From patchwork Wed Aug 14 13:07:32 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Brauner X-Patchwork-Id: 34082 Received: (qmail 109259 invoked by alias); 14 Aug 2019 13:10:54 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 109241 invoked by uid 89); 14 Aug 2019 13:10:53 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-14.7 required=5.0 tests=AWL, BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, SEM_URI, SEM_URIRED autolearn=ham version=3.3.1 spammy= X-HELO: youngberry.canonical.com From: Christian Brauner To: linux-kernel@vger.kernel.org, libc-alpha@sourceware.org Cc: oleg@redhat.com, alistair23@gmail.com, ebiederm@xmission.com, arnd@arndb.de, dalias@libc.org, torvalds@linux-foundation.org, adhemerval.zanella@linaro.org, fweimer@redhat.com, palmer@sifive.com, macro@wdc.com, zongbox@gmail.com, akpm@linux-foundation.org, viro@zeniv.linux.org.uk, hpa@zytor.com, Christian Brauner Subject: [PATCH v2 1/1] waitid: Add support for waiting for the current process group Date: Wed, 14 Aug 2019 15:07:32 +0200 Message-Id: <20190814130732.23572-2-christian.brauner@ubuntu.com> In-Reply-To: <20190814130732.23572-1-christian.brauner@ubuntu.com> References: <20190814130732.23572-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 From: "Eric W. Biederman" 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" Signed-off-by: Christian Brauner Cc: "H. Peter Anvin" Cc: Arnd Bergmann Cc: Palmer Dabbelt Cc: Rich Felker Cc: Alistair Francis Cc: Zong Li Cc: Andrew Morton Cc: Oleg Nesterov Cc: Linus Torvalds Cc: Al Viro Cc: Florian Weimer Cc: Adhemerval Zanella Cc: GNU C Library --- v1: - Christian Brauner : - 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 v2: - Oleg Nesterov : - take rcu_read_lock() around task_pgrp() so that we don't race with another thread changing the pgrp - Christian Brauner : - introduce find_get_pgrp() --- kernel/exit.c | 23 +++++++++++++++++++---- 1 file changed, 19 insertions(+), 4 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 5b4a5dcce8f8..d02715a39f68 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1554,6 +1554,20 @@ static long do_wait(struct wait_opts *wo) return retval; } +static struct pid *find_get_pgrp(pid_t nr) +{ + struct pid *pid; + + if (nr) + return find_get_pid(nr); + + rcu_read_lock(); + pid = get_pid(task_pgrp(current)); + rcu_read_unlock(); + + return pid; +} + static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop, int options, struct rusage *ru) { @@ -1576,19 +1590,20 @@ 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; + + pid = find_get_pgrp(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;