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

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Message-ID: <20250220195940.ely2l2fpsozd2tuv@jpoimboe>
Date: Thu, 20 Feb 2025 11:59:40 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Sathvika Vasireddy <sv@...ux.ibm.com>
Cc: peterz@...radead.org, christophe.leroy@...roup.eu, npiggin@...il.com,
	maddy@...ux.ibm.com, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH] objtool: Skip unannotated intra-function call
 warning for bl+mflr pattern

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?

> 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@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202502180818.XnFdv8I8-lkp@intel.com/
> Signed-off-by: Sathvika Vasireddy <sv@...ux.ibm.com>

This should have a Fixes tag as well.

>  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.

-- 
Josh