Re: [PATCH v3 15/20] objtool: cache relocations, do less work, eliminate relocation hash

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Thu, Sep 17, 2026 at 05:06:25PM +0100, Lorenzo Stoakes (ARM) wrote:
> This relies upon the entries within a section being sorted, which is the
> case for all sections supplied to objtool by the link step during the
> kernel build.

This is wrong (or at least actively misleading).  Objtool doesn't *only*
run on linked objects.  In some configs it runs on individual .o files.
And GCC doesn't sort relocs:

  Relocation section '.rela.text' at offset 0xc0d0 contains 724 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
      ...
  0000000000005c50  0000009000000004 R_X86_64_PLT32         0000000000000000 _raw_spin_lock - 4
  0000000000005c65  0000009100000004 R_X86_64_PLT32         0000000000000000 _raw_spin_unlock - 4
  0000000000005c6d  0000019f00000004 R_X86_64_PLT32         0000000000000000 put_files_struct - 4
  000000000000004d  0000008f00000004 R_X86_64_PLT32         0000000000000000 __x86_return_thunk - 4
  0000000000000075  0000008f00000004 R_X86_64_PLT32         0000000000000000 __x86_return_thunk - 4
  00000000000000cd  0000008f00000004 R_X86_64_PLT32         0000000000000000 __x86_return_thunk - 4

(JMP target relocations are emitted in a second pass, for whatever
reason)

So the hash may actually be needed as a fallback after all.  Or some
other scheme.

Either way it's overkill to have more than a single fallback.  No second
fallback for "just in case".  Attempting to search an unhashed section
(DWARF) can just be a fatal error instead of the "just in case"
WARN+linear fallback thing.

> @@ -1168,19 +1286,26 @@ static int read_relocs(struct elf *elf)
>  				return -1;
>  			}
>  
> -			elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
>  			set_sym_next_reloc(reloc, sym->relocs);
>  			sym->relocs = reloc;
>  
>  			nr_reloc++;
>  		}
>  		max_reloc = max(max_reloc, nr_reloc);
> +
> +		/* DWARF relocs are never looked up, so are not worth indexing. */
> +		if (is_dwarf_section(rsec->base))
> +			continue;

This DWARF reloc skipping is a standalone improvement, can you split
this out to another patch?

-- 
Josh