Re: procfs does not return error code when file name is already in use

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Oct 26, 2012 at 01:57:43PM +0400, ??????? ???????? wrote:
> I'm working a kernel module, and one of my tasks is to disallow a
> process to open my character device(/dev/xxx) more than once. I tried
> to solve this problem by creating /proc/apu directory, and creating a
> /proc/xxx/{pid} file for when process opens /dev/xxx. This file will
> act as per-process mutex, and provide a way to control resource usage.

Whoever had set that "task" upon you deserves to be painfully educated
until they realize that thus stated the limitation is absolutely worthless -
file does not have to be opened by the same process that uses it.  IOW,
limiting opens to one per process does not provide any kind of resource
control - it's trivially bypassed by creating an AF_UNIX socketpair,
then forking children in a loop, each doing open + send SCM_RIGHTS
datagram to parent over that socket with opened fd in it + exit.  And
receiving those datagrams until you get as many opened descriptors for
your device as you wanted.