[PATCH] arm64: kprobe: fix an error in single stepping support

[1534428646 at qq.com]

From: "Yiren Xie" <1534428646 at qq.com>

It is obvious a conflict between the code and the comment.
The function aarch64_insn_is_steppable is used to check if a mrs
instruction can be safe in single-stepping environment, in the
comment it says only reading DAIF bits by mrs is safe in 
single-stepping environment, and other mrs instructions are not. So 
aarch64_insn_is_steppable should returen "TRUE" if the mrs instruction
being single stepped is reading DAIF bits.

And have verified using a kprobe kernel module which reads the DAIF bits by
function arch_local_irq_save with offset setting to 0x4, confirmed that
without this modification, it encounters
"kprobe_init: register_kprobe failed, returned -22" error while inserting
the kprobe kernel module. and with this modification, it can read the DAIF
bits in single-stepping environment.

Fixes: 2dd0e8d2d2a1 ("arm64: Kprobes with single stepping support")
Cc: stable at vger.kernel.org
Signed-off-by: Yiren Xie <1534428646 at qq.com>
---
 arch/arm64/kernel/probes/decode-insn.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
index 6438bf62e753..22383eb1c22c 100644
--- a/arch/arm64/kernel/probes/decode-insn.c
+++ b/arch/arm64/kernel/probes/decode-insn.c
@@ -40,7 +40,7 @@ static bool __kprobes aarch64_insn_is_steppable(u32 insn)
 		 */
 		if (aarch64_insn_is_mrs(insn))
 			return aarch64_insn_extract_system_reg(insn)
-			     != AARCH64_INSN_SPCLREG_DAIF;
+			     == AARCH64_INSN_SPCLREG_DAIF;
 
 		/*
 		 * The HINT instruction is steppable only if it is in whitelist
-- 
2.34.1