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ipc/msg: Fix msgctl(..., IPC_STAT, ...) between pid namespaces
Today msg_lspid and msg_lrpid are remembered in the pid namespace of the creator and the processes that last send or received a sysvipc message. If you have processes in multiple pid namespaces that is just wrong. The process ids reported will not make the least bit of sense. This fix is slightly more susceptible to a performance problem than the related fix for System V shared memory. By definition the pids are updated by msgsnd and msgrcv, the fast path of System V message queues. The only concern over the previous implementation is the incrementing and decrementing of the pid reference count. As that is the only difference and multiple updates by of the task_tgid by threads in the same process have been shown in af_unix sockets to create a cache line ping-pong between cpus of the same processor. In this case I don't expect cache lines holding pid reference counts to ping pong between cpus. As senders and receivers update different pids there is a natural separation there. Further if multiple threads of the same process either send or receive messages the pid will be updated to the same value and ipc_update_pid will avoid the reference count update. Which means in the common case I expect msg_lspid and msg_lrpid to remain constant, and reference counts not to be updated when messages are sent. In rare cases it may be possible to trigger the issue which was observed for af_unix sockets, but it will require multiple processes with multiple threads to be either sending or receiving messages. It just does not feel likely that anyone would do that in practice. This change updates msgctl(..., IPC_STAT, ...) to return msg_lspid and msg_lrpid in the pid namespace of the process calling stat. This change also updates cat /proc/sysvipc/msg to return print msg_lspid and msg_lrpid in the pid namespace of the process that opened the proc file. Fixes: b488893a ("pid namespaces: changes to show virtual ids to user") Reviewed-by:Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com> Signed-off-by: