[PATCH RESEND] 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 fdinfo of eventfd as identifier. Address
for eventfd context is used as id. Typical applicaiton utilizing
the information is lsof.

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

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 08d3bd602f73..fc63ad43d962 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -297,6 +297,7 @@ 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: %p\n", ctx);
 }
 #endif
 
-- 
2.17.2