Re: Feature request: improved build-id generation

[Nick Clifton <nickc at redhat dot com>]

Hi Mark,

> That might be an interesting alternative. Could you use this for e.g.
> inserting a .comment section fragment with an unique (version) string?
> That would be stripped away, but should still count for the build-id
> hash calculation.

If you know the value you want to store ahead of time, then it is easy:

  % cat comment.t
SECTIONS
{
	.comment (INFO) : 
	{
		BYTE (0x12);
		BYTE (0x34);
		BYTE (0x56);
		BYTE (0x78);
	}
}

  % gcc hello.c -Wl,comment.t
  % readelf -x.comment a.out

Hex dump of section '.comment':
  0x00000000 4743433a 2028474e 55292037 2e332e31 GCC: (GNU) 7.3.1
  0x00000010 20323031 38303133 30202852 65642048  20180130 (Red H
  0x00000020 61742037 2e332e31 2d322900 4743433a at 7.3.1-2).GCC:
  0x00000030 2028474e 55292037 2e322e31 20323031  (GNU) 7.2.1 201
  0x00000040 37303931 35202852 65642048 61742037 70915 (Red Hat 7
  0x00000050 2e322e31 2d322900 12345678          .2.1-2)..4Vx

  (Note how the value has been appended to the .comment section).

  Unfortunately the linker does not have a STRING() operator to insert
  ascii codes into a section, so you have to construct the bytes by
  hand.  Eg:

  % cat comment.t
SECTIONS
{
	.comment (INFO) : 
	{
		BYTE (0x41);
		BYTE (0x42);
		BYTE (0x43);
		BYTE (0x00);
	}
}

  % gcc hello.c -Wl,comment.t
  % readelf -p.comment a.out

String dump of section '.comment':
  [     0]  GCC: (GNU) 7.3.1 20180130 (Red Hat 7.3.1-2)
  [    2c]  GCC: (GNU) 7.2.1 20170915 (Red Hat 7.2.1-2)
  [    58]  ABC

  A simple perl or python script could be used to create the comment.t 
  linker script fragment.

Cheers
  Nick