iov_iter: Fix iov_iter_extract_pages() with zero-sized entries

iov_iter_extract_pages() doesn't correctly handle skipping over initial
zero-length entries in ITER_KVEC and ITER_BVEC-type iterators.

The problem is that it accidentally reduces maxsize to 0 when it
skipping and thus runs to the end of the array and returns 0.

Fix this by sticking the calculated size-to-copy in a new variable
rather than back in maxsize.

Fixes: 7d58fe731028 ("iov_iter: Add a function to extract a page list from an iterator")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Hildenbrand <david@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index b31597b0ca20ed3a3671f7fadd0ebb0382742065..27234a820eeb390f99e26f55002e51b2f95864e7 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1654,14 +1654,14 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
                                           size_t *offset0)
 {
        struct page **p, *page;
-       size_t skip = i->iov_offset, offset;
+       size_t skip = i->iov_offset, offset, size;
        int k;
 
        for (;;) {
                if (i->nr_segs == 0)
                        return 0;
-               maxsize = min(maxsize, i->bvec->bv_len - skip);
-               if (maxsize)
+               size = min(maxsize, i->bvec->bv_len - skip);
+               if (size)
                        break;
                i->iov_offset = 0;
                i->nr_segs--;
@@ -1674,16 +1674,16 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
        offset = skip % PAGE_SIZE;
        *offset0 = offset;
 
-       maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+       maxpages = want_pages_array(pages, size, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        p = *pages;
        for (k = 0; k < maxpages; k++)
                p[k] = page + k;
 
-       maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
-       iov_iter_advance(i, maxsize);
-       return maxsize;
+       size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
+       iov_iter_advance(i, size);
+       return size;
 }
 
 /*
@@ -1698,14 +1698,14 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
 {
        struct page **p, *page;
        const void *kaddr;
-       size_t skip = i->iov_offset, offset, len;
+       size_t skip = i->iov_offset, offset, len, size;
        int k;
 
        for (;;) {
                if (i->nr_segs == 0)
                        return 0;
-               maxsize = min(maxsize, i->kvec->iov_len - skip);
-               if (maxsize)
+               size = min(maxsize, i->kvec->iov_len - skip);
+               if (size)
                        break;
                i->iov_offset = 0;
                i->nr_segs--;
@@ -1717,13 +1717,13 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
        offset = (unsigned long)kaddr & ~PAGE_MASK;
        *offset0 = offset;
 
-       maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+       maxpages = want_pages_array(pages, size, offset, maxpages);
        if (!maxpages)
                return -ENOMEM;
        p = *pages;
 
        kaddr -= offset;
-       len = offset + maxsize;
+       len = offset + size;
        for (k = 0; k < maxpages; k++) {
                size_t seg = min_t(size_t, len, PAGE_SIZE);
 
@@ -1737,9 +1737,9 @@ static ssize_t iov_iter_extract_kvec_pages(struct iov_iter *i,
                kaddr += PAGE_SIZE;
        }
 
-       maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
-       iov_iter_advance(i, maxsize);
-       return maxsize;
+       size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
+       iov_iter_advance(i, size);
+       return size;
 }
 
 /*