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 <[email protected]>
> ---
> 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 <[email protected]>
--
Joe
