fold checks into iterate_and_advance()

they are open-coded in all users except iov_iter_advance(), and there
they wouldn't be a bad idea either - as it is, iov_iter_advance(i, 0)
ends up dereferencing potentially past the end of iovec array.  It
doesn't do anything with the value it reads, and very unlikely to
trigger an oops on dereference, but it is not impossible.
Reported-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -99,6 +99,9 @@
 }
 
 #define iterate_and_advance(i, n, v, I, B, K) {			\
+	if (unlikely(i->count < n))				\
+		n = i->count;					\
+	if (n) {						\
 		size_t skip = i->iov_offset;			\
 		if (unlikely(i->type & ITER_BVEC)) {		\
 			const struct bio_vec *bvec;		\
@@ -133,6 +136,7 @@
 		}						\
 		i->count -= n;					\
 		i->iov_offset = skip;				\
+	}							\
 }
 
 static size_t copy_page_to_iter_iovec(struct page *page, size_t offset, size_t bytes,
@@ -386,12 +390,6 @@ static void memzero_page(struct page *page, size_t offset, size_t len)
 size_t copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 {
 	const char *from = addr;
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	iterate_and_advance(i, bytes, v,
 		__copy_to_user(v.iov_base, (from += v.iov_len) - v.iov_len,
 			       v.iov_len),
@@ -407,12 +405,6 @@ EXPORT_SYMBOL(copy_to_iter);
 size_t copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
 {
 	char *to = addr;
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	iterate_and_advance(i, bytes, v,
 		__copy_from_user((to += v.iov_len) - v.iov_len, v.iov_base,
 				 v.iov_len),
@@ -428,12 +420,6 @@ EXPORT_SYMBOL(copy_from_iter);
 size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 {
 	char *to = addr;
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	iterate_and_advance(i, bytes, v,
 		__copy_from_user_nocache((to += v.iov_len) - v.iov_len,
 					 v.iov_base, v.iov_len),
@@ -474,12 +460,6 @@ EXPORT_SYMBOL(copy_page_from_iter);
 
 size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	iterate_and_advance(i, bytes, v,
 		__clear_user(v.iov_base, v.iov_len),
 		memzero_page(v.bv_page, v.bv_offset, v.bv_len),
@@ -666,12 +646,6 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
 	char *to = addr;
 	__wsum sum, next;
 	size_t off = 0;
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	sum = *csum;
 	iterate_and_advance(i, bytes, v, ({
 		int err = 0;
@@ -710,12 +684,6 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, __wsum *csum,
 	const char *from = addr;
 	__wsum sum, next;
 	size_t off = 0;
-	if (unlikely(bytes > i->count))
-		bytes = i->count;
-
-	if (unlikely(!bytes))
-		return 0;
-
 	sum = *csum;
 	iterate_and_advance(i, bytes, v, ({
 		int err = 0;