[PATCH 44/48] objtool/klp: Fix position-dependent checksums for non-relocated jumps/calls

Josh Poimboeuf
2026-04-23T04:04:47Z

When computing klp checksums, instructions with non-relocated jump/call
destination offsets are problematic because the offset values can change
when surrounding code has moved, causing the function to be incorrectly
marked as changed.

Specifically, that includes jumps from alternatives to the end of the
alternative, which from objtool's perspective are jumps to the end of
the alternative instruction block in the original function.

Note that 'jump_dest' jumps don't include sibling calls (those use
call_dest), nor do they include jumps to/from .cold sub functions (those
are cross-section and need a reloc).

Fix it by hashing the opcode bytes (excluding the immediate operand)
along with a position-independent representation of the destination.
For calls, use the function name, and for jumps, use the destination's
offset within its function.

[Note the "9 bit hole" comment was wrong: it has been 8 bits since
commit 70589843b36f ("objtool: Add option to trace function validation")
added the 'trace' field. Adding the 4-bit 'immediate_len' field now
leaves a 4-bit hole.]

Fixes: 0d83da43b1e1 ("objtool/klp: Add --checksum option to generate per-function checksums")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
tools/objtool/arch/x86/decode.c | 17 ++++++++-
tools/objtool/include/objtool/arch.h | 3 ++
tools/objtool/include/objtool/check.h | 3 +-
tools/objtool/klp-checksum.c | 53 ++++++++++++++++++++++++---
4 files changed, 67 insertions(+), 9 deletions(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 350b8ee6e776..1b387d5a195b 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -805,14 +805,27 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
break;
}

- if (ins.immediate.nbytes)
+ if (ins.immediate.nbytes) {
insn->immediate = ins.immediate.value;
- else if (ins.displacement.nbytes)
+ insn->immediate_len = ins.immediate.nbytes;
+ } else if (ins.displacement.nbytes) {
insn->immediate = ins.displacement.value;
+ insn->immediate_len = ins.displacement.nbytes;
+ }

return 0;
}

+size_t arch_jump_opcode_bytes(struct objtool_file *file, struct instruction *insn,
+ unsigned char *buf)
+{
+ size_t len;
+
+ len = insn->len - insn->immediate_len;
+ memcpy(buf, insn->sec->data->d_buf + insn->offset, len);
+ return len;
+}
+
void arch_initial_func_cfi_state(struct cfi_init_state *state)
{
int i;
diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
index 8866158975fc..96d828a8401f 100644
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -79,6 +79,9 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
unsigned long offset, unsigned int maxlen,
struct instruction *insn);

+size_t arch_jump_opcode_bytes(struct objtool_file *file, struct instruction *insn,
+ unsigned char *buf);
+
bool arch_callee_saved_reg(unsigned char reg);

unsigned long arch_jump_destination(struct instruction *insn);
diff --git a/tools/objtool/include/objtool/check.h b/tools/objtool/include/objtool/check.h
index 0c53476528a8..bc99355e66e9 100644
--- a/tools/objtool/include/objtool/check.h
+++ b/tools/objtool/include/objtool/check.h
@@ -68,6 +68,7 @@ struct instruction {
s8 instr;

u32 idx : INSN_CHUNK_BITS,
+ immediate_len : 4,
dead_end : 1,
ignore_alts : 1,
hint : 1,
@@ -81,7 +82,7 @@ struct instruction {
hole : 1,
fake : 1,
trace : 1;
- /* 9 bit hole */
+ /* 4 bit hole */

struct alt_group *alt_group;
struct instruction *jump_dest;
diff --git a/tools/objtool/klp-checksum.c b/tools/objtool/klp-checksum.c
index adfd02447a45..4100b4dfba86 100644
--- a/tools/objtool/klp-checksum.c
+++ b/tools/objtool/klp-checksum.c
@@ -66,17 +66,58 @@ static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
if (insn->fake)
return;

- __checksum_update_insn(func, insn, insn->sec->data->d_buf + insn->offset, insn->len);
-
if (!reloc) {
struct symbol *call_dest = insn_call_dest(insn);
+ struct instruction *jump_dest = insn->jump_dest;

- if (call_dest)
- __checksum_update_insn(func, insn, call_dest->demangled_name,
- strlen(call_dest->demangled_name));
- goto alts;
+ /*
+ * For a jump/call non-relocated dest offset embedded in the
+ * instruction, the offset may vary due to changes in
+ * surrounding code. Just hash the opcode and a
+ * position-independent representation of the destination.
+ */
+
+ if (call_dest || jump_dest) {
+ unsigned char buf[16];
+ size_t len;
+
+ len = arch_jump_opcode_bytes(file, insn, buf);
+ __checksum_update_insn(func, insn, buf, len);
+
+ if (call_dest) {
+ __checksum_update_insn(func, insn, call_dest->demangled_name,
+ strlen(call_dest->demangled_name));
+
+ } else if (jump_dest) {
+ struct symbol *dest_sym;
+ unsigned long offset;
+
+ /*
+ * use insn->_sym instead of insn_sym() here.
+ * For alternative replacements, the latter
+ * would give the function of the code being
+ * replaced.
+ */
+ dest_sym = jump_dest->_sym;
+ if (!dest_sym)
+ goto alts;
+
+ __checksum_update_insn(func, insn, dest_sym->demangled_name,
+ strlen(dest_sym->demangled_name));
+
+ offset = jump_dest->offset - dest_sym->offset;
+ __checksum_update_insn(func, insn, &offset, sizeof(offset));
+ }
+
+ goto alts;
+ }
}

+ __checksum_update_insn(func, insn, insn->sec->data->d_buf + insn->offset, insn->len);
+
+ if (!reloc)
+ goto alts;
+
sym = reloc->sym;
offset = arch_insn_adjusted_addend(insn, reloc);

--
2.53.0