From: "Eric W. Biederman" <ebied...@xmission.com> It was recently discovered that the linux version of waitid is not a superset of the other wait functions because it does not include support for waiting for the current process group. This has two downsides. An extra system call is needed to get the current process group, and a signal could come in between the system call that retrieved the process gorup and the call to waitid that changes the current process group.
Allow userspace to avoid both of those issues by defining idtype == P_PGID and id == 0 to mean wait for the caller's process group at the time of the call. Arguments can be made for using a different choice of idtype and id for this case but the BSDs already use this P_PGID and 0 to indicate waiting for the current process's process group. So be nice to user space programmers and don't introduce an unnecessary incompatibility. Some people have noted that the posix description is that waitpid will wait for the current process group, and that in the presence of pthreads that process group can change. To get clarity on this issue I looked at XNU, FreeBSD, and Luminos. All of those flavors of unix waited for the current process group at the time of call and as written could not adapt to the process group changing after the call. At one point Linux did adapt to the current process group changing but that stopped in 161550d74c07 ("pid: sys_wait... fixes"). It has been over 11 years since Linux has that behavior, no programs that fail with the change in behavior have been reported, and I could not find any other unix that does this. So I think it is safe to clarify the definition of current process group, to current process group at the time of the wait function. Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com> Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com> Cc: "H. Peter Anvin" <h...@zytor.com> Cc: Arnd Bergmann <a...@arndb.de> Cc: Palmer Dabbelt <pal...@sifive.com> Cc: Rich Felker <dal...@libc.org> Cc: Alistair Francis <alistai...@gmail.com> Cc: Zong Li <zong...@gmail.com> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Oleg Nesterov <o...@redhat.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Al Viro <v...@zeniv.linux.org.uk> Cc: Florian Weimer <fwei...@redhat.com> Cc: Adhemerval Zanella <adhemerval.zane...@linaro.org> Cc: GNU C Library <libc-al...@sourceware.org> --- v1: - Christian Brauner <christian.brau...@ubuntu.com>: - move find_get_pid() calls into the switch statements to minimize merge conflicts with P_PIDFD changes and adhere to coding style discussions we had for P_PIDFD --- kernel/exit.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 5b4a5dcce8f8..e70083b14f31 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1576,19 +1576,23 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop, type = PIDTYPE_PID; if (upid <= 0) return -EINVAL; + + pid = find_get_pid(upid); break; case P_PGID: type = PIDTYPE_PGID; - if (upid <= 0) + if (upid < 0) return -EINVAL; + + if (upid == 0) + pid = get_pid(task_pgrp(current)); + else + pid = find_get_pid(upid); break; default: return -EINVAL; } - if (type < PIDTYPE_MAX) - pid = find_get_pid(upid); - wo.wo_type = type; wo.wo_pid = pid; wo.wo_flags = options; -- 2.22.0