pagemap: require aligned-length, non-null reads of /proc/pid/pagemap

The intention of commit aae8679b0ebcaa92f99c1c3cb0cd651594a43915
("pagemap: fix bug in add_to_pagemap, require aligned-length reads of
/proc/pid/pagemap") was to force reads of /proc/pid/pagemap to be a
multiple of 8 bytes, but now it allows to read 0 bytes, which actually
puts some data to user's buffer.  According to POSIX, if count is zero,
read() should return zero and has no other results.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Cc: Thomas Tuttle <ttuttle@google.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 39e4ad4f59f4..6f61b7cc32e0 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -665,6 +665,10 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
                 goto out_task;
 
         ret = 0;
+
+        if (!count)
+                goto out_task;
+
         mm = get_task_mm(task);
         if (!mm)
                 goto out_task;