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

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Kamalesh Babulal wrote:
> From: Josh Poimboeuf <jpoimboe@redhat.com>
>=20
> When attempting to load a livepatch module, I got the following error:
>=20
>   module_64: patch_module: Expect noop after relocate, got 3c820000
>=20
> The error was triggered by the following code in
> unregister_netdevice_queue():
>=20
>   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
>=20
> 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.
>=20
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/module_64.c | 4 ++++
>  1 file changed, 4 insertions(+)
>=20
> 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 modul=
e *me)
>  	if (is_early_mcount_callsite(instruction - 1))
>  		return 1;
>=20
> +	/* Sibling calls don't return, so they don't need to restore r2 */
> +	if (instruction[-1] =3D=3D 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

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