Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call

[Naveen N. Rao]

Kamalesh Babulal wrote:
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
> When attempting to load a livepatch module, I got the following error:
>
>   module_64: patch_module: Expect noop after relocate, got 3c820000
>
> The error was triggered by the following code in
> unregister_netdevice_queue():
>
>   14c:   00 00 00 48     b       14c <unregister_netdevice_queue+0x14c>
>                          14c: R_PPC64_REL24      net_set_todo
>   150:   00 00 82 3c     addis   r4,r2,0
>
> GCC didn't insert a nop after the branch to net_set_todo() because it's
> a sibling call, so it never returns.  The nop isn't needed after the
> branch in that case.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/powerpc/kernel/module_64.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 39b01fd..9e5391f 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct module *me)
>  	if (is_early_mcount_callsite(instruction - 1))
>  		return 1;
>
> +	/* Sibling calls don't return, so they don't need to restore r2 */
> +	if (instruction[-1] == PPC_INST_BRANCH)
> +		return 1;
> +

This looks quite fragile, unless we know for sure that gcc will _always_
emit this instruction form for sibling calls with relocations.

As an alternative, does it make sense to do the following check instead?
	if ((instr_is_branch_iform(insn) || instr_is_branch_bform(insn))
		&& !(insn & 0x1))


- Naveen