[PATCH] Use PTRACE_GETREGS on i386

[Denys Vlasenko]

While discussing ptrace speedup, in one email Linus said
that it's stupid that strace doesn't even use already existing
speedups, such as PTRACE_GETREGS.

I decided to give it a try.
Stracing a strace running a strace running a strace running a static binary,
before patch:

$ time ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace ./true
execve("/bin/true", ["true"], [/* 51 vars */]) = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
getuid32()                              = 0
_exit(0)                                = ?
+++ exited with 0 +++

real	0m33.178s
user	0m6.753s
sys	0m28.798s


After the patch:

$ time ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace -s999 -o/dev/null ./strace ./true
execve("/bin/true", ["true"], [/* 51 vars */]) = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
getuid32()                              = 0
_exit(0)                                = ?
+++ exited with 0 +++

real	0m14.608s
user	0m2.959s
sys	0m12.828s


Patch follows. Please review.

I guess we need to add a check on startup to make sure PTRACE_GETREGS works?

-- 
vda

diff -d -urpN strace.5/syscall.c strace.6/syscall.c
--- strace.5/syscall.c	2011-08-25 10:39:36.000000000 +0200
+++ strace.6/syscall.c	2011-08-25 11:51:32.148543603 +0200
@@ -716,7 +716,28 @@ struct tcb *tcp_last = NULL;
 
 #ifdef LINUX
 # if defined (I386)
-static long eax;
+struct i386_user_regs_struct {
+	long ebx;
+	long ecx;
+	long edx;
+	long esi;
+	long edi;
+	long ebp;
+	long eax;
+	long xds;
+	long xes;
+	long xfs;
+	long xgs;
+	long orig_eax;
+	long eip;
+	long xcs;
+	long eflags;
+	long esp;
+	long xss;
+	/* Just in case we forgot a few fields and kernel would write more... */
+	long paranoia[8];
+};
+static struct i386_user_regs_struct i386_regs;
 # elif defined (IA64)
 long r8, r10, psr; /* TODO: make static? */
 long ia32 = 0; /* not static */
@@ -895,8 +916,9 @@ get_scno(struct tcb *tcp)
 	if (upeek(tcp, PT_ORIG_P0, &scno))
 		return -1;
 # elif defined (I386)
-	if (upeek(tcp, 4*ORIG_EAX, &scno) < 0)
+	if (ptrace(PTRACE_GETREGS, tcp->pid, NULL, (long)&i386_regs) < 0)
 		return -1;
+	scno = i386_regs.orig_eax;
 # elif defined (X86_64)
 	if (upeek(tcp, 8*ORIG_RAX, &scno) < 0)
 		return -1;
@@ -969,7 +991,7 @@ get_scno(struct tcb *tcp)
 	if (upeek(tcp, PT_CR_IPSR, &psr) >= 0)
 		ia32 = (psr & IA64_PSR_IS) != 0;
 	if (ia32) {
-		if (upeek(tcp, PT_R1, &scno) < 0)	/* orig eax */
+		if (upeek(tcp, PT_R1, &scno) < 0)
 			return -1;
 	} else {
 		if (upeek(tcp, PT_R15, &scno) < 0)
@@ -1291,21 +1313,17 @@ syscall_fixup_on_sysenter(struct tcb *tc
 #ifdef LINUX
 	/* A common case of "not a syscall entry" is post-execve SIGTRAP */
 #if defined (I386)
+	if (i386_regs.eax != -ENOSYS) {
+		if (debug)
+			fprintf(stderr, "not a syscall entry (eax = %ld)\n", i386_regs.eax);
+		return 0;
+	}
+#elif defined (X86_64)
 	/* With PTRACE_O_TRACEEXEC, post-execve SIGTRAP is disabled.
-	 * Every extra ptrace call is expensive, so check EAX
+	 * Every extra ptrace call is expensive, so check RAX
 	 * on syscall entry only if PTRACE_O_TRACEEXEC is not enabled:
 	 */
 	if (!(ptrace_setoptions & PTRACE_O_TRACEEXEC)) {
-		if (upeek(tcp, 4*EAX, &eax) < 0)
-			return -1;
-		if (eax != -ENOSYS) {
-			if (debug)
-				fprintf(stderr, "not a syscall entry (eax = %ld)\n", eax);
-			return 0;
-		}
-	}
-#elif defined (X86_64)
-	if (!(ptrace_setoptions & PTRACE_O_TRACEEXEC)) {
 		if (upeek(tcp, 8*RAX, &rax) < 0)
 			return -1;
 		if (current_personality == 1)
@@ -1582,7 +1600,16 @@ syscall_enter(struct tcb *tcp)
 	for (i = 0; i < nargs; ++i)
 		if (upeek(tcp, (i < 5 ? i : i + 2)*4, &tcp->u_arg[i]) < 0)
 			return -1;
-# else /* Other architecture (like i386) (32bits specific) */
+# elif defined(I386)
+	(void)i;
+	(void)nargs;
+	tcp->u_arg[0] = i386_regs.ebx;
+	tcp->u_arg[1] = i386_regs.ecx;
+	tcp->u_arg[2] = i386_regs.edx;
+	tcp->u_arg[3] = i386_regs.esi;
+	tcp->u_arg[4] = i386_regs.edi;
+	tcp->u_arg[5] = i386_regs.ebp;
+# else /* Other architecture (32bits specific) */
 	for (i = 0; i < nargs; ++i)
 		if (upeek(tcp, i*4, &tcp->u_arg[i]) < 0)
 			return -1;
@@ -1882,7 +1909,7 @@ get_syscall_result(struct tcb *tcp)
 	if (upeek(tcp, PT_R0, &r0) < 0)
 		return -1;
 # elif defined (I386)
-	if (upeek(tcp, 4*EAX, &eax) < 0)
+	if (ptrace(PTRACE_GETREGS, tcp->pid, NULL, (long)&i386_regs) < 0)
 		return -1;
 # elif defined (X86_64)
 	if (upeek(tcp, 8*RAX, &rax) < 0)
@@ -2053,12 +2080,12 @@ get_error(struct tcb *tcp)
 		tcp->u_rval = gpr2;
 	}
 # elif defined(I386)
-	if (check_errno && is_negated_errno(eax)) {
+	if (check_errno && is_negated_errno(i386_regs.eax)) {
 		tcp->u_rval = -1;
-		u_error = -eax;
+		u_error = -i386_regs.eax;
 	}
 	else {
-		tcp->u_rval = eax;
+		tcp->u_rval = i386_regs.eax;
 	}
 # elif defined(X86_64)
 	if (check_errno && is_negated_errno(rax)) {