[Bug symtab/28094] New: Support executable with only line debug section

[Bug symtab/28094] New: Support executable with only line debug section

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

            Bug ID: 28094
           Summary: Support executable with only line debug section
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

In the dwarf 5 standard we find:
...
Prior to DWARF Version 5, the current directory was not represented in the
directories field and a directory index of 0 implicitly referred to that
directory as found in the DW_AT_comp_dir attribute of the compilation unit
debugging information entry.  In DWARF Version 5, the current directory is
explicitly present in the directories field. This is needed to support the
common practice of stripping all but the line number sections (.debug_line and
.debug_line_str) from an executable.
...

FWIW, the change comes from issue
http://dwarfstd.org/ShowIssue.php?issue=140724.1 .

I tried an experiment where I compiled a hello world with gcc (and dwarf-4) and
stripped all but the .debug_line section:
...
$ gcc -g hello.c
$ for s in $(readelf -S -W a.out | grep debug | awk '{print $2}'| grep -v
.debug_line); do objcopy --remove-section=$s a.out; done
...

But gdb doesn't do well with that:
...
$ gdb -q a.out -ex start
Reading symbols from a.out...
(No debugging symbols found in a.out)
Temporary breakpoint 1 at 0x40054b
Starting program: /home/vries/a.out 

Temporary breakpoint 1, 0x000000000040054b in main ()
(gdb) 
...

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[Bug symtab/28094] Support executable with only line debug section

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
I also found that gold has some option (
https://sourceware.org/legacy-ml/binutils/2008-05/msg00232.html ):
...
$ gcc -fuse-ld=gold -Wl,--strip-debug-non-line hello.c -g
...
which gives better results:
...
$ gdb -q a.out -ex start
Reading symbols from a.out...
Temporary breakpoint 1 at 0x4005cf: file hello.c, line 7.
Starting program: /home/vries/a.out 

Temporary breakpoint 1, main () at hello.c:7
7         void *p = malloc (10);
(gdb) 
...

We could use a test-case with strip-debug-non-line.

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[Bug symtab/28094] Support executable with only line debug section(s)

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Support executable with     |Support executable with
                   |only line debug section     |only line debug section(s)

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
Update $summary.  For dwarf 5, we have .debug_line and .debug_line_str.

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[Bug symtab/28094] Support executable with only line debug section(s)

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #1)
> I also found that gold has some option (
> https://sourceware.org/legacy-ml/binutils/2008-05/msg00232.html ):

FWIW, this is a patch for lld to add the same option, which describes the
functionality a bit more verbosely: https://reviews.llvm.org/D46628?id=206198

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[Bug symtab/28094] Support executable with only line debug section(s)

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

--- Comment #4 from Tom de Vries <vries at gcc dot gnu.org> ---
I've also tried strip-debug-non-line in combination with dwarf-5, but even with
gold build from trunk:
...
$ PATH=~/binutils/install/bin:$PATH gcc -fuse-ld=gold
-Wl,--strip-debug-non-line hello.c -g -gdwarf-5
/home/vries/binutils/install/bin/ld.gold: warning: Invalid DIE in debug info;
failed to reduce debug info
/home/vries/binutils/install/bin/ld.gold: internal error in finalize_data_size,
at /home/vries/binutils/src/gold/output.h:355
collect2: error: ld returned 1 exit status
...

It looks like the option didn't get its dwarf-5 update yet.

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[Bug symtab/28094] Support executable with only line debug section(s)

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

--- Comment #5 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #4)
> I've also tried strip-debug-non-line in combination with dwarf-5, but even
> with gold build from trunk:
> ...
> $ PATH=~/binutils/install/bin:$PATH gcc -fuse-ld=gold
> -Wl,--strip-debug-non-line hello.c -g -gdwarf-5
> /home/vries/binutils/install/bin/ld.gold: warning: Invalid DIE in debug
> info; failed to reduce debug info
> /home/vries/binutils/install/bin/ld.gold: internal error in
> finalize_data_size, at /home/vries/binutils/src/gold/output.h:355
> collect2: error: ld returned 1 exit status
> ...
> 
> It looks like the option didn't get its dwarf-5 update yet.

Filed PR28095 - "--strip-debug-non-line doesn't work with dwarf-5"

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[Bug symtab/28094] Support executable with only line debug section(s)

[pedro at palves dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

Pedro Alves <pedro at palves dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pedro at palves dot net

--- Comment #6 from Pedro Alves <pedro at palves dot net> ---
Also, Clang has "-gline-tables-only".  I think the "--strip-debug-non-line" lld
patch was never merged because it was considered legacy, as in, it's better to
not emit the debug info in the first place, with "-gline-tables-only".

I'm not aware of a an equivalent option in GCC.

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[Bug symtab/28094] Support executable with only line debug section(s)

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28094

--- Comment #7 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Pedro Alves from comment #6)
> Also, Clang has "-gline-tables-only".  I think the "--strip-debug-non-line"
> lld patch was never merged because it was considered legacy, as in, it's
> better to not emit the debug info in the first place, with
> "-gline-tables-only".
> 

Ack, interesting, thanks.

> I'm not aware of a an equivalent option in GCC.

Filed gcc PR https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101470 .

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