[gdb/symtab] Fix version check in dwarf compilation unit header

  [ was: Subject: Re: [PATCH][gdb/testsuite] Fix error message test in
dw2-error.exp ]

On Tue, Jul 03, 2018 at 03:59:17PM +0100, Pedro Alves wrote:
> On 06/19/2018 10:00 AM, Tom de Vries wrote:
> 
> > the executable used in dw2-error.exp is compiled from a .s that was generated
> > with dwarf2 debug information but has been hand-edited to set the version in
> > the compilation unit header to 0x99:
> > ...
> > .Ldebug_info0:
> >         .long   0x4e    # Length of Compilation Unit Info
> >         .value  0x99    # DWARF version number
> >         .long   .Ldebug_abbrev0 # Offset Into Abbrev. Section
> > ...
> > 
> > Consequently, dwarf2read.c:read_comp_unit_head() interprets the compilation
> > unit header as dwarf5, 
> 
> 
> That right there looks like the real bug to me.
> 
> I went looking for the history behind the testcase, and
> got surprised that the testcase is expecting that "wrong unit_type in
> compilation unit header" error instead of the same error that had been
> reported in the original bug report at
> <https://sourceware.org/bugzilla/show_bug.cgi?id=14931>:
> 
>  ~~~~~
>  Dwarf Error: wrong version in compilation unit header (is 4, should be 2) [in module ....build/gdb/gdb]
>  ~~~~~
> 
> read_and_check_comp_unit_head calls error_check_comp_unit_head 
> after calling read_comp_unit_head, and thus AFAICT error_check_comp_unit_head
> would error out with the "wrong version" error, the one that had been
> reported in the original bug report.  That seems like a much better
> error to me.
> 
> static void
> error_check_comp_unit_head (struct dwarf2_per_objfile *dwarf2_per_objfile,
> 			    struct comp_unit_head *header,
> 			    struct dwarf2_section_info *section,
> 			    struct dwarf2_section_info *abbrev_section)
> {
>   const char *filename = get_section_file_name (section);
> 
>   if (header->version < 2 || header->version > 5)
>     error (_("Dwarf Error: wrong version in compilation unit header "
> 	   "(is %d, should be 2, 3, 4 or 5) [in module %s]"), header->version,
> 	   filename);
> 
> 
> 
> So it seems to me that read_comp_unit_head shouldn't be
> trying to interpret contents of a dwarf version that 
> gdb doesn't understand.  Seems like that error_check_comp_unit_head 
> version check is too late?  How about moving it into
> read_and_check_comp_unit_head?  Of course, the testcase would then
> be adjusted to expect the new message, and it would expect 153/0x99
> exactly instead of any number, which ensures that gdb reads and
> prints the version number correctly.
> 

Agreed, that analysis makes sense.

OK for trunk?

Thanks,
- Tom

[gdb/symtab] Fix version check in dwarf compilation unit header

The version check of the dwarf compilation unit header in
error_check_comp_unit_head is done too late, and consequently dwarf code with
an unsupported version in the compilation unit header is interpreted as dwarf5
code in read_comp_unit_head.

Fixed by moving the check earlier.

Build and reg-tested on x86_64-linux.

2018-07-04  Tom de Vries  <tdevries@suse.de>

	* dwarf2read.c (error_check_comp_unit_head): Move dwarf version
	check ...
	(read_comp_unit_head): ... here.

	* gdb.dwarf2/dw2-error.exp: Update expected error message.

---
 gdb/dwarf2read.c                       | 9 ++++-----
 gdb/testsuite/gdb.dwarf2/dw2-error.exp | 2 +-
 2 files changed, 5 insertions(+), 6 deletions(-)
  

On 07/04/2018 07:54 AM, Tom de Vries wrote:

> Agreed, that analysis makes sense.
> 
> OK for trunk?

OK.

Thanks,
Pedro Alves