues/detail?id=461&sort=-id

This is the third patch in the series. I think it makes patches much easier to
review when they're split, but rietveld doesn't really support that process very
well.

>
http://breakpad.appspot.com/343003/diff/3001/src/common/dwarf/dwarf2reader.cc...
> src/common/dwarf/dwarf2reader.cc:207: fprintf(stderr,"Unhandled form
> type");
> remove the default case?

Good idea --- done.

[jimb]

On 2012/01/27 01:57:55, jimb wrote:
> Good idea --- done.

I made this change in issue 343002:
http://breakpad.appspot.com/343002/show

[Lei Zhang (chromium)]

On 2012/01/27 01:57:55, jimb wrote:
> On Thu, Jan 26, 2012 at 3:41 PM,  <mailto:thestig@chromium.org> wrote:
> > Will this make dump_syms work on newer distros like Fedora 16?
> 
> I don't know, but it's possible --- in what way does it fail on those distros?

Same issue as https://bugzilla.mozilla.org/show_bug.cgi?id=714129, with Fedora
16.

[Ted Mielczarek]

On 2012/01/27 02:11:21, thestig wrote:
> On 2012/01/27 01:57:55, jimb wrote:
> > On Thu, Jan 26, 2012 at 3:41 PM,  <mailto:thestig@chromium.org> wrote:
> > > Will this make dump_syms work on newer distros like Fedora 16?
> > 
> > I don't know, but it's possible --- in what way does it fail on those
distros?
> 
> Same issue as https://bugzilla.mozilla.org/show_bug.cgi?id=714129, with Fedora
> 16.

The original reporter there was using Fedora 16, so this should fix that issue.

[jimb]

On 2012/01/27 12:05:48, Ted Mielczarek wrote:
> The original reporter there was using Fedora 16, so this should fix that
issue.

I think it's really weird that Fedora 16 GCC 4.6.2 is producing DWARF 4 without
being invited to.

Maybe we can get Wikipedia to black itself out to raise awareness of this issue.

[Ted Mielczarek]

LGTM. Can you land this patch in mozilla-central as well as Breakpad SVN?

[jimb]

On 2012/02/01 13:04:53, Ted Mielczarek wrote:
> LGTM. Can you land this patch in mozilla-central as well as Breakpad SVN?

Sure thing. Thanks for the review!