| Message ID | 20190814154400.6371-2-christian.brauner@ubuntu.com |
|---|---|
| State | New |
| Headers | show |
| Series | waitid: process group enhancement | expand |
On 08/14, Christian Brauner wrote: > > 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. I noticed this typo only because I spent 2 minutes or more trying to understand this sentence ;) But yes, a signal handler or another thread can change pgrp in between. Reviewed-by: Oleg Nesterov <oleg@redhat.com>
On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote: > On 08/14, Christian Brauner wrote: > > > > 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. > > I noticed this typo only because I spent 2 minutes or more trying to > understand this sentence ;) But yes, a signal handler or another thread I'll try to rewrite it. :) > can change pgrp in between. > > Reviewed-by: Oleg Nesterov <oleg@redhat.com> >
On Wed, Aug 14, 2019 at 06:15:17PM +0200, Christian Brauner wrote: > On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote: > > On 08/14, Christian Brauner wrote: > > > > > > 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. > > > > I noticed this typo only because I spent 2 minutes or more trying to > > understand this sentence ;) But yes, a signal handler or another thread > > I'll try to rewrite it. :) Ok, here's what I changed it to: 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: 1. An extra system call is needed to get the current process group. 2. After the current process group is received and before it is passed to waitid a signal could arrive causing the current process group to change. Christian
On Wed, Aug 14, 2019 at 06:34:44PM +0200, Christian Brauner wrote: > On Wed, Aug 14, 2019 at 06:15:17PM +0200, Christian Brauner wrote: > > On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote: > > > On 08/14, Christian Brauner wrote: > > > > > > > > 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. > > > > > > I noticed this typo only because I spent 2 minutes or more trying to > > > understand this sentence ;) But yes, a signal handler or another thread > > > > I'll try to rewrite it. :) > > Ok, here's what I changed it to: > > 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: > 1. An extra system call is needed to get the current process group. > 2. After the current process group is received and before it is passed > to waitid a signal could arrive causing the current process group to change. I don't think "downsides" sufficiently conveys that this is hard breakage of a requirement for waitpid. How about something like the following? "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. Userspace cannot simply emulate this functionality with an additional getpgid syscall due to inherent race conditions that violate the async-signal safety requirements for waitpid." Rich
On Wed, Aug 14, 2019 at 12:55:01PM -0400, Rich Felker wrote: > On Wed, Aug 14, 2019 at 06:34:44PM +0200, Christian Brauner wrote: > > On Wed, Aug 14, 2019 at 06:15:17PM +0200, Christian Brauner wrote: > > > On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote: > > > > On 08/14, Christian Brauner wrote: > > > > > > > > > > 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. > > > > > > > > I noticed this typo only because I spent 2 minutes or more trying to > > > > understand this sentence ;) But yes, a signal handler or another thread > > > > > > I'll try to rewrite it. :) > > > > Ok, here's what I changed it to: > > > > 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: > > 1. An extra system call is needed to get the current process group. > > 2. After the current process group is received and before it is passed > > to waitid a signal could arrive causing the current process group to change. > > I don't think "downsides" sufficiently conveys that this is hard > breakage of a requirement for waitpid. How about something like the > following? > > "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. Userspace cannot > simply emulate this functionality with an additional getpgid syscall > due to inherent race conditions that violate the async-signal safety > requirements for waitpid." I like the rather specific example in there. How about we add that after this section like so: 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: 1. An extra system call is needed to get the current process group. 2. After the current process group is received and before it is passed to waitid a signal could arrive causing the current process group to change. Such inherent race-conditions as mentioned in 2. make it impossible for userspace to emulate this functionaly and thus violate the async-signal safety requirements for waitpid. Christian
