Feature request: improved build-id generation

[Laura Abbott]

On 03/15/2018 06:32 AM, Nick Clifton wrote:
> 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
> 

I think this approach looks promising. I'm going to see about prototyping
this approach.

Thanks,
Laura