[PATCH v2] eventfd: prepare id to userspace via fdinfo

[Masatake YAMATO]

Finding endpoints of an IPC channel is one of essential task to
understand how a user program works. Procfs and netlink socket provide
enough hints to find endpoints for IPC channels like pipes, unix
sockets, and pseudo terminals. However, there is no simple way to find
endpoints for an eventfd file from userland. An inode number doesn't
hint. Unlike pipe, all eventfd files share the same inode object.

To provide the way to find endpoints of an eventfd file, this patch
adds "eventfd-id" field to /proc/PID/fdinfo of eventfd as identifier.
Integers managed by an IDA are used as ids.

A tool like lsof can utilize the information to print endpoints.

Change in v2:
Use integers as ids instead of memory addresses of eventfd contexts.

Signed-off-by: Masatake YAMATO <yamato@xxxxxxxxxx>
---
 fs/eventfd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 08d3bd602f73..ce8fa15cebe5 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -21,6 +21,9 @@
 #include <linux/eventfd.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
+#include <linux/idr.h>
+
+DEFINE_IDA(eventfd_ida);
 
 struct eventfd_ctx {
 	struct kref kref;
@@ -35,6 +38,7 @@ struct eventfd_ctx {
 	 */
 	__u64 count;
 	unsigned int flags;
+	int id;
 };
 
 /**
@@ -69,6 +73,8 @@ EXPORT_SYMBOL_GPL(eventfd_signal);
 
 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
 {
+	if (ctx->id >= 0)
+		ida_simple_remove(&eventfd_ida, ctx->id);
 	kfree(ctx);
 }
 
@@ -297,6 +303,7 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 	seq_printf(m, "eventfd-count: %16llx\n",
 		   (unsigned long long)ctx->count);
 	spin_unlock_irq(&ctx->wqh.lock);
+	seq_printf(m, "eventfd-id: %d\n", ctx->id);
 }
 #endif
 
@@ -400,6 +407,7 @@ static int do_eventfd(unsigned int count, int flags)
 	init_waitqueue_head(&ctx->wqh);
 	ctx->count = count;
 	ctx->flags = flags;
+	ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
 
 	fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
 			      O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
-- 
2.17.2