Hi Manfred,

On 20 December 2017 at 09:32, Dr. Manfred Spraul
[off-list ref] wrote:

Hi Michal,

On 12/19/2017 10:48 AM, Michal Hocko wrote:

quoted

Hi,
we have been contacted by our partner about the following permission
discrepancy
1. Create a shared memory segment with permissions 600 with user A using
    shmget(key, 1024, 0600 | IPC_CREAT)
2. ipcs -m should return an output as follows:

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x58b74326 759562241  A          600        1024       0

3. Try to read the metadata with shmctl(0, SHM_STAT,...) as user B.
4. shmctl will return -EACCES

The supper set information provided by shmctl can be retrieved by
reading /proc/sysvipc/shm which does not require read permissions
because it is 444.

It seems that the discrepancy is there since ae7817745eef ("[PATCH] ipc:
add generic struct ipc_ids seq_file iteration") when the proc interface
has been introduced. The changelog is really modest on information or
intention but I suspect this just got overlooked during review. SHM_STAT
has always been about read permission and it is explicitly documented
that way.

Are you sure that this patch changed the behavior?
The proc interface is much older.

Yes, I think that's correct. The /proc/sysvipc interface appeared in
2.3.x, and AFAIK the behavior was already different from *_STAT back
then.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/