seq_file: fix mishandling of consecutive pread() invocations.

The following program illustrates the problem:

    char buf[8192];

    int fd = open("/proc/self/maps", O_RDONLY);

    n = pread(fd, buf, sizeof(buf), 0);
    printf("%d\n", n);

    /* lseek(fd, 0, SEEK_CUR); */ /* Uncomment to work around */

    n = pread(fd, buf, sizeof(buf), 0);
    printf("%d\n", n);

The second printf() prints zero, but uncommenting the lseek() corrects its
behaviour.

To fix, make seq_read() mirror seq_lseek() when processing changes in
*ppos.  Restore m->version first, then if required traverse and update
read_pos on success.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=11856

Signed-off-by: Earl Chew <echew@ixiacom.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 13 deletions

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 4023d6be939b..aa242dc99373 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -140,9 +140,21 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 
         mutex_lock(&m->lock);
 
+        /*
+         * seq_file->op->..m_start/m_stop/m_next may do special actions
+         * or optimisations based on the file->f_version, so we want to
+         * pass the file->f_version to those methods.
+         *
+         * seq_file->version is just copy of f_version, and seq_file
+         * methods can treat it simply as file version.
+         * It is copied in first and copied out after all operations.
+         * It is convenient to have it as  part of structure to avoid the
+         * need of passing another argument to all the seq_file methods.
+         */
+        m->version = file->f_version;
+
         /* Don't assume *ppos is where we left it */
         if (unlikely(*ppos != m->read_pos)) {
-                m->read_pos = *ppos;
                 while ((err = traverse(m, *ppos)) == -EAGAIN)
                         ;
                 if (err) {
@@ -152,21 +164,11 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
                         m->index = 0;
                         m->count = 0;
                         goto Done;
+                } else {
+                        m->read_pos = *ppos;
                 }
         }
 
-        /*
-         * seq_file->op->..m_start/m_stop/m_next may do special actions
-         * or optimisations based on the file->f_version, so we want to
-         * pass the file->f_version to those methods.
-         *
-         * seq_file->version is just copy of f_version, and seq_file
-         * methods can treat it simply as file version.
-         * It is copied in first and copied out after all operations.
-         * It is convenient to have it as  part of structure to avoid the
-         * need of passing another argument to all the seq_file methods.
-         */
-        m->version = file->f_version;
         /* grab buffer if we didn't have one */
         if (!m->buf) {
                 m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);