Re: [RFC PATCH] objtool: Skip unannotated intra-function call warning for bl+mflr pattern
From: Christophe Leroy <hidden>
Date: 2025-02-24 08:50:02
Also in:
lkml, llvm
Le 19/02/2025 à 17:20, Sathvika Vasireddy a écrit :
Architectures like PowerPC use a pattern where the compiler generates a branch-and-link (bl) instruction that targets the very next instruction, followed by loading the link register (mflr) later. This pattern appears in the code like: bl .+4 li r5,0 mflr r30
What compiler do you use ? Is it a very old version of GCC ?
That sequence is not correct and should never be used by modern
compilers. It should be bcl 20,31,+4 instead.
All such hand writen sequences have been removed from kernel assembly,
see commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption in
__get_datapage()") for details
Objtool currently warns about this as an "unannotated intra-function call" because find_call_destination() fails to find any symbol at the target offset. Add a check to skip the warning when a branch targets the immediate next instruction in the same function.
I think this should be done in arch_decode_instruction(), just set insn->type to INSN_OTHER when you see bl .+4 Something like:
diff --git a/tools/objtool/arch/powerpc/decode.c b/tools/objtool/arch/powerpc/decode.c index 53b55690f320..ca264c97ee8d 100644
--- a/tools/objtool/arch/powerpc/decode.c
+++ b/tools/objtool/arch/powerpc/decode.c@@ -55,7 +55,9 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
switch (opcode) {
case 18: /* b[l][a] */
- if ((ins & 3) == 1) /* bl */
+ if (ins == 0x48000005) /* bl .+4 */
+ typ = INSN_OTHER;
+ else if ((ins & 3) == 1) /* bl */
typ = INSN_CALL;
imm = ins & 0x3fffffc;
quoted hunk ↗ jump to hunk
Reported-by: kernel test robot <redacted> Closes: https://lore.kernel.org/oe-kbuild-all/202502180818.XnFdv8I8-lkp@intel.com/ (local) Signed-off-by: Sathvika Vasireddy <redacted> --- tools/objtool/check.c | 6 ++++++ 1 file changed, 6 insertions(+)diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 753dbc4f8198..3f7cf2c917b5 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c@@ -1613,6 +1613,7 @@ static struct symbol *find_call_destination(struct section *sec, unsigned long o */ static int add_call_destinations(struct objtool_file *file) { + struct instruction *next_insn; struct instruction *insn; unsigned long dest_off; struct symbol *dest;@@ -1625,6 +1626,11 @@ static int add_call_destinations(struct objtool_file *file) reloc = insn_reloc(file, insn); if (!reloc) { dest_off = arch_jump_destination(insn); + + next_insn = next_insn_same_func(file, insn); + if (next_insn && dest_off == next_insn->offset) + continue; + dest = find_call_destination(insn->sec, dest_off); add_call_dest(file, insn, dest, false); --2.39.3