[CRIU] [PATCH] ptrace: add ability to retrieve signals without removing them from a queue

[Michael Kerrisk (man-pages)]

On Wed, Feb 13, 2013 at 4:16 PM, Andrey Vagin <avagin at openvz.org> wrote:
> This patch adds a new ptrace request PTRACE_PEEKSIGINFO.
>
> This request is used to retrieve information about a signal with the
> specified sequence number. A siginfo_t structure is copied from the child
> to location data in the parent.
>
> The low 16 bits of addr contains a sequence number of signal in a queue:

I think 16 bits is probably not enough.... Already, on the "out of the
box" system that I have at hand, one can queue more than 2^16-1
signals:

    $ cat /proc/$$/status | grep SigQ
    SigQ:	2/126065

Cheers,

Michael


> All other bits of addr is used for flags. Currently here is only one
> flag PTRACE_PEEK_SHARED for dumping signals from process-wide shared
> queue. If this flag is not set, a signal is read from a per-thread
> queue.  A result siginfo contains a kernel part of si_code which usually
> striped, but it's required for queuing the same siginfo back during
> restore of pending signals.
>
> If a signal with the specified sequence number doesn't exist, ptrace
> returns ENOENT.
>
> This functionality is required for checkpointing pending signals.
>
> The prototype of this code was developed by Oleg Nesterov.
>
> Cc: Roland McGrath <roland at redhat.com>
> Cc: Oleg Nesterov <oleg at redhat.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> 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: "Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com>
> Cc: Pavel Emelyanov <xemul at parallels.com>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  include/uapi/linux/ptrace.h |  9 +++++++
>  kernel/ptrace.c             | 64 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 73 insertions(+)
>
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index 022ab18..5d851d5 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -52,6 +52,15 @@
>  #define PTRACE_INTERRUPT       0x4207
>  #define PTRACE_LISTEN          0x4208
>
> +#define PTRACE_PEEKSIGINFO     0x4209
> +
> +/*
> + * The lower 16 bits of addr is a sequence number of a signal.
> + * All other bits can be used for flags.
> + */
> +#define PTRACE_PEEKSIGINFO_FLAGS_MASK  (~0UL << 16)
> +#define PTRACE_PEEK_SHARED             (1UL << 31)
> +
>  /* Wait extended result codes for the above trace options.  */
>  #define PTRACE_EVENT_FORK      1
>  #define PTRACE_EVENT_VFORK     2
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 1599157..27fd31a 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -579,6 +579,40 @@ static int ptrace_setsiginfo(struct task_struct *child, const siginfo_t *info)
>         return error;
>  }
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +static int ptrace_peek_siginfo(struct task_struct *child,
> +                               unsigned long addr, siginfo_t *siginfo)
> +{
> +       struct sigpending *pending;
> +       struct sigqueue *q;
> +       unsigned long flags;
> +       unsigned int nr;
> +       int ret = -ENOENT;
> +
> +       nr = addr & ~PTRACE_PEEKSIGINFO_FLAGS_MASK;
> +       flags = addr & PTRACE_PEEKSIGINFO_FLAGS_MASK;
> +
> +       if (flags & PTRACE_PEEK_SHARED)
> +               pending = &child->signal->shared_pending;
> +       else
> +               pending = &child->pending;
> +
> +       if (flags & ~PTRACE_PEEK_SHARED)
> +               return -EINVAL; /* unknown flags */
> +
> +       spin_lock_irq(&child->sighand->siglock);
> +       list_for_each_entry(q, &pending->list, list) {
> +               if (!nr--) {
> +                       copy_siginfo(siginfo, &q->info);
> +                       ret = 0;
> +                       break;
> +               }
> +       }
> +       spin_unlock_irq(&child->sighand->siglock);
> +
> +       return ret;
> +}
> +#endif
>
>  #ifdef PTRACE_SINGLESTEP
>  #define is_singlestep(request)         ((request) == PTRACE_SINGLESTEP)
> @@ -703,6 +737,21 @@ int ptrace_request(struct task_struct *child, long request,
>                 ret = put_user(child->ptrace_message, datalp);
>                 break;
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +       case PTRACE_PEEKSIGINFO:
> +       {
> +               siginfo_t __user *uinfo = (siginfo_t __user *) data;
> +
> +               ret = ptrace_peek_siginfo(child, addr, &siginfo);
> +
> +               if (!ret)
> +                       ret = copy_siginfo_to_user(uinfo, &siginfo);
> +               if (!ret)
> +                       ret = __put_user(siginfo.si_code, &uinfo->si_code);
> +               break;
> +       }
> +#endif
> +
>         case PTRACE_GETSIGINFO:
>                 ret = ptrace_getsiginfo(child, &siginfo);
>                 if (!ret)
> @@ -959,6 +1008,21 @@ int compat_ptrace_request(struct task_struct *child, compat_long_t request,
>                 ret = put_user((compat_ulong_t) child->ptrace_message, datap);
>                 break;
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +       case PTRACE_PEEKSIGINFO:
> +       {
> +               compat_siginfo_t __user *uinfo = compat_ptr(data);
> +
> +               ret = ptrace_peek_siginfo(child, addr, &siginfo);
> +
> +               if (!ret)
> +                       ret = copy_siginfo_to_user32(uinfo, &siginfo);
> +               if (!ret)
> +                       ret = __put_user(siginfo.si_code, &uinfo->si_code);
> +               break;
> +       }
> +#endif
> +
>         case PTRACE_GETSIGINFO:
>                 ret = ptrace_getsiginfo(child, &siginfo);
>                 if (!ret)
> --
> 1.7.11.7
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/