This isn't functionally apparent for some reason, but
when we test io at extreme offsets at the end of the loff_t
rang, such as in fstests xfs/071, the calculation of
"max" in dax_io() can be wrong due to pos + size overflowing.
For example,
# xfs_io -c "pwrite 9223372036854771712 512" /mnt/test/file
enters dax_io with:
start 0x7ffffffffffff000
end 0x7ffffffffffff200
and the rounded up "size" variable is 0x1000. This yields:
pos + size 0x8000000000000000 (overflows loff_t)
end 0x7ffffffffffff200
Due to the overflow, the min() function picks the wrong
value for the "max" variable, and when we send (max - pos)
into i.e. copy_from_iter_pmem() it is also the wrong value.
This somehow(tm) gets magically absorbed without incident,
probably because iter->count is correct. But it seems best
to fix it up properly by comparing the two values as
unsigned.
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---
diff --git a/fs/dax.c b/fs/dax.c
index 761495b..e207f8f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -208,7 +208,12 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
dax.addr += first;
size = map_len - first;
}
- max = min(pos + size, end);
+ /*
+ * pos + size is one past the last offset for IO,
+ * so pos + size can overflow loff_t at extreme offsets.
+ * Cast to u64 to catch this and get the true minimum.
+ */
+ max = min_t(u64, pos + size, end);
}
if (iov_iter_rw(iter) == WRITE) {
--
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