[CRIU] [PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v2)

[Michael Kerrisk (man-pages)]

On Jan 30, 2013 8:05 AM, "Andrey Vagin" <avagin at openvz.org> wrote:
>
> If signalfd is created with the flag SFD_PEEK, it reads siginfo-s
> without dequeuing signals.
>
> For reading not first siginfo pread(fd, buf, size, pos) can be used,
> where ppos / sizeof(signalfd_siginfo) is a sequence number of a signal
> in a queue.

Andrey,

Is it perhaps worth erroring (EINVAL) if  ((ppos % sizeof
(signalfd_siginfo)) != 0) ?

Cheers,

Michael

> This functionality is required for checkpointing pending signals.
>
> v2: * signals can be dumped only from one queue.
>     * treat pos as offset in bytes, not in elements, so pos should be
>       aligned to the size of signalfd_siginfo.
>
> Cc: Oleg Nesterov <oleg at redhat.com>
> Cc: Alexander Viro <viro at zeniv.linux.org.uk>
> Cc: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Cc: David Howells <dhowells at redhat.com>
> Cc: Dave Jones <davej at redhat.com>
> Cc: Andrey Vagin <avagin at openvz.org>
> Cc: Michael Kerrisk <mtk.manpages at gmail.com>
> Cc: Pavel Emelyanov <xemul at parallels.com>
> CC: Cyrill Gorcunov <gorcunov at openvz.org>
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  fs/signalfd.c                 | 61
++++++++++++++++++++++++++++++++++++++++---
>  include/uapi/linux/signalfd.h |  2 ++
>  2 files changed, 60 insertions(+), 3 deletions(-)
>
> diff --git a/fs/signalfd.c b/fs/signalfd.c
> index 8019ec9..0da6a30 100644
> --- a/fs/signalfd.c
> +++ b/fs/signalfd.c
> @@ -51,6 +51,47 @@ struct signalfd_ctx {
>         sigset_t sigmask;
>  };
>
> +static int peek_signal(struct sigpending *pending, sigset_t *mask,
> +                               siginfo_t *info, loff_t *pseq)
> +{
> +       struct sigqueue *q;
> +       int ret = 0;
> +
> +       spin_lock_irq(&current->sighand->siglock);
> +
> +       list_for_each_entry(q, &pending->list, list) {
> +               if (sigismember(mask, q->info.si_signo))
> +                       continue;
> +
> +               if ((*pseq)-- == 0) {
> +                       copy_siginfo(info, &q->info);
> +                       ret = info->si_signo;
> +                       break;
> +               }
> +       }
> +
> +       spin_unlock_irq(&current->sighand->siglock);
> +
> +       return ret;
> +}
> +
> +static ssize_t signalfd_peek(struct signalfd_ctx *ctx,
> +                               siginfo_t *info, loff_t *ppos, int
queue_mask)
> +{
> +       loff_t seq = *ppos / sizeof(struct signalfd_siginfo);
> +       int signr = 0;
> +
> +       if (queue_mask & SIGQUEUE_PRIVATE)
> +               signr = peek_signal(&current->pending,
> +                                       &ctx->sigmask, info, &seq);
> +       else if (queue_mask & SIGQUEUE_SHARED)
> +               signr = peek_signal(&current->signal->shared_pending,
> +                                        &ctx->sigmask, info, &seq);
> +       (*ppos) += sizeof(struct signalfd_siginfo);
> +
> +       return signr;
> +}
> +
>  static int signalfd_release(struct inode *inode, struct file *file)
>  {
>         kfree(file->private_data);
> @@ -257,9 +298,15 @@ static ssize_t signalfd_read(struct file *file, char
__user *buf, size_t count,
>         if (!count)
>                 return -EINVAL;
>
> +       if (*ppos % sizeof(struct signalfd_siginfo))
> +               return -EINVAL;
> +
>         siginfo = (struct signalfd_siginfo __user *) buf;
>         do {
> -               ret = signalfd_dequeue(ctx, &info, nonblock, qmask);
> +               if (file->f_flags & SFD_PEEK)
> +                       ret = signalfd_peek(ctx, &info, ppos, qmask);
> +               else
> +                       ret = signalfd_dequeue(ctx, &info, nonblock,
qmask);
>
>                 if (unlikely(ret <= 0))
>                         break;
> @@ -315,7 +362,12 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user
*, user_mask,
>         BUILD_BUG_ON(SFD_CLOEXEC != O_CLOEXEC);
>         BUILD_BUG_ON(SFD_NONBLOCK != O_NONBLOCK);
>
> -       if (flags & ~(SFD_CLOEXEC | SFD_NONBLOCK | SFD_RAW | SFD_QUEUES))
> +       if (flags & ~(SFD_CLOEXEC | SFD_NONBLOCK |
> +                       SFD_RAW | SFD_PEEK | SFD_QUEUES))
> +               return -EINVAL;
> +
> +       /* SFD_PEEK can be used for one queue only */
> +       if ((flags & SFD_PEEK) && ((flags & SFD_QUEUES) == SFD_QUEUES))
>                 return -EINVAL;
>
>         if (sizemask != sizeof(sigset_t) ||
> @@ -352,7 +404,10 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user
*, user_mask,
>                 }
>
>                 file->f_flags |= (flags & SFD_QUEUES) ? : SFD_QUEUES;
> -               file->f_flags |= flags & SFD_RAW;
> +               file->f_flags |= flags & (SFD_RAW | SFD_PEEK);
> +
> +               if (file->f_flags & SFD_PEEK)
> +                       file->f_mode |= FMODE_PREAD;
>
>                 fd_install(ufd, file);
>         } else {
> diff --git a/include/uapi/linux/signalfd.h b/include/uapi/linux/signalfd.h
> index 481b658..24c5d2d 100644
> --- a/include/uapi/linux/signalfd.h
> +++ b/include/uapi/linux/signalfd.h
> @@ -20,6 +20,8 @@
>  #define SFD_SHARED_QUEUE O_DIRECTORY
>  /* Read signals from a per-thread queue */
>  #define SFD_PER_THREAD_QUEUE O_EXCL
> +/* Read signals without removing them from a queue */
> +#define SFD_PEEK O_APPEND
>
>  struct signalfd_siginfo {
>         __u32 ssi_signo;
> --
> 1.7.11.7
>
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