Messages in this thread Patch in this message | | | From | Manfred Spraul <> | Subject | [RFC, PATCH] ipc/util.c: use idr_alloc_cyclic() for ipc allocations | Date | Tue, 2 Oct 2018 18:19:42 +0200 |
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A bit related to the patch series that increases IPC_MNI:
(User space) id reuse create the risk of data corruption:
Process A: calls ipc function Process A: sleeps just at the beginning of the syscall Process B: Frees the ipc object (i.e.: calls ...ctl(IPC_RMID) Process B: Creates a new ipc object (i.e.: calls ...get()) <If new object and old object have the same id> Process A: is woken up, and accesses the new object
To reduce the probability that the new and the old object have the same id, the current implementation adds a sequence number to the index of the object in the idr tree.
To further reduce the probability for a reuse, switch from idr_alloc to idr_alloc_cyclic.
The patch cycles over at least RADIX_TREE_MAP_SIZE, i.e. if there is only a small number of objects, the accesses continue to be direct.
As an option, this could be made dependent on the extended mode: In extended mode, cycle over e.g. at least 16k ids.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com> ---
Open questions: - Is there a significant performance advantage, especially there are many ipc ids? - Over how many ids should the code cycle always? - Further review remarks?
ipc/util.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/ipc/util.c b/ipc/util.c index 0af05752969f..6f83841f6761 100644 --- a/ipc/util.c +++ b/ipc/util.c @@ -216,10 +216,30 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new) */ if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */ + int idr_max; + new->seq = ids->seq++; if (ids->seq > IPCID_SEQ_MAX) ids->seq = 0; - idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT); + + /* + * If a user space visible id is reused, then this creates a + * risk for data corruption. To reduce the probability that + * a number is reduced, two approaches are used: + * 1) the idr index is allocated cyclically. + * 2) the use space id is build by concatenating the + * internal idr index with a sequence number + * To avoid that both numbers have the same cycle time, try + * to set the size for the cyclic alloc to an odd number. + */ + idr_max = ids->in_use*2+1; + if (idr_max < RADIX_TREE_MAP_SIZE-1) + idr_max = RADIX_TREE_MAP_SIZE-1; + if (idr_max > IPCMNI) + idr_max = IPCMNI; + + idx = idr_alloc_cyclic(&ids->ipcs_idr, new, 0, idr_max, + GFP_NOWAIT); } else { new->seq = ipcid_to_seqx(next_id); idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id), -- 2.17.1
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