[PATCH 3/3] signalfd: add ability to read siginfo-s without dequeuing signals (v4)

[Andrey Vagin]

pread(fd, buf, size, pos) with non-zero pos returns siginfo-s
without dequeuing signals.

A sequence number and a queue are encoded in pos.

pos = seq + SFD_*_OFFSET

seq is a sequence number of a signal in a queue.

SFD_PER_THREAD_QUEUE_OFFSET - read signals from a per-thread queue.
SFD_SHARED_QUEUE_OFFSET - read signals from a shared (process wide) queue.

This functionality is required for checkpointing pending signals.

v2: llseek() can't be used here, because peek_offset/f_pos/whatever
has to be shared with all processes which have this file opened.

Suppose that the task forks after sys_signalfd(). Now if parent or child
do llseek this affects them both. This is insane because signalfd is
"strange" to say at least, fork/dup/etc inherits signalfd_ctx but not
the" source" of the data. // Oleg Nesterov

v3,v4: minor cleanups

Cc: Oleg Nesterov <o...@redhat.com>
Cc: Alexander Viro <v...@zeniv.linux.org.uk>
Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
Cc: David Howells <dhowe...@redhat.com>
Cc: Dave Jones <da...@redhat.com>
Cc: Andrey Vagin <ava...@openvz.org>
Cc: Michael Kerrisk <mtk.manpa...@gmail.com>
Cc: Pavel Emelyanov <xe...@parallels.com>
CC: Cyrill Gorcunov <gorcu...@openvz.org>
Signed-off-by: Andrey Vagin <ava...@openvz.org>
---
 fs/signalfd.c                 | 45 ++++++++++++++++++++++++++++++++++++++++++-
 include/uapi/linux/signalfd.h |  5 +++++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/fs/signalfd.c b/fs/signalfd.c
index 4439a81..1eb9b87 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -51,6 +51,44 @@ struct signalfd_ctx {
        sigset_t sigmask;
 };
 
+static ssize_t signalfd_peek(struct signalfd_ctx *ctx,
+                               siginfo_t *info, loff_t *ppos)
+{
+       struct sigpending *pending;
+       struct sigqueue *q;
+       loff_t seq;
+       int ret = 0;
+
+       if (*ppos >= SFD_SHARED_QUEUE_OFFSET) {
+               pending = &current->signal->shared_pending;
+               seq = *ppos - SFD_SHARED_QUEUE_OFFSET;
+       } else if (*ppos >= SFD_PER_THREAD_QUEUE_OFFSET) {
+               pending = &current->pending;
+               seq = *ppos - SFD_PER_THREAD_QUEUE_OFFSET;
+       } else
+               return -EINVAL;
+
+       spin_lock_irq(&current->sighand->siglock);
+
+       list_for_each_entry(q, &pending->list, list) {
+               if (sigismember(&ctx->sigmask, q->info.si_signo))
+                       continue;
+
+               if (seq-- == 0) {
+                       copy_siginfo(info, &q->info);
+                       ret = info->si_signo;
+                       break;
+               }
+       }
+
+       spin_unlock_irq(&current->sighand->siglock);
+
+       if (ret)
+               (*ppos)++;
+
+       return ret;
+}
+
 static int signalfd_release(struct inode *inode, struct file *file)
 {
        kfree(file->private_data);
@@ -248,7 +286,11 @@ static ssize_t signalfd_read(struct file *file, char 
__user *buf, size_t count,
 
        siginfo = (struct signalfd_siginfo __user *) buf;
        do {
-               ret = signalfd_dequeue(ctx, &info, nonblock);
+               if (*ppos == 0)
+                       ret = signalfd_dequeue(ctx, &info, nonblock);
+               else
+                       ret = signalfd_peek(ctx, &info, ppos);
+
                if (unlikely(ret <= 0))
                        break;
 
@@ -338,6 +380,7 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, 
user_mask,
                }
 
                file->f_flags |= flags & SFD_RAW;
+               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 bc31849..0953785 100644
--- a/include/uapi/linux/signalfd.h
+++ b/include/uapi/linux/signalfd.h
@@ -17,6 +17,11 @@
 #define SFD_NONBLOCK O_NONBLOCK
 #define SFD_RAW O_DIRECT
 
+/* Read signals from a shared (process wide) queue */
+#define SFD_SHARED_QUEUE_OFFSET (1LL << 62)
+/* Read signals from a per-thread queue */
+#define SFD_PER_THREAD_QUEUE_OFFSET 1
+
 struct signalfd_siginfo {
        __u32 ssi_signo;
        __s32 ssi_errno;
-- 
1.7.11.7

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