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

[Sathvika Vasireddy]

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 <lkp@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202502180818.XnFdv8I8-lkp@xxxxxxxxx/
> > Signed-off-by: Sathvika Vasireddy <sv@xxxxxxxxxxxxx>
> 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