Re: [PATCH 5.15 746/913] ARM: ftrace: avoid redundant loads or clobbering IP

From: Sasha Levin
Date: Wed Apr 06 2022 - 10:49:22 EST

On Tue, Apr 05, 2022 at 12:01:19PM +0200, Ard Biesheuvel wrote:
On Tue, 5 Apr 2022 at 11:54, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

[ Upstream commit d11967870815b5ab89843980e35aab616c97c463 ]

Tweak the ftrace return paths to avoid redundant loads of SP, as well as
unnecessary clobbering of IP.

This also fixes the inconsistency of using MOV to perform a function
return, which is sub-optimal on recent micro-architectures but more
importantly, does not perform an interworking return, unlike compiler
generated function returns in Thumb2 builds.

Let's fix this by popping PC from the stack like most ordinary code

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

Please drop all the 32-bit ARM patches authored by me from the stable
queues except the ones that have fixes tags. These are highly likely

I can drop you from future selections as well.

to cause an explosion of regressions, and they should have never been
selected, as I don't remember anyone proposing these for stable.

They were proposed by the bot last week