readv/writev: do the same MAX_RW_COUNT truncation that read/write does

We used to protect against overflow, but rather than return an error, do
what read/write does, namely to limit the total size to MAX_RW_COUNT.
This is not only more consistent, but it also means that any broken
low-level read/write routine that still keeps counts in 'int' can't
break.
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/compat.c

@@ -606,14 +606,14 @@ ssize_t compat_rw_copy_check_uvector(int type,
 	/*
 	 * Single unix specification:
 	 * We should -EINVAL if an element length is not >= 0 and fitting an
-	 * ssize_t.  The total length is fitting an ssize_t
+	 * ssize_t.
 	 *
-	 * Be careful here because iov_len is a size_t not an ssize_t
+	 * In Linux, the total length is limited to MAX_RW_COUNT, there is
+	 * no overflow possibility.
 	 */
 	tot_len = 0;
 	ret = -EINVAL;
 	for (seg = 0; seg < nr_segs; seg++) {
-		compat_ssize_t tmp = tot_len;
 		compat_uptr_t buf;
 		compat_ssize_t len;
 
@@ -624,13 +624,13 @@ ssize_t compat_rw_copy_check_uvector(int type,
 		}
 		if (len < 0)	/* size_t not fitting in compat_ssize_t .. */
 			goto out;
-		tot_len += len;
-		if (tot_len < tmp) /* maths overflow on the compat_ssize_t */
-			goto out;
 		if (!access_ok(vrfy_dir(type), compat_ptr(buf), len)) {
 			ret = -EFAULT;
 			goto out;
 		}
+		if (len > MAX_RW_COUNT - tot_len)
+			len = MAX_RW_COUNT - tot_len;
+		tot_len += len;
 		iov->iov_base = compat_ptr(buf);
 		iov->iov_len = (compat_size_t) len;
 		uvector++;

fs/read_write.c

@@ -243,8 +243,6 @@ SYSCALL_DEFINE5(llseek, unsigned int, fd, unsigned long, offset_high,
  * them to something that fits in "int" so that others
  * won't have to do range checks all the time.
  */
-#define MAX_RW_COUNT (INT_MAX & PAGE_CACHE_MASK)
-
 int rw_verify_area(int read_write, struct file *file, loff_t *ppos, size_t count)
 {
 	struct inode *inode;
@@ -584,7 +582,7 @@ ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
 			      unsigned long nr_segs, unsigned long fast_segs,
 			      struct iovec *fast_pointer,
 			      struct iovec **ret_pointer)
-  {
+{
 	unsigned long seg;
 	ssize_t ret;
 	struct iovec *iov = fast_pointer;
@@ -624,6 +622,9 @@ ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
 	 * if an element length is < 0 when cast to ssize_t or if the
 	 * total length would overflow the ssize_t return value of the
 	 * system call.
+	 *
+	 * Linux caps all read/write calls to MAX_RW_COUNT, and avoids the
+	 * overflow case.
 	 */
 	ret = 0;
 	for (seg = 0; seg < nr_segs; seg++) {
@@ -632,7 +633,7 @@ ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
 
 		/* see if we we're about to use an invalid len or if
 		 * it's about to overflow ssize_t */
-		if (len < 0 || (ret + len < ret)) {
+		if (len < 0) {
 			ret = -EINVAL;
 			goto out;
 		}
@@ -640,7 +641,10 @@ ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,
 			ret = -EFAULT;
 			goto out;
 		}
-
+		if (len > MAX_RW_COUNT - ret) {
+			len = MAX_RW_COUNT - ret;
+			iov[seg].iov_len = len;
+		}
 		ret += len;
 	}
 out:

include/linux/fs.h

@@ -1867,6 +1867,7 @@ extern int current_umask(void);
 /* /sys/fs */
 extern struct kobject *fs_kobj;
 
+#define MAX_RW_COUNT (INT_MAX & PAGE_CACHE_MASK)
 extern int rw_verify_area(int, struct file *, loff_t *, size_t);
 
 #define FLOCK_VERIFY_READ  1