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Subject[PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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_NEG_OFFSET to allow negative file offsets.

Changelog: v5->v6
- use FMODE_NEG_OFFSET (suggested by Al)
- rebased onto 2.6.33-rc1

Changelog: v4->v5
- clean up patches dor /dev/mem.
- rebased onto 2.6.32-rc1

Changelog: v3->v4
- make changes in mem.c aligned.
- change __negative_fpos_check() to return int.
- fixed bug in "pos" check.
- added comments.

Changelog: v2->v3
- fixed bug in rw_verify_area (it cannot be compiled)

CC: Al Viro <viro@ZenIV.linux.org.uk>
CC: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
drivers/char/mem.c | 4 ++++
fs/proc/base.c | 2 ++
fs/read_write.c | 21 +++++++++++++++++++--
include/linux/fs.h | 3 +++
4 files changed, 28 insertions(+), 2 deletions(-)
--- linux.orig/fs/read_write.c 2010-01-14 21:28:00.000000000 +0800
+++ linux/fs/read_write.c 2010-01-14 21:30:41.000000000 +0800
@@ -205,6 +205,20 @@ bad:
}
#endif

+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_NEG_OFFSET)
+ return 0;
+ return -EINVAL;
+}
+
/*
* rw_verify_area doesn't like huge counts. We limit
* them to something that fits in "int" so that others
@@ -222,8 +236,11 @@ int rw_verify_area(int read_write, struc
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(
--- linux.orig/include/linux/fs.h 2010-01-14 21:28:00.000000000 +0800
+++ linux/include/linux/fs.h 2010-01-14 21:32:24.000000000 +0800
@@ -93,6 +93,9 @@ struct inodes_stat_t {
/* Expect random access pattern */
#define FMODE_RANDOM ((__force fmode_t)0x1000)

+/* File is huge (eg. /dev/kmem): treat loff_t as unsigned */
+#define FMODE_NEG_OFFSET ((__force fmode_t)0x2000)
+
/*
* The below are the various read and write types that we support. Some of
* them include behavioral modifiers that send information down to the
--- linux.orig/drivers/char/mem.c 2010-01-14 21:28:00.000000000 +0800
+++ linux/drivers/char/mem.c 2010-01-14 21:33:20.000000000 +0800
@@ -861,6 +861,10 @@ static int memory_open(struct inode *ino
if (dev->dev_info)
filp->f_mapping->backing_dev_info = dev->dev_info;

+ /* Is /dev/mem or /dev/kmem ? */
+ if (dev->dev_info == &directly_mappable_cdev_bdi)
+ filp->f_mode |= FMODE_NEG_OFFSET;
+
if (dev->fops->open)
return dev->fops->open(inode, filp);

--- linux.orig/fs/proc/base.c 2010-01-14 21:28:00.000000000 +0800
+++ linux/fs/proc/base.c 2010-01-14 21:37:08.000000000 +0800
@@ -861,6 +861,8 @@ static const struct file_operations proc
static int mem_open(struct inode* inode, struct file* file)
{
file->private_data = (void*)((long)current->self_exec_id);
+ /* OK to pass negative loff_t, we can catch out-of-range */
+ file->f_mode |= FMODE_NEG_OFFSET;
return 0;
}




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