x86/ptrace: make genregs[32]_get/set more robust

The loop condition is fragile: we compare an unsigned value to zero, and
then decrement it by something larger than one in the loop.  All the
callers should be passing in appropriately aligned buffer lengths, but
it's better to just not rely on it, and have some appropriate defensive
loop limits.
Acked-by: NRoland McGrath <roland@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/ptrace.c

@@ -509,14 +509,14 @@ static int genregs_get(struct task_struct *target,
 {
 	if (kbuf) {
 		unsigned long *k = kbuf;
-		while (count > 0) {
+		while (count >= sizeof(*k)) {
 			*k++ = getreg(target, pos);
 			count -= sizeof(*k);
 			pos += sizeof(*k);
 		}
 	} else {
 		unsigned long __user *u = ubuf;
-		while (count > 0) {
+		while (count >= sizeof(*u)) {
 			if (__put_user(getreg(target, pos), u++))
 				return -EFAULT;
 			count -= sizeof(*u);
@@ -535,14 +535,14 @@ static int genregs_set(struct task_struct *target,
 	int ret = 0;
 	if (kbuf) {
 		const unsigned long *k = kbuf;
-		while (count > 0 && !ret) {
+		while (count >= sizeof(*k) && !ret) {
 			ret = putreg(target, pos, *k++);
 			count -= sizeof(*k);
 			pos += sizeof(*k);
 		}
 	} else {
 		const unsigned long  __user *u = ubuf;
-		while (count > 0 && !ret) {
+		while (count >= sizeof(*u) && !ret) {
 			unsigned long word;
 			ret = __get_user(word, u++);
 			if (ret)
@@ -1458,14 +1458,14 @@ static int genregs32_get(struct task_struct *target,
 {
 	if (kbuf) {
 		compat_ulong_t *k = kbuf;
-		while (count > 0) {
+		while (count >= sizeof(*k)) {
 			getreg32(target, pos, k++);
 			count -= sizeof(*k);
 			pos += sizeof(*k);
 		}
 	} else {
 		compat_ulong_t __user *u = ubuf;
-		while (count > 0) {
+		while (count >= sizeof(*u)) {
 			compat_ulong_t word;
 			getreg32(target, pos, &word);
 			if (__put_user(word, u++))
@@ -1486,14 +1486,14 @@ static int genregs32_set(struct task_struct *target,
 	int ret = 0;
 	if (kbuf) {
 		const compat_ulong_t *k = kbuf;
-		while (count > 0 && !ret) {
+		while (count >= sizeof(*k) && !ret) {
 			ret = putreg32(target, pos, *k++);
 			count -= sizeof(*k);
 			pos += sizeof(*k);
 		}
 	} else {
 		const compat_ulong_t __user *u = ubuf;
-		while (count > 0 && !ret) {
+		while (count >= sizeof(*u) && !ret) {
 			compat_ulong_t word;
 			ret = __get_user(word, u++);
 			if (ret)