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objtool/klp: Fix checksums for constant pool references

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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: 0d83da43b1 ("objtool/klp: Add --checksum option to generate per-function checksums")
Link: https://patch.msgid.link/f3a9e74ceebc6475ce94bcfe985401140857814a.1787939301.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Josh Poimboeuf
2026-08-31 17:50:16 -07:00
parent 4825ef699c
commit ac323c9467
1 changed files with 21 additions and 0 deletions
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tools/objtool/klp-checksum.c
+21
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@@ -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)