What is NT_GNU_BUILD_ID used for?

Question

I was reading the help guide for golang ld and one of the options is

-B value
    Add a NT_GNU_BUILD_ID note when using ELF.  The value
    should start with 0x and be an even number of hex digits.

Does anyone knows why one would use that flag ?

searching for NT_GNU_BUILD_ID does not provide any insightful answer.

Answer 1

This comes from the massive conversion from C to Go of cmd/new5l (Feb. 2015), translated from src/cmd/ld/pobj.c

That information was introduced in commit 7d507dc6e (Dec. 2013, for Go 1.3), a preparation for the new linker structure

NT_GNU_BUILD_ID is mentioned here as a unique build ID bitstring.
You see it employed for instance in Fedora release build

To embed an ID into both the stripped object and its .debug file, I've chosen to use an ELF note section.
strip et al can keep the section intact in both files when its type is SHT_NOTE.

The new section is canonically called .note.gnu.build-id, but the name is not normative, and the section can be merged with other SHT_NOTE sections.
The ELF note headers give name "GNU" and type 3 (NT_GNU_BUILD_ID) for a build ID note, of which there can be only one in a linked object (or an ET_REL file of the .ko style).

You can see it introduced in this patch in 2007:

This patch adds a new option to ld for ELF targets, --build-id.
It generates a synthetic ELF note section containing "unique build ID" bits chosen by ld.

This is done as an ld option to be efficient and foolproof to enable for every compilation (vs some script adding a generated object into the link).
It's done the way it is so it can use no more and no less than the exact final ELF bits of the output to contribute to selecting a unique ID.

This is the best way to ensure that deterministic styles of ID generation (i.e. cryptographic hash) will always yield identical results for repeated builds reproduced precisely.