Re: [PATCH v2] objtool/klp: Fix checksums for constant pool references

[Joe Lawrence <joe.lawrence@redhat.com>]

On 8/28/26 1:48 PM, Josh Poimboeuf wrote:
> Adding a line of code to __link_shadow_page() with a literal string
> causes a false positive changed function with GCC:
> 
>   arch/x86/kvm/kvm.ko.o: changed function: kvm_tdp_mmu_map_private_pfn
> 
> While the patch only touched __link_shadow_page(), the string addition
> triggered a rename of .LC64 -> .LC65 in kvm_tdp_mmu_map_private_pfn()
> even though the underlying referenced constant data didn't change.
> 
> So for .LC* symbols, the suffix is arbitrary but the data isn't.  Add
> the underlying data to the checksum calculation rather than the symbol
> name.
> 
> Clang also uses .LC* symbols, but also uses anonymous data.  Both
> compilers put this data in .rodata.cst<num> sections.
> 
> Fixes: 0d83da43b1e1 ("objtool/klp: Add --checksum option to generate per-function checksums")
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>  tools/objtool/klp-checksum.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/tools/objtool/klp-checksum.c b/tools/objtool/klp-checksum.c
> index b8e47f28997e9..ebe25f9c5260a 100644
> --- a/tools/objtool/klp-checksum.c
> +++ b/tools/objtool/klp-checksum.c
> @@ -54,6 +54,19 @@ static int checksum_debug_init(struct objtool_file *file)
>  	return 0;
>  }
>  
> +/*
> + * Detect a reference to anonymous constant pool data which the compiler places
> + * in .rodata.cst<num> and which either has an .LC<num> symbol associated with
> + * it or (with Clang) no symbol at all.  These are typically initializers for
> + * local function stack data, so they're considered part of the function rather
> + * than data per se.
> + */
> +static bool is_anonymous_const_data(struct symbol *sym)
> +{
> +	return strstarts(sym->sec->name, ".rodata.cst") &&
> +	       (is_sec_sym(sym) || strstarts(sym->name, ".LC"));
> +}
> +
>  static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
>  				 struct instruction *insn)
>  {
> @@ -129,6 +142,14 @@ static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
>  		goto alts;
>  	}
>  
> +	if (is_anonymous_const_data(sym)) {
> +		void *cst;
> +
> +		cst = sym->sec->data->d_buf + sym->offset + offset;
> +		__checksum_update_insn(func, insn, cst, sym->sec->sh.sh_entsize);
> +		goto alts;
> +	}
> +
>  	if (is_sec_sym(sym)) {
>  		sym = find_symbol_containing(reloc->sym->sec, offset);
>  		if (!sym)

Acked-by: Joe Lawrence <joe.lawrence@redhat.com>

-- 
Joe