ipc: conserve sequence numbers in ipcmni_extend mode
Rewrite, based on the patch from Waiman Long:
The mixing in of a sequence number into the IPC IDs is probably to avoid
ID reuse in userspace as much as possible. With ipcmni_extend mode, the
number of usable sequence numbers is greatly reduced leading to higher
chance of ID reuse.
To address this issue, we need to conserve the sequence number space as
much as possible. Right now, the sequence number is incremented for
every new ID created. In reality, we only need to increment the
sequence number when new allocated ID is not greater than the last one
allocated. It is in such case that the new ID may collide with an
existing one. This is being done irrespective of the ipcmni mode.
In order to avoid any races, the index is first allocated and then the
pointer is replaced.
Changes compared to the initial patch:
- Handle failures from idr_alloc().
- Avoid that concurrent operations can see the wrong sequence number.
(This is achieved by using idr_replace()).
- IPCMNI_SEQ_SHIFT is not a constant, thus renamed to
ipcmni_seq_shift().
- IPCMNI_SEQ_MAX is not a constant, thus renamed to ipcmni_seq_max().
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Manfred Spraul <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
Acked-by: Waiman Long <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: "Luis R. Rodriguez" <[email protected]>
Cc: Takashi Iwai <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
3 files changed
