[PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

[guoren at kernel.org]

From: Guo Ren <guoren at linux.alibaba.com>

The previous implementation was based on the stop_matchine mechanism,
which reduced the speed of arm/disarm_kprobe. Using minimum ebreak
instruction would get accurate atomicity.

This patch removes the patch_text of riscv, which is based on
stop_machine. Then riscv only reserved patch_text_nosync, and developers
need to be more careful in dealing with patch_text atomicity.

When CONFIG_RISCV_ISA_C=n, the ebreak could replace the whole
instruction. When CONFIG_RISCV_ISA_C=y, the patch uses 16-bit length
c.ebreak instruction, which may occupy the first part of the 32-bit
instruction and leave half the rest of the broken instruction. Because
ebreak could detour the flow to skip it, leaving it in the kernel text
memory is okay.

Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
Signed-off-by: Guo Ren <guoren at kernel.org>
---
 arch/riscv/include/asm/patch.h     |  1 -
 arch/riscv/kernel/patch.c          | 33 ------------------------------
 arch/riscv/kernel/probes/kprobes.c | 29 ++++++++++++++++++--------
 3 files changed, 21 insertions(+), 42 deletions(-)

diff --git a/arch/riscv/include/asm/patch.h b/arch/riscv/include/asm/patch.h
index 9a7d7346001e..2500782e6f5b 100644
--- a/arch/riscv/include/asm/patch.h
+++ b/arch/riscv/include/asm/patch.h
@@ -7,6 +7,5 @@
 #define _ASM_RISCV_PATCH_H
 
 int patch_text_nosync(void *addr, const void *insns, size_t len);
-int patch_text(void *addr, u32 insn);
 
 #endif /* _ASM_RISCV_PATCH_H */
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 765004b60513..8bd51ed8b806 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -98,36 +98,3 @@ int patch_text_nosync(void *addr, const void *insns, size_t len)
 	return ret;
 }
 NOKPROBE_SYMBOL(patch_text_nosync);
-
-static int patch_text_cb(void *data)
-{
-	struct patch_insn *patch = data;
-	int ret = 0;
-
-	if (atomic_inc_return(&patch->cpu_count) == num_online_cpus()) {
-		ret =
-		    patch_text_nosync(patch->addr, &patch->insn,
-					    GET_INSN_LENGTH(patch->insn));
-		atomic_inc(&patch->cpu_count);
-	} else {
-		while (atomic_read(&patch->cpu_count) <= num_online_cpus())
-			cpu_relax();
-		smp_mb();
-	}
-
-	return ret;
-}
-NOKPROBE_SYMBOL(patch_text_cb);
-
-int patch_text(void *addr, u32 insn)
-{
-	struct patch_insn patch = {
-		.addr = addr,
-		.insn = insn,
-		.cpu_count = ATOMIC_INIT(0),
-	};
-
-	return stop_machine_cpuslocked(patch_text_cb,
-				       &patch, cpu_online_mask);
-}
-NOKPROBE_SYMBOL(patch_text);
diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index 475989f06d6d..27f8960c321c 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -24,12 +24,18 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
 static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
 {
 	unsigned long offset = GET_INSN_LENGTH(p->opcode);
+#ifdef CONFIG_RISCV_ISA_C
+	u32 opcode = __BUG_INSN_16;
+#else
+	u32 opcode = __BUG_INSN_32;
+#endif
 
 	p->ainsn.api.restore = (unsigned long)p->addr + offset;
 
-	patch_text(p->ainsn.api.insn, p->opcode);
-	patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
-		   __BUG_INSN_32);
+	patch_text_nosync(p->ainsn.api.insn, &p->opcode, offset);
+	patch_text_nosync((void *)((unsigned long)(p->ainsn.api.insn) + offset),
+			  &opcode, GET_INSN_LENGTH(opcode));
+
 }
 
 static void __kprobes arch_prepare_simulate(struct kprobe *p)
@@ -114,16 +120,23 @@ void *alloc_insn_page(void)
 /* install breakpoint in text */
 void __kprobes arch_arm_kprobe(struct kprobe *p)
 {
-	if ((p->opcode & __INSN_LENGTH_MASK) == __INSN_LENGTH_32)
-		patch_text(p->addr, __BUG_INSN_32);
-	else
-		patch_text(p->addr, __BUG_INSN_16);
+#ifdef CONFIG_RISCV_ISA_C
+	u32 opcode = __BUG_INSN_16;
+#else
+	u32 opcode = __BUG_INSN_32;
+#endif
+	patch_text_nosync(p->addr, &opcode, GET_INSN_LENGTH(opcode));
 }
 
 /* remove breakpoint from text */
 void __kprobes arch_disarm_kprobe(struct kprobe *p)
 {
-	patch_text(p->addr, p->opcode);
+#ifdef CONFIG_RISCV_ISA_C
+	u32 opcode = __BUG_INSN_16;
+#else
+	u32 opcode = __BUG_INSN_32;
+#endif
+	patch_text_nosync(p->addr, &p->opcode, GET_INSN_LENGTH(opcode));
 }
 
 void __kprobes arch_remove_kprobe(struct kprobe *p)
-- 
2.36.1