splice: __generic_file_splice_read: fix i_size_read() length checks

__generic_file_splice_read's partial page check, at eof after readpage,
not only got its calculations wrong, but also reused the loff variable:
causing data corruption when splicing from a non-0 offset in the file's
last page (revealed by ext2 -b 1024 testing on a loop of a tmpfs file).

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 6349d3189e3f..123fcdb2e4d9 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -272,7 +272,6 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
         struct page *page;
         pgoff_t index, end_index;
         loff_t isize;
-        size_t total_len;
         int error, page_nr;
         struct splice_pipe_desc spd = {
                 .pages = pages,
@@ -298,7 +297,6 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
          * Now fill in the holes:
          */
         error = 0;
-        total_len = 0;
 
         /*
          * Lookup the (hopefully) full range of pages we need.
@@ -429,29 +427,33 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
                          * the length and stop
                          */
                         if (end_index == index) {
-                                loff = PAGE_CACHE_SIZE - (isize & ~PAGE_CACHE_MASK);
-                                if (total_len + loff > isize)
+                                unsigned int plen;
+
+                                /*
+                                 * max good bytes in this page
+                                 */
+                                plen = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+                                if (plen <= loff)
                                         break;
+
                                 /*
                                  * force quit after adding this page
                                  */
+                                this_len = min(this_len, plen - loff);
                                 len = this_len;
-                                this_len = min(this_len, loff);
-                                loff = 0;
                         }
                 }
 fill_it:
                 partial[page_nr].offset = loff;
                 partial[page_nr].len = this_len;
                 len -= this_len;
-                total_len += this_len;
                 loff = 0;
                 spd.nr_pages++;
                 index++;
         }
 
         /*
-         * Release any pages at the end, if we quit early. 'i' is how far
+         * Release any pages at the end, if we quit early. 'page_nr' is how far
          * we got, 'nr_pages' is how many pages are in the map.
          */
         while (page_nr < nr_pages)