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 <[email protected]>
Cc: Thomas Tuttle <[email protected]>
Acked-by: Matt Mackall <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 39e4ad4f59f4ef84fff3e809b1494a1eedb41267..6f61b7cc32e0e09c39aea2dd31cf64e2bc049060 100644 (file)
--- 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;