Bug 25611

Summary: [DWARF-5] support for checksums in .file directives
Product: binutils Reporter: Nick Desaulniers <ndesaulniers>
Component: gasAssignee: Not yet assigned to anyone <unassigned>
Severity: normal CC: nickc
Priority: P2    
Version: 2.35   
Target Milestone: ---   
Host: Target:
Build: Last reconfirmed: 2020-03-03 00:00:00

Description Nick Desaulniers 2020-02-28 21:47:29 UTC
I was playing around trying to enable -gdwarf-5 in the Linux kernel, and hit an issue where it looks like clang is emitting .file directives like:

.file 1 "/home/nick/linux/init/do_mounts.c" md5 0x62e8a195aa9d4d8b466f84b7775ea4cd

with GAS producing errors like:
do_mounts.s:19: Error: junk at end of line, first unrecognized character is `m'

A quick grep through the DWARF-5 spec [0] doesn't mention anything about assembly directives.  The docs on .file directives also doesn't mention this. [1]

I assume this is maybe an extension that Clang implemented? IIUC, it's used by debuggers to tell when/if a file has been modified.

Is this something that can be implemented in GNU as? I'm happy to also pursue a command line flag in Clang to disable the emission of these checksums.

See also [2].

[0] http://www.dwarfstd.org/doc/DWARF5.pdf
[1] https://sourceware.org/binutils/docs/as/File.html#File
[2] https://bugs.llvm.org/show_bug.cgi?id=45040
Comment 1 Nick Desaulniers 2020-02-28 22:47:51 UTC
Pawing more through the spec (previously linked), it looks like "1.4 Changes from Version 4 to Version 5" on pdf pg 26 kind of hints at this:

20 • Replace the line number program header format with a new format that
21 provides the ability to use an MD5 hash to validate the source file version in
22 use, allows pooling of directory and file name strings and makes provision
23 for vendor-defined extensions. Also add a string section specific to the line
24 number table (.debug_line_str) to properly support the common practice
25 of stripping all DWARF sections except for line number information.
Comment 2 Nick Desaulniers 2020-02-28 22:59:51 UTC
Also, documentation here would have to be updated. https://sourceware.org/binutils/docs/as/File.html#File
Comment 3 Nick Clifton 2020-03-03 13:39:51 UTC
Hi Nick,

  OK, so this looks like it would be a useful extension to support.

  I'll have a look at it in my copious free time (tm) unless of
  course you want to have a go yourself ?

Comment 4 Nick Clifton 2020-03-11 10:25:58 UTC
Support for this feature has now been added to the assembler: commit 5496f3c635
Comment 5 cvs-commit@gcc.gnu.org 2020-03-31 22:03:04 UTC
The master branch has been updated by Maciej W. Rozycki <macro@sourceware.org>:


commit 876678f05e70235ed71d5b9ec63e4f6db1ec76e4
Author: Maciej W. Rozycki <macro@linux-mips.org>
Date:   Tue Mar 31 23:01:36 2020 +0100

    PR 25611, PR 25614: GAS: Remove a double inclusion of "bignum.h"
    Correct an issue with commit 5496f3c635dc ("Add support for generating
    DWARF-5 format directory and file name tables from the assembler.") and
    remove a duplicate direct inclusion of "bignum.h" from dwarf2dbg.c that
    causes a GAS compilation error:
    In file included from .../gas/dwarf2dbg.c:33:
    .../gas/bignum.h:42: error: redefinition of typedef 'LITTLENUM_TYPE'
    .../gas/bignum.h:42: error: previous declaration of 'LITTLENUM_TYPE' was here
    make[4]: *** [dwarf2dbg.o] Error 1
    with some GCC versions, as this header has been already included via
    "as.h" and then "flonum.h".
            PR 25611
            PR 25614
            * dwarf2dbg.c: Do not include "bignum.h".