[llvm] [llvm][ELF]Add Shdr check for getBuildID (PR #126537)

[James Henderson via llvm-commits]

jh7370 wrote:

> and the generated ELF's Program Header has an Offset valued `0x8000`(the section data itself is in the file properly), which means it is not pointing to the real section data.
> Did I just misunderstand something😢?

Nope, that YAML looks perfect. Unfortunately, it looks like there's a bug in yaml2obj: it decides the program header offset (and other properties) based on the final section header sh_offset, rather than the actual intended properties.

The fix should be fairly easy, if you're willing to tackle it (in a separate PR). Specifically, there are three points that are of interest. This line is the place where values are obtained from section headers that are later used in setting the program header values of the program header they are in. Note that it uses sh_offset, sh_size and sh_addralign, all of which, I believe can be overridden by dedicated fields in the YAML.
https://github.com/llvm/llvm-project/blob/36cd60144b15dd35c8e0081100421c3511242e02/llvm/lib/ObjectYAML/ELFEmitter.cpp#L1159

This line is the start of the bit of code where the section sh_* field values are calculated.
https://github.com/llvm/llvm-project/blob/main/llvm/lib/ObjectYAML/ELFEmitter.cpp#L796
Note how it initially calculates the "real" values, before finally calling `overrideFields` (there are a few places `overrideFields` is called, depending on the exact nature of the section).

Finally, `overrideFields` is defined at https://github.com/llvm/llvm-project/blob/5c375c3283fcd2bf4f98fe8627658e056e25dc44/llvm/lib/ObjectYAML/ELFEmitter.cpp#L634.

My feeling is that you could look at moving the calls to `overrideFields` to much later, so that it happens after the program headers have been laid out, perhaps right before the section header data is actually written.

https://github.com/llvm/llvm-project/pull/126537