Re: [PATCH] proc: show locks in /proc/pid/fdinfo/X

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  5 Mar 2015 18:37:18 +0300 Andrey Vagin <avagin@openvz.org> wrote:

> Let's show locks which are associated with a file descriptor in
> its fdinfo file.
> 
> Currently we don't have a reliable way to determine who holds a lock.
> We can find some information in /proc/locks, but PID which is reported
> there can be wrong. For example, a process takes a lock, then forks a
> child and dies. In this case /proc/locks contains the parent pid, which
> can be reused by another process.
> 
> $ cat /proc/locks
> ...
> 6: FLOCK  ADVISORY  WRITE 324 00:13:13431 0 EOF
> ...
> 
> $ ps -C rpcbind
>   PID TTY          TIME CMD
>   332 ?        00:00:00 rpcbind
> 
> $ cat /proc/332/fdinfo/4
> pos:	0
> flags:	0100000
> mnt_id:	22
> lock:	1: FLOCK  ADVISORY  WRITE 324 00:13:13431 0 EOF
> 
> $ ls -l /proc/332/fd/4
> lr-x------ 1 root root 64 Mar  5 14:43 /proc/332/fd/4 -> /run/rpcbind.lock
> 
> $ ls -l /proc/324/fd/
> total 0
> lrwx------ 1 root root 64 Feb 27 14:50 0 -> /dev/pts/0
> lrwx------ 1 root root 64 Feb 27 14:50 1 -> /dev/pts/0
> lrwx------ 1 root root 64 Feb 27 14:49 2 -> /dev/pts/0
> 
> You can see that the process with the 324 pid doesn't hold the lock.
> 
> This information is required for proper dumping and restoring file
> locks.
> 
> ...
>
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -8,6 +8,7 @@
>  #include <linux/security.h>
>  #include <linux/file.h>
>  #include <linux/seq_file.h>
> +#include <linux/fs.h>
>  
>  #include <linux/proc_fs.h>
>  
> @@ -48,16 +49,24 @@ static int seq_show(struct seq_file *m, void *v)
>  		put_files_struct(files);
>  	}
>  
> -	if (!ret) {
> -		seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\n",
> -			   (long long)file->f_pos, f_flags,
> -			   real_mount(file->f_path.mnt)->mnt_id);
> -		if (file->f_op->show_fdinfo)
> -			file->f_op->show_fdinfo(m, file);
> -		ret = seq_has_overflowed(m);
> -		fput(file);
> -	}
> +	if (ret)
> +		return ret;
> +
> +	seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\n",
> +		   (long long)file->f_pos, f_flags,
> +		   real_mount(file->f_path.mnt)->mnt_id);
> +
> +	show_fd_locks(m, file, files);
> +	ret = seq_has_overflowed(m);
> +	if (ret)
> +		goto out;

seq_has_overflowed() returns a boolean, but fs/seq_file.c:traverse() is
looking for a -ve errno from ->show().

Documentation/filesystems/seq_file.txt says

If all is well, the show() function should return zero.  A negative error
code in the usual manner indicates that something went wrong; it will be
passed back to user space.  This function can also return SEQ_SKIP, which
causes the current item to be skipped; if the show() function has already
generated output before returning SEQ_SKIP, that output will be dropped.


> +	if (file->f_op->show_fdinfo)
> +		file->f_op->show_fdinfo(m, file);
> +	ret = seq_has_overflowed(m);
>  
> +out:
> +	fput(file);
>  	return ret;
>  }