linux-kernel - Re: [PATCH resend] eventfd: make eventfd files distinguishable in /proc/$PID/fd

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Date:   Tue, 11 Dec 2018 17:09:14 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Masatake YAMATO <yamato@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] eventfd: make eventfd files distinguishable in
 /proc/$PID/fd

On Mon, Dec 10, 2018 at 03:35:46AM +0900, Masatake YAMATO wrote:
> 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 shares one inode object.
> 
> To provide the way to find endpoints of an eventfd file, this patch
> adds eventfd identifiers to the output of 'ls -l /proc/$pid/fd' like:
> 
>   ...
>   lrwx------. 1 qemu qemu 64 May 20 04:49 93 -> 'anon_inode:[eventfd:130]'
>   lrwx------. 1 qemu qemu 64 May 20 04:49 94 -> 'anon_inode:[eventfd:131]'
>   ...
> 
> Here "130" and "131" are added as identifiers newly added.
> In the case that ida_simple_get returns an error, this change doesn't add
> an identifier; just use "[eventfd]" as before.
> 
> Signed-off-by: Masatake YAMATO <yamato@...hat.com>

I'm going to love this when I need it :)  Thanks.

Acked-by: Serge Hallyn <serge@...lyn.com>

> ---
>  fs/eventfd.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 08d3bd602f73..c18952948110 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -21,6 +21,11 @@
>  #include <linux/eventfd.h>
>  #include <linux/proc_fs.h>
>  #include <linux/seq_file.h>
> +#include <linux/idr.h>
> +
> +/* Worst case buffer size needed for holding an integer. */
> +#define ITOA_MAX_LEN 12
> +DEFINE_IDA(eventfd_ida);
>  
>  struct eventfd_ctx {
>  	struct kref kref;
> @@ -35,6 +40,7 @@ struct eventfd_ctx {
>  	 */
>  	__u64 count;
>  	unsigned int flags;
> +	int id;
>  };
>  
>  /**
> @@ -69,6 +75,8 @@ 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);
>  }
>  
> @@ -384,6 +392,7 @@ static int do_eventfd(unsigned int count, int flags)
>  {
>  	struct eventfd_ctx *ctx;
>  	int fd;
> +	char name[1 + 8 + ITOA_MAX_LEN + 1 + 1] = "[eventfd]";
>  
>  	/* Check the EFD_* constants for consistency.  */
>  	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
> @@ -400,8 +409,11 @@ 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,
> +	if (ctx->id >= 0)
> +		snprintf(name, sizeof(name), "[eventfd:%d]", ctx->id);
> +	fd = anon_inode_getfd(name, &eventfd_fops, ctx,
>  			      O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
>  	if (fd < 0)
>  		eventfd_free_ctx(ctx);
> -- 
> 2.17.0