Re: [RFC PATCH] objtool: Skip unannotated intra-function call warning for bl+mflr pattern

[Christophe Leroy]

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;




Reported-by: kernel test robot <l...@intel.com>
Closes: 
https://lore.kernel.org/oe-kbuild-all/202502180818.xnfdv8i8-...@intel.com/
Signed-off-by: Sathvika Vasireddy <s...@linux.ibm.com>
---
  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