On Wed, Aug 14, 2019 at 9:55 AM Rich Felker <dalias@libc.org> wrote: > > I don't think "downsides" sufficiently conveys that this is hard > breakage of a requirement for waitpid. Well, let's be honest here. Who has _ever_ seen a signal handler changing the current process group? In fact, the SYSV version of setpgid() takes a process ID to set it *for somebody else*, so the signal safety is not even necessarily relevant, since it might be racing with _another_ thread doing it (which even the kernel side won't fix - it's just user space doing odd things). So yes - it's technically true that it's impossible to emulate properly in user space. But I doubt it makes _any_ difference what-so-ever, and glibc might as well do something like ret = waitid(P_PGID, 0, ..); if (ret == -EINVAL) { do the emulation } which makes it work with older kernels, and has zero downside in practice. Hmm? Linus
On Wed, Aug 14, 2019 at 10:06:19AM -0700, Linus Torvalds wrote: > On Wed, Aug 14, 2019 at 9:55 AM Rich Felker <dalias@libc.org> wrote: > > > > I don't think "downsides" sufficiently conveys that this is hard > > breakage of a requirement for waitpid. > > Well, let's be honest here. Who has _ever_ seen a signal handler > changing the current process group? > > In fact, the SYSV version of setpgid() takes a process ID to set it > *for somebody else*, so the signal safety is not even necessarily > relevant, since it might be racing with _another_ thread doing it > (which even the kernel side won't fix - it's just user space doing odd > things). For that case, the operations are inherently unordered with respect to each other, and assuming the interpretation that waitpid is allowed to wait on "the pgid at the time of the call" rather than at the time of child exit/status-change -- which was discussed thoroughly in the thread leading up to this patch -- there is no conformance distinction. On the other hand, with changing your own pgid from a signal handler, there is a clear observable ordering between the events. For example, if the signal handler changes the pgid and forks a child with the new pgid, waitpid for "own pgid" can be assumed to include the new child in its wait set. I agree this is not common usage, so impact of breakage is probably low, but I'd rather not have wrong/racy hacks be something we're committed to supporting indefinitely on the userspace side. > So yes - it's technically true that it's impossible to emulate > properly in user space. > > But I doubt it makes _any_ difference what-so-ever, and glibc might as > well do something like > > ret = waitid(P_PGID, 0, ..); > if (ret == -EINVAL) { do the emulation } > > which makes it work with older kernels, and has zero downside in practice. > > Hmm? It only affects RV32 anyway; other archs all have a waitpid syscall that can be used. Since there's not yet any official libc release with RV32 support and AIUI the ABI is not considered "frozen" yet, emulation doesn't seem useful here. Whatever kernel version fixes this (or some later one, if nobody gets things together on upstreaming libc support of RV32) will just become the minimum version for RV32. Rich
On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote: > On 08/14, Christian Brauner wrote: > > > > 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. > > I noticed this typo only because I spent 2 minutes or more trying to > understand this sentence ;) But yes, a signal handler or another thread > can change pgrp in between. > > Reviewed-by: Oleg Nesterov <oleg@redhat.com> Applied-to: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=pidfd on top of P_PIDFD changes (cf. [1]) with merge conflict resolved (cf. [2]). (All changes on top of v5.3-rc1.) Merged-into: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=for-next and should show up in linux-next tomorrow. Thanks! Christian /* References */ [1]: https://lore.kernel.org/r/20190727222229.6516-2-christian@brauner.io [2]: patch after resolved merge-conflict: kernel/exit.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 64bb6893a37d..d2d74a7b81d1 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1596,10 +1596,13 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop, break; case P_PGID: type = PIDTYPE_PGID; - if (upid <= 0) + if (upid < 0) return -EINVAL; - pid = find_get_pid(upid); + if (upid) + pid = find_get_pid(upid); + else + pid = get_task_pid(current, PIDTYPE_PGID); break; case P_PIDFD: type = PIDTYPE_PID;
diff --git a/kernel/exit.c b/kernel/exit.c index 5b4a5dcce8f8..8e4e357fddc8 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) + pid = find_get_pid(upid); + else + pid = get_task_pid(current, PIDTYPE_PGID); 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;