----- On Apr 13, 2016, at 8:57 AM, Richard W.M. Jones rjones@xxxxxxxxxx wrote: > v1 -> v2: > > - Use current_umask() instead of current->fs->umask. > > - Retested it. > > ---------------------------------------------------------------------- > > It's not possible to read the process umask without also modifying it, > which is what umask(2) does. A library cannot read umask safely, > especially if the main program might be multithreaded. > > This patch series adds a trivial system call "getumask" which returns > the umask of the current process. In addition to this system call, we could extend a variation of my thread_local_abi system call (https://lkml.org/lkml/2016/4/4/455) (could be without features flags, or an entirely new system call specifically for a umask cache) to register a "current umask" cache located in a TLS area. Basically, reading the current umask value would be a simple load from a TLS variable. This could also allow quickly blocking and unblocking signal delivery from user-space by storing a mask to this TLS area. The kernel could then look into the signal mask in this TLS area whenever it needs to deliver a signal (assuming this code path can take user-space faults), in addition to the mask kept within the task struct. This "tls cache" idea could also apply to setting a CPU affinity to the currently running CPU for short user-space critical sections. The benefit here is to get _very_ fast operations on the thread umask and cpu affinity. Are those ideas too far-fetched ? Thanks, Mathieu > > Another approach to this has been attempted before, adding something > to /proc, although it didn't go anywhere. See: > > http://comments.gmane.org/gmane.linux.kernel/1292109 > > Another way to solve this would be to add a thread-safe getumask to > glibc. Since glibc could own the mutex, this would permit libraries > linked to this glibc to read umask safely. > > I should also note that man-pages documents getumask(3), but no > version of glibc has ever implemented it. > > Typical test script: > > #include <stdio.h> > #include <stdlib.h> > #include <linux/unistd.h> > #include <sys/syscall.h> > > int main(int argc, char *argv[]) > { > int r = syscall(329); > if (r == -1) { > perror("getumask"); > exit(1); > } > printf("umask = %o\n", r); > exit(0); > } > > $ ./getumask > umask = 22 > > Rich. -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
