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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
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