On Mon, Mar 18, 2019 at 07:50:52PM -0400, Joel Fernandes wrote: > On Mon, Mar 18, 2019 at 01:29:51AM +0100, Christian Brauner wrote: > > On Sun, Mar 17, 2019 at 08:40:19AM -0700, Daniel Colascione wrote: > > > On Sun, Mar 17, 2019 at 4:42 AM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > > > On Sat, Mar 16, 2019 at 09:53:06PM -0400, Joel Fernandes wrote: > > > > > On Sat, Mar 16, 2019 at 12:37:18PM -0700, Suren Baghdasaryan wrote: > > > > > > On Sat, Mar 16, 2019 at 11:57 AM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Sat, Mar 16, 2019 at 11:00:10AM -0700, Daniel Colascione wrote: > > > > > > > > On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > > On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote: > > > > > > > > > > [..] > > > > > > > > > > > > why do we want to add a new syscall (pidfd_wait) though? Why not just use > > > > > > > > > > > > standard poll/epoll interface on the proc fd like Daniel was suggesting. > > > > > > > > > > > > AFAIK, once the proc file is opened, the struct pid is essentially pinned > > > > > > > > > > > > even though the proc number may be reused. Then the caller can just poll. > > > > > > > > > > > > We can add a waitqueue to struct pid, and wake up any waiters on process > > > > > > > > > > > > death (A quick look shows task_struct can be mapped to its struct pid) and > > > > > > > > > > > > also possibly optimize it using Steve's TIF flag idea. No new syscall is > > > > > > > > > > > > needed then, let me know if I missed something? > > > > > > > > > > > > > > > > > > > > > > Huh, I thought that Daniel was against the poll/epoll solution? > > > > > > > > > > > > > > > > > > > > Hmm, going through earlier threads, I believe so now. Here was Daniel's > > > > > > > > > > reasoning about avoiding a notification about process death through proc > > > > > > > > > > directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html > > > > > > > > > > > > > > > > > > > > May be a dedicated syscall for this would be cleaner after all. > > > > > > > > > > > > > > > > > > Ah, I wish I've seen that discussion before... > > > > > > > > > syscall makes sense and it can be non-blocking and we can use > > > > > > > > > select/poll/epoll if we use eventfd. > > > > > > > > > > > > > > > > Thanks for taking a look. > > > > > > > > > > > > > > > > > I would strongly advocate for > > > > > > > > > non-blocking version or at least to have a non-blocking option. > > > > > > > > > > > > > > > > Waiting for FD readiness is *already* blocking or non-blocking > > > > > > > > according to the caller's desire --- users can pass options they want > > > > > > > > to poll(2) or whatever. There's no need for any kind of special > > > > > > > > configuration knob or non-blocking option. We already *have* a > > > > > > > > non-blocking option that works universally for everything. > > > > > > > > > > > > > > > > As I mentioned in the linked thread, waiting for process exit should > > > > > > > > work just like waiting for bytes to appear on a pipe. Process exit > > > > > > > > status is just another blob of bytes that a process might receive. A > > > > > > > > process exit handle ought to be just another information source. The > > > > > > > > reason the unix process API is so awful is that for whatever reason > > > > > > > > the original designers treated processes as some kind of special kind > > > > > > > > of resource instead of fitting them into the otherwise general-purpose > > > > > > > > unix data-handling API. Let's not repeat that mistake. > > > > > > > > > > > > > > > > > Something like this: > > > > > > > > > > > > > > > > > > evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC); > > > > > > > > > // register eventfd to receive death notification > > > > > > > > > pidfd_wait(pid_to_kill, evfd); > > > > > > > > > // kill the process > > > > > > > > > pidfd_send_signal(pid_to_kill, ...) > > > > > > > > > // tend to other things > > > > > > > > > > > > > > > > Now you've lost me. pidfd_wait should return a *new* FD, not wire up > > > > > > > > an eventfd. > > > > > > > > > > > > > > > > > > > > Ok, I probably misunderstood your post linked by Joel. I though your > > > > > > original proposal was based on being able to poll a file under > > > > > > /proc/pid and then you changed your mind to have a separate syscall > > > > > > which I assumed would be a blocking one to wait for process exit. > > > > > > Maybe you can describe the new interface you are thinking about in > > > > > > terms of userspace usage like I did above? Several lines of code would > > > > > > explain more than paragraphs of text. > > > > > > > > > > Hey, Thanks Suren for the eventfd idea. I agree with Daniel on this. The idea > > > > > from Daniel here is to wait