On 12/28, Andrey Vagin wrote:
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
I think we should cc Linus.
This patch adds the hack and it makes signalfd even more strange.
Yes, this hack was suggested by me because I can't suggest something
better. But if Linus dislikes this user-visible API it would be better
to get his nack right now.
quoted hunk
+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;
+
+ spin_lock_irq(¤t->sighand->siglock);
+
+ if (*ppos >= SFD_SHARED_QUEUE_OFFSET) {
+ pending = ¤t->signal->shared_pending;
+ seq = *ppos - SFD_SHARED_QUEUE_OFFSET;
+ } else {
+ pending = ¤t->pending;
+ seq = *ppos - SFD_PER_THREAD_QUEUE_OFFSET;
+ }
You can do this outside of spin_lock_irq().
And I think it would be better to check SFD_PRIVATE_QUEUE_OFFSET too
although this is not strictly necessary. Otherwise this code assumes
that sys_pread() cheks pos >= 0 and SFD_PRIVATE_QUEUE_OFFSET == 1.
quoted hunk
+ 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(¤t->sighand->siglock);
+
+ if (ret)
+ (*ppos)++;
We can change it unconditionally but I won't argue.
quoted hunk
@@ -338,6 +379,7 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
}
file->f_flags |= flags & SFD_RAW;
+ file->f_mode |= FMODE_PREAD;
Again, this is not needed or the code was broken by the previous patch.
Given that 2/3 passes O_RDWR to anon_inode_getfile() I think FMODE_PREAD
should be already set. Note OPEN_FMODE(flags) in anon_inode_getfile().
Oleg.