riscv: jump_label: Fixup unaligned arch_static_branch function

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: ebc00dd ("riscv: Add jump-label implementation")
Link: https://lore.kernel.org/linux-riscv/[email protected]/
Reviewed-by: Greentime Hu <[email protected]>
Signed-off-by: Andy Chiu <[email protected]>
Signed-off-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: AndybnACT

arch/riscv/include/asm/jump_label.h

@@ -18,6 +18,7 @@ static __always_inline bool arch_static_branch(struct static_key * const key,
 					       const 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 * const ke
 						    const bool branch)
 {
 	asm_volatile_goto(
+		"	.align		2			\n\t"
 		"	.option push				\n\t"
 		"	.option norelax				\n\t"
 		"	.option norvc				\n\t"