iov_iter_fault_in_readable fix

[Dmitriy Monakhov <dmonakhov@sw.ru>]

 	Function prerform check for signgle region, with out respect to
 	segment nature of iovec, For example writev no longer works :)

	/* TESTCASE BEGIN */
	#include <stdio.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>
	#include <sys/uio.h>
	#include <sys/mman.h>
	#define SIZE  (4096 * 2)
	int main(int argc, char* argv[])
	{	
		char* ptr[4];
		struct iovec iov[2];
		int fd, ret;
		ptr[0] = mmap(NULL, SIZE, PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
		ptr[1] = mmap(NULL, SIZE, PROT_NONE,
			MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
		ptr[2] = mmap(NULL, SIZE, PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
		ptr[3] = mmap(NULL, SIZE, PROT_NONE, 
			MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);

		iov[0].iov_base = ptr[0] + (SIZE -1);
		iov[0].iov_len = 1;
		memset(ptr[0], 1, SIZE);

		iov[1].iov_base = ptr[2];
		iov[1].iov_len = SIZE;
		memset(ptr[2], 2, SIZE);

		fd = open(argv[1], O_CREAT|O_RDWR|O_TRUNC, 0666);
		ret = writev(fd, iov, sizeof(iov) / sizeof(struct iovec));
		return 0;
	}	
	/* TESTCASE END*/
	We will get folowing result:
		writev(3, [{"\1", 1}, {"\2"..., 8192}], 2) = -1 EFAULT (Bad address)
 	
	this is hidden bug, and it was invisiable because XXXX_fault_in_readable
 	return value was ignored before. Lets iov_iter_fault_in_readable
 	perform checks for all segments.

Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index fef19fc..7e025ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -433,7 +433,7 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
 size_t iov_iter_copy_from_user(struct page *page,
 		struct iov_iter *i, unsigned long offset, size_t bytes);
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
-int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes);
+int iov_iter_fault_in_readable(struct iov_iter *i, size_t *bytes);
 size_t iov_iter_single_seg_count(struct iov_iter *i);
 
 static inline void iov_iter_init(struct iov_iter *i,
diff --git a/mm/filemap.c b/mm/filemap.c
index 8d59ed9..8600c3e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1817,10 +1817,32 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
 }
 EXPORT_SYMBOL(iov_iter_advance);
 
-int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)
+int iov_iter_fault_in_readable(struct iov_iter *i, size_t* bytes)
 {
-	char __user *buf = i->iov->iov_base + i->iov_offset;
-	return fault_in_pages_readable(buf, bytes);
+	size_t len = *bytes;
+	int ret;
+	if (likely(i->nr_segs == 1)) {
+		ret = fault_in_pages_readable(i->iov->iov_base, len);
+		if (ret)
+			*bytes = 0;
+	} else {
+		const struct iovec *iov = i->iov;
+		size_t base = i->iov_offset;
+		*bytes = 0;
+		while (len) {
+			int copy = min(len, iov->iov_len - base);
+			if ((ret = fault_in_pages_readable(iov->iov_base + base, copy)))
+				break;
+			*bytes += copy;
+			len -= copy;
+			base += copy;
+			if (iov->iov_len == base) {
+				iov++;
+				base = 0;
+			}
+		}
+	}
+	return ret;	
 }
 EXPORT_SYMBOL(iov_iter_fault_in_readable);
 
@@ -2110,7 +2132,7 @@ static ssize_t generic_perform_write_2copy(struct file *file,
 		 * to check that the address is actually valid, when atomic
 		 * usercopies are used, below.
 		 */
-		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
+		if (unlikely(iov_iter_fault_in_readable(i, &bytes) && !bytes)) {
 			status = -EFAULT;
 			break;
 		}
@@ -2284,7 +2306,7 @@ again:
 		 * to check that the address is actually valid, when atomic
 		 * usercopies are used, below.
 		 */
-		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
+		if (unlikely(iov_iter_fault_in_readable(i, &bytes)&& !bytes)) {
 			status = -EFAULT;
 			break;
 		}