[PATCH 2/2] signalfd: add ability to get signal without removing from the queue

[Andrey Vagin]

lseek sets a sequence number of signal in a queue, then read() returns
siginfo if a signal is exists, otherwise it returns 0. All signals
remain in a queue.

If lseek sets a positive position, signals are taken from a shared queue.
If lseek sets a negative position, signals are taken from a private queue.
If ppos is zero (default), signalfd dequeues signals.

Signed-off-by: Andrey Vagin <avagin at openvz.org>
---
 fs/signalfd.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/fs/signalfd.c b/fs/signalfd.c
index ee60d5f..0ce6fb3 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -48,8 +48,21 @@ void signalfd_cleanup(struct sighand_struct *sighand)
 
 struct signalfd_ctx {
 	sigset_t sigmask;
+	loff_t peek_offset;
 };
 
+static ssize_t signalfd_peek(struct signalfd_ctx *ctx, siginfo_t *info)
+{
+       int ret;
+
+       spin_lock_irq(&current->sighand->siglock);
+       ret = peek_signal(current, &ctx->sigmask, info,
+			abs(ctx->peek_offset) - 1, ctx->peek_offset > 0);
+       spin_unlock_irq(&current->sighand->siglock);
+
+       return ret;
+}
+
 static int signalfd_release(struct inode *inode, struct file *file)
 {
 	kfree(file->private_data);
@@ -230,7 +243,11 @@ 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 (ctx->peek_offset == 0)
+			ret = signalfd_dequeue(ctx, &info, nonblock);
+		else
+			ret = signalfd_peek(ctx, &info);
+
 		if (unlikely(ret <= 0))
 			break;
 
@@ -242,6 +259,13 @@ static ssize_t signalfd_read(struct file *file, char __user *buf, size_t count,
 		if (ret < 0)
 			break;
 
+		if (ctx->peek_offset) {
+			if (ctx->peek_offset > 0)
+				ctx->peek_offset++;
+			else
+				ctx->peek_offset--;
+		}
+
 		siginfo++;
 		total += ret;
 		nonblock = 1;
@@ -264,6 +288,24 @@ static int signalfd_show_fdinfo(struct seq_file *m, struct file *f)
 }
 #endif
 
+loff_t signalfd_llseek(struct file *f, loff_t offset, int whence)
+{
+	struct signalfd_ctx *ctx = f->private_data;
+
+	switch (whence) {
+	case SEEK_SET:
+		ctx->peek_offset = offset;
+		break;
+	case SEEK_CUR:
+		ctx->peek_offset += offset;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return ctx->peek_offset;
+}
+
 static const struct file_operations signalfd_fops = {
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= signalfd_show_fdinfo,
@@ -271,7 +313,7 @@ static const struct file_operations signalfd_fops = {
 	.release	= signalfd_release,
 	.poll		= signalfd_poll,
 	.read		= signalfd_read,
-	.llseek		= noop_llseek,
+	.llseek		= signalfd_llseek,
 };
 
 SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
@@ -300,6 +342,7 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
 			return -ENOMEM;
 
 		ctx->sigmask = sigmask;
+		ctx->peek_offset = 0;
 
 		ufd = get_unused_fd_flags(flags);
 		if (ufd < 0) {
@@ -321,6 +364,7 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
 		}
 
 		file->f_flags |= flags & SFD_RAW;
+		file->f_mode |= FMODE_LSEEK;
 
 		fd_install(ufd, file);
 	} else {
-- 
1.7.11.7


--M9NhX3UHpAaciwkO--