vfs: introduce FMODE_UNSIGNED_OFFSET for allowing negative f_pos

Now, rw_verify_area() checsk f_pos is negative or not.  And if negative,
returns -EINVAL.

But, some special files as /dev/(k)mem and /proc/<pid>/mem etc..  has
negative offsets.  And we can't do any access via read/write to the
file(device).

So introduce FMODE_UNSIGNED_OFFSET to allow negative file offsets.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 24 insertions, 4 deletions

diff --git a/fs/read_write.c b/fs/read_write.c
index e757ef26e4ce..9cd9d148105d 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -31,6 +31,20 @@ const struct file_operations generic_ro_fops = {
 
 EXPORT_SYMBOL(generic_ro_fops);
 
+static int
+__negative_fpos_check(struct file *file, loff_t pos, size_t count)
+{
+        /*
+         * pos or pos+count is negative here, check overflow.
+         * too big "count" will be caught in rw_verify_area().
+         */
+        if ((pos < 0) && (pos + count < pos))
+                return -EOVERFLOW;
+        if (file->f_mode & FMODE_UNSIGNED_OFFSET)
+                return 0;
+        return -EINVAL;
+}
+
 /**
  * generic_file_llseek_unlocked - lockless generic llseek implementation
  * @file:       file structure to seek on
@@ -62,7 +76,9 @@ generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
                 break;
         }
 
-        if (offset < 0 || offset > inode->i_sb->s_maxbytes)
+        if (offset < 0 && __negative_fpos_check(file, offset, 0))
+                return -EINVAL;
+        if (offset > inode->i_sb->s_maxbytes)
                 return -EINVAL;
 
         /* Special lock needed here? */
@@ -137,7 +153,7 @@ loff_t default_llseek(struct file *file, loff_t offset, int origin)
                         offset += file->f_pos;
         }
         retval = -EINVAL;
-        if (offset >= 0) {
+        if (offset >= 0 || !__negative_fpos_check(file, offset, 0)) {
                 if (offset != file->f_pos) {
                         file->f_pos = offset;
                         file->f_version = 0;
@@ -221,6 +237,7 @@ bad:
 }
 #endif
 
+
 /*
  * rw_verify_area doesn't like huge counts. We limit
  * them to something that fits in "int" so that others
@@ -238,8 +255,11 @@ int rw_verify_area(int read_write, struct file *file, loff_t *ppos, size_t count
         if (unlikely((ssize_t) count < 0))
                 return retval;
         pos = *ppos;
-        if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
-                return retval;
+        if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
+                retval = __negative_fpos_check(file, pos, count);
+                if (retval)
+                        return retval;
+        }
 
         if (unlikely(inode->i_flock && mandatory_lock(inode))) {
                 retval = locks_mandatory_area(