for process death and exit events by just > > > > > referring to a stable fd, independent of whatever is going on in /proc. > > > > > > > > > > What is needed is something like this (in highly pseudo-code form): > > > > > > > > > > pidfd = opendir("/proc/<pid>",..); > > > > > wait_fd = pidfd_wait(pidfd); > > > > > read or poll wait_fd (non-blocking or blocking whichever) > > > > > > > > > > wait_fd will block until the task has either died or reaped. In both these > > > > > cases, it can return a suitable string such as "dead" or "reaped" although an > > > > > integer with some predefined meaning is also Ok. > > > > > > I want to return a siginfo_t: we already use this structure in other > > > contexts to report exit status. > > > > > Fine with me. I did a prototype (code is below) as a string but I can change > that to siginfo_t in the future. > > > > > Having pidfd_wait() return another fd will make the syscall harder to > > > > swallow for a lot of people I reckon. > > > > What exactly prevents us from making the pidfd itself readable/pollable > > > > for the exit staus? They are "special" fds anyway. I would really like > > > > to avoid polluting the api with multiple different types of fds if possible. > > > > > > If pidfds had been their own file type, I'd agree with you. But pidfds > > > are directories, which means that we're beholden to make them behave > > > like directories normally do. I'd rather introduce another FD than > > > heavily overload the semantics of a directory FD in one particular > > > context. In no other circumstances are directory FDs also weird > > > IO-data sources. Our providing a facility to get a new FD to which we > > > *can* give pipe-like behavior does no harm and *usage* cleaner and > > > easier to reason about. > > > > I have two things I'm currently working on: > > - hijacking translate_pid() > > - pidfd_clone() essentially > > > > My first goal is to talk to Eric about taking the translate_pid() > > syscall that has been sitting in his tree and expanding it. > > translate_pid() currently allows you to either get an fd for the pid > > namespace a pid resides in or the pid number of a given process in > > another pid namespace relative to a passed in pid namespace fd. > > That's good to know. More comments below: Sorry for the delay I'm still traveling. I'll be back on a fully functional schedule starting Monday. > > > I would > > like to make it possible for this syscall to also give us back pidfds. > > One question I'm currently struggling with is exactly what you said > > above: what type of file descriptor these are going to give back to us. > > It seems that a regular file instead of directory would make the most > > sense and would lead to a nicer API and I'm very much leaning towards > > that. > > How about something like the following? We can plumb the new file as a pseudo > file that is invisible and linked to the fd. This is extremely rough (does > not do error handling, synchronizatoin etc) but just wanted to share the idea > of what the "frontend" could look like. It is also missing all the actual pid > status messages. It just takes care of the creating new fd from the pidfd > part and providing file read ops returning the "status" string. It is also > written in signal.c and should likely go into proc fs files under fs. > Appreciate any suggestions (a test program did prove it works). > > Also, I was able to translate a pidfd to a pid_namespace by referring to some > existing code but perhaps you may be able to suggest something better for > such translation.. Yeah, there's better ways but I think there's another issue. See below. > > ---8<----------------------- > > From: Joel Fernandes <joelaf@xxxxxxxxxx> > Subject: [PATCH] Partial skeleton prototype of pidfd_wait frontend > > Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx> > --- > arch/x86/entry/syscalls/syscall_32.tbl | 1 + > arch/x86/entry/syscalls/syscall_64.tbl | 1 + > include/linux/syscalls.h | 1 + > include/uapi/asm-generic/unistd.h | 4 +- > kernel/signal.c | 62 ++++++++++++++++++++++++++ > kernel/sys_ni.c | 3 ++ > 6 files changed, 71 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl > index 1f9607ed087c..2a63f1896b63 100644 > --- a/arch/x86/entry/syscalls/syscall_32.tbl > +++ b/arch/x86/entry/syscalls/syscall_32.tbl > @@ -433,3 +433,4 @@ > 425 i386 io_uring_setup sys_io_uring_setup __ia32_sys_io_uring_setup > 426 i386 io_uring_enter sys_io_uring_enter __ia32_sys_io_uring_enter > 427 i386 io_uring_register sys_io_uring_register __ia32_sys_io_uring_register > +428 i386 pidfd_wait sys_pidfd_wait __ia32_sys_pidfd_wait > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl > index 92ee0b4378d4..cf2e08a8053b 100644 > --- a/arch/x86/entry/syscalls/syscall_64.tbl > +++ b/arch/x86/entry/syscalls/syscall_64.tbl > @@ -349,6 +349,7 @@ > 425 common io_uring_setup __x64_sys_io_uring_setup > 426 common io_uring_enter __x64_sys_io_uring_enter > 427 common io_uring_register __x64_sys_io_uring_register > +428 common pidfd_wait __x64_sys_pidfd_wait > > # > # x32-specific system call numbers start at 512 to avoid cache impact > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index e446806a561f..62160970ed3f 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -988,6 +988,7 @@ asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len, > asmlinkage long sys_pidfd_send_signal(int pidfd, int sig, > siginfo_t __user *info, > unsigned int flags); > +asmlinkage long sys_pidfd_wait(int pidfd); > > /* > * Architecture-specific system calls > diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h > index dee7292e1df6..137aa8662230 100644 > --- a/include/uapi/asm-generic/unistd.h > +++ b/include/uapi/asm-generic/unistd.h > @@ -832,9 +832,11 @@ __SYSCALL(__NR_io_uring_setup, sys_io_uring_setup) > __SYSCALL(__NR_io_uring_enter, sys_io_uring_enter) > #define __NR_io_uring_register 427 > __SYSCALL(__NR_io_uring_register, sys_io_uring_register) > +#define __NR_pidfd_wait 428 > +__SYSCALL(__NR_pidfd_wait, sys_pidfd_wait) > > #undef __NR_syscalls > -#define __NR_syscalls 428 > +#define __NR_syscalls 429 > > /* > * 32 bit systems traditionally used different > diff --git a/kernel/signal.c b/kernel/signal.c > index b7953934aa99..ebb550b87044 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -3550,6 +3550,68 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info) > return copy_siginfo_from_user(kinfo, info); > } > > +static ssize_t pidfd_wait_read_iter(struct kiocb *iocb, struct iov_iter *to) > +{ > + /* > + * This is just a test string, it will contain the actual > + * status of the pidfd in the future. > + */ > + char buf[] = "status"; > + > + return copy_to_iter(buf, strlen(buf)+1, to); > +} > + > +static const struct file_operations pidfd_wait_file_ops = { > + .read_iter = pidfd_wait_read_iter, > +}; > + > +static struct inode *pidfd_wait_get_inode(struct super_block *sb) > +{ > + struct inode *inode = new_inode(sb); > + > + inode->i_ino = get_next_ino(); > + inode_init_owner(inode, NULL, S_IFREG); > + > + inode->i_op = &simple_dir_inode_operations; > + inode->i_fop = &pidfd_wait_file_ops; > + > + return inode; > +} > + > +SYSCALL_DEFINE1(pidfd_wait, int, pidfd) > +{ > + struct fd f; > + struct inode *inode; > + struct file *file; > + int new_fd; > + struct pid_namespace *pid_ns; > + struct super_block *sb; > + struct vfsmount *mnt; > + > + f = fdget_raw(pidfd); > + if (!f.file) > + return -EBADF; > + > + sb = file_inode(f.file)->i_sb; > + pid_ns = sb->s_fs_info; > + > + inode = pidfd_wait_get_inode(sb); > + > + mnt = pid_ns->proc_mnt; > + > + file = alloc_file_pseudo(inode, mnt, "pidfd_wait", O_RDONLY, > + &pidfd_wait_file_ops); So I dislike the idea of allocating new inodes from the procfs super block. I would like to avoid pinning the whole pidfd concept exclusively to proc. The idea is that the pidfd API will be useable through procfs via open("/proc/<pid>") because that is what users expect and really wanted to have for a long time. So it makes sense to have this working. But it should really be useable without it. That's why translate_pid() and pidfd_clone() are on the table. What I'm saying is, once the pidfd api is "complete" you should be able to set CONFIG_PROCFS=N - even though that's crazy - and still be able to use pidfds. This is also a point akpm asked about when I did the pidfd_send_signal work. So instead of going throught proc we should probably do what David has been doing in the mount API and come to rely on anone_inode. So something like: fd = anon_inode_getfd("pidfd", &pidfd_fops, file_priv_data, flags); and stash information such as pid namespace etc. in a pidfd struct or something that we then can stash file->private_data of the new file. This also lets us avoid all this open coding done here. Another advantage is that anon_inodes is its own kernel-internal filesystem. Christian > + > + file->f_mode |= FMODE_PREAD; > + > + new_fd = get_unused_fd_flags(0); > + fd_install(new_fd, file); > + > + fdput(f); > + > + return new_fd; > +} > + > /** > * sys_pidfd_send_signal - send a signal to a process through a task file > * descriptor > diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c > index d21f4befaea4..f52c4d864038 100644 > --- a/kernel/sys_ni.c > +++ b/kernel/sys_ni.c > @@ -450,3 +450,6 @@ COND_SYSCALL(setuid16); > > /* restartable sequence */ > COND_SYSCALL(rseq); > + > +/* pidfd */ > +COND_SYSCALL(pidfd_wait); > -- > 2.21.0.225.g810b269d1ac-goog > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
