linux-kernel - [PATCH -next V2 1/2] riscv: jump_label: Fixup unaligned arch_static

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Message-Id: <20221210100927.835145-2-guoren@kernel.org>
Date:   Sat, 10 Dec 2022 05:09:26 -0500
From:   guoren@...nel.org
To:     palmer@...osinc.com, conor.Dooley@...rochip.com, guoren@...nel.org,
        andy.chiu@...ive.com, greentime.hu@...ive.com,
        paul.walmsley@...ive.com, peterz@...radead.org,
        rostedt@...dmis.org, jrtc27@...c27.com
Cc:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: [PATCH -next V2 1/2] riscv: jump_label: Fixup unaligned arch_static_branch function

From: Andy Chiu <andy.chiu@...ive.com>

Runtime code patching must be done at a naturally aligned address, or we
may execute on a partial instruction.

We have encountered problems traced back to static jump functions during
the test. We switched the tracer randomly for every 1~5 seconds on a
dual-core QEMU setup and found the kernel sucking at a static branch
where it jumps to itself.

The reason is that the static branch was 2-byte but not 4-byte aligned.
Then, the kernel would patch the instruction, either J or NOP, with two
half-word stores if the machine does not have efficient unaligned
accesses. Thus, moments exist where half of the NOP mixes with the other
half of the J when transitioning the branch. In our particular case, on
a little-endian machine, the upper half of the NOP was mixed with the
lower part of the J when enabling the branch, resulting in a jump that
jumped to itself. Conversely, it would result in a HINT instruction when
disabling the branch, but it might not be observable.

ARM64 does not have this problem since all instructions must be 4-byte
aligned.

Fixes: ebc00dde8a97 ("riscv: Add jump-label implementation")
Link: https://lore.kernel.org/linux-riscv/20220913094252.3555240-6-andy.chiu@sifive.com/
Signed-off-by: Andy Chiu <andy.chiu@...ive.com>
Reviewed-by: Greentime Hu <greentime.hu@...ive.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/riscv/include/asm/jump_label.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/riscv/include/asm/jump_label.h b/arch/riscv/include/asm/jump_label.h
index 38af2ec7b9bf..729991e8f782 100644
--- a/arch/riscv/include/asm/jump_label.h
+++ b/arch/riscv/include/asm/jump_label.h
@@ -18,6 +18,7 @@ static __always_inline bool arch_static_branch(struct static_key *key,
 					       bool branch)
 {
 	asm_volatile_goto(
+		"	.align		2			\n\t"
 		"	.option push				\n\t"
 		"	.option norelax				\n\t"
 		"	.option norvc				\n\t"
@@ -39,6 +40,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key,
 						    bool branch)
 {
 	asm_volatile_goto(
+		"	.align		2			\n\t"
 		"	.option push				\n\t"
 		"	.option norelax				\n\t"
 		"	.option norvc				\n\t"
-- 
2.36.1