An error about IPC permission checking

[李尚杰]

in the source code kern/sysv_shm.c:

729         error = ipcperm(td, &shmseg->u.shm_perm, mode);
730 #ifdef MAC
731         error = mac_check_sysv_shmget(td->td_ucred, shmseg, uap->shmflg);
732         if (error != 0)
733                 MPRINTF(("mac_check_sysv_shmget returned %d\n", error));
734 #endif
735         if (error)
736                 return (error);

The return value of ipcperm() call is not be checked in time, and
interrupted by mac checking, if Mac is enabled.

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