[PATCH] uprobes/x86: emulate push insns for uprobe on x86

[Yonghong Song <yhs@fb.com>]

Uprobe is a tracing mechanism for userspace programs.
Typical uprobe will incur overhead of two traps.
First trap is caused by replaced trap insn, and
the second trap is to execute the original displaced
insn in user space.

To reduce the overhead, kernel provides hooks
for architectures to emulate the original insn
and skip the second trap. In x86, emulation
is done for certain branch insns.

This patch extends the emulation to "push <reg>"
insns. These insns are typical in the beginning
of the function. For example, bcc
in https://github.com/iovisor/bcc repo provides
tools to measure funclantency, detect memleak, etc.
The tools will place uprobes in the beginning of
function and possibly uretprobes at the end of function.
This patch is able to reduce the trap overhead for
uprobe from 2 to 1.

Without this patch, uretprobe will typically incur
three traps. With this patch, if the function starts
with "push" insn, the number of traps can be
reduced from 3 to 2.

An experiment was conducted on two local VMs,
fedora 26 64-bit VM and 32-bit VM, both 4 processors
and 4GB memory, booted with latest tip repo (and this patch).
The host is MacBook with intel i7 processor.

The test program looks like
  #include <stdio.h>
  #include <stdlib.h>
  #include <time.h>
  #include <sys/time.h>

  static void test() __attribute__((noinline));
  void test() {}
  int main() {
    struct timeval start, end;

    gettimeofday(&start, NULL);
    for (int i = 0; i < 1000000; i++) {
      test();
    }
    gettimeofday(&end, NULL);

    printf("%ld\n", ((end.tv_sec * 1000000 + end.tv_usec)
                     - (start.tv_sec * 1000000 + start.tv_usec)));
    return 0;
  }

The program is compiled without optimization, and
the first insn for function "test" is "push %rbp".
The host is relatively idle.

Before the test run, the uprobe is inserted as below for uprobe:
  echo 'p <binary>:<test_func_offset>' > /sys/kernel/debug/tracing/uprobe_events
  echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable
and for uretprobe:
  echo 'r <binary>:<test_func_offset>' > /sys/kernel/debug/tracing/uprobe_events
  echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable

Unit: microsecond(usec) per loop iteration

x86_64          W/ this patch   W/O this patch
uprobe          1.55            3.1
uretprobe       2.0             3.6

x86_32          W/ this patch   W/O this patch
uprobe          1.41            3.5
uretprobe       1.75            4.0

You can see that this patch significantly reduced the overhead,
50% for uprobe and 44% for uretprobe on x86_64, and even more
on x86_32.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 arch/x86/include/asm/uprobes.h |   4 ++
 arch/x86/kernel/uprobes.c      | 110 ++++++++++++++++++++++++++++++++++++++---
 2 files changed, 107 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/uprobes.h b/arch/x86/include/asm/uprobes.h
index 74f4c2f..a90090c 100644
--- a/arch/x86/include/asm/uprobes.h
+++ b/arch/x86/include/asm/uprobes.h
@@ -53,6 +53,10 @@ struct arch_uprobe {
 			u8	fixups;
 			u8	ilen;
 		} 			defparam;
+		struct {
+			u8	src_offset;	/* to the start of pt_regs */
+			u8	ilen;
+		}			push;
 	};
 };
 
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index a3755d2..1ee8b59 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -528,11 +528,11 @@ static int default_pre_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
 	return 0;
 }
 
-static int push_ret_address(struct pt_regs *regs, unsigned long ip)
+static int emulate_push_stack(struct pt_regs *regs, unsigned long val)
 {
 	unsigned long new_sp = regs->sp - sizeof_long();
 
-	if (copy_to_user((void __user *)new_sp, &ip, sizeof_long()))
+	if (copy_to_user((void __user *)new_sp, &val, sizeof_long()))
 		return -EFAULT;
 
 	regs->sp = new_sp;
@@ -566,7 +566,7 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
 		regs->ip += correction;
 	} else if (auprobe->defparam.fixups & UPROBE_FIX_CALL) {
 		regs->sp += sizeof_long(); /* Pop incorrect return address */
-		if (push_ret_address(regs, utask->vaddr + auprobe->defparam.ilen))
+		if (emulate_push_stack(regs, utask->vaddr + auprobe->defparam.ilen))
 			return -ERESTART;
 	}
 	/* popf; tell the caller to not touch TF */
@@ -655,7 +655,7 @@ static bool branch_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
 		 *
 		 * But there is corner case, see the comment in ->post_xol().
 		 */
-		if (push_ret_address(regs, new_ip))
+		if (emulate_push_stack(regs, new_ip))
 			return false;
 	} else if (!check_jmp_cond(auprobe, regs)) {
 		offs = 0;
@@ -665,6 +665,16 @@ static bool branch_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
 	return true;
 }
 
+static bool push_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
+{
+	void *src_ptr = (void *)regs + auprobe->push.src_offset;
+
+	if (emulate_push_stack(regs, *(unsigned long *)src_ptr))
+		return false;
+	regs->ip += auprobe->push.ilen;
+	return true;
+}
+
 static int branch_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
 {
 	BUG_ON(!branch_is_call(auprobe));
@@ -703,13 +713,99 @@ static const struct uprobe_xol_ops branch_xol_ops = {
 	.post_xol = branch_post_xol_op,
 };
 
-/* Returns -ENOSYS if branch_xol_ops doesn't handle this insn */
-static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
+static const struct uprobe_xol_ops push_xol_ops = {
+	.emulate  = push_emulate_op,
+};
+
+static int uprobe_setup_push_ops(struct arch_uprobe *auprobe, struct insn *insn,
+				 u8 opc1)
+{
+	u8 src_offset = 0;
+
+	if (insn->length > 2)
+		return -ENOSYS;
+	if (insn->length == 2) {
+		/* only support rex_prefix 0x41 (x64 only) */
+#ifdef CONFIG_X86_64
+		if (insn->rex_prefix.nbytes != 1 ||
+		    insn->rex_prefix.bytes[0] != 0x41)
+			return -ENOSYS;
+
+		auprobe->push.ilen = 2;
+		switch (opc1) {
+		case 0x50:
+			src_offset = offsetof(struct pt_regs, r8);
+			break;
+		case 0x51:
+			src_offset = offsetof(struct pt_regs, r9);
+			break;
+		case 0x52:
+			src_offset = offsetof(struct pt_regs, r10);
+			break;
+		case 0x53:
+			src_offset = offsetof(struct pt_regs, r11);
+			break;
+		case 0x54:
+			src_offset = offsetof(struct pt_regs, r12);
+			break;
+		case 0x55:
+			src_offset = offsetof(struct pt_regs, r13);
+			break;
+		case 0x56:
+			src_offset = offsetof(struct pt_regs, r14);
+			break;
+		case 0x57:
+			src_offset = offsetof(struct pt_regs, r15);
+			break;
+		}
+#else
+		return -ENOSYS;
+#endif
+	} else {
+		auprobe->push.ilen = 1;
+		switch (opc1) {
+		case 0x50:
+			src_offset = offsetof(struct pt_regs, ax);
+			break;
+		case 0x51:
+			src_offset = offsetof(struct pt_regs, cx);
+			break;
+		case 0x52:
+			src_offset = offsetof(struct pt_regs, dx);
+			break;
+		case 0x53:
+			src_offset = offsetof(struct pt_regs, bx);
+			break;
+		case 0x54:
+			src_offset = offsetof(struct pt_regs, sp);
+			break;
+		case 0x55:
+			src_offset = offsetof(struct pt_regs, bp);
+			break;
+		case 0x56:
+			src_offset = offsetof(struct pt_regs, si);
+			break;
+		case 0x57:
+			src_offset = offsetof(struct pt_regs, di);
+			break;
+		}
+	}
+
+	auprobe->push.src_offset = src_offset;
+	auprobe->ops = &push_xol_ops;
+	return 0;
+}
+
+/* Returns -ENOSYS if {branch|push}_xol_ops doesn't handle this insn */
+static int uprobe_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 {
 	u8 opc1 = OPCODE1(insn);
 	int i;
 
 	switch (opc1) {
+	case 0x50 ... 0x57:
+		return uprobe_setup_push_ops(auprobe, insn, opc1);
+
 	case 0xeb:	/* jmp 8 */
 	case 0xe9:	/* jmp 32 */
 	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
@@ -767,7 +863,7 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	if (ret)
 		return ret;
 
-	ret = branch_setup_xol_ops(auprobe, &insn);
+	ret = uprobe_setup_xol_ops(auprobe, &insn);
 	if (ret != -ENOSYS)
 		return ret;
 
-- 
2.9.5