Hi Josh, Thanks for the review.
On 2/21/25 1:29 AM, Josh Poimboeuf wrote:
On Wed, Feb 19, 2025 at 09:50:14PM +0530, Sathvika Vasireddy wrote:
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
If I understand correctly, this is basically a fake call which is used
to get the value of the program counter?
Yes, that's correct.
Also, just out of curiosity, how does x86 do it? Does it not use a
branch to next instruction approach?
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.
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>
This should have a Fixes tag as well.
Thanks for catching that. I'll add the Fixes tag.
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;
+
This won't work on x86, where an intra-function call is converted to a
stack-modifying JUMP. So this should probably be checked in an
arch-specific function.
Thanks for letting me know, I'll introduce arch_skip_call_warning() to
handle architecture specific cases in the next patch I send.
Regards,
Sathvika
