Re: O_NOATIME and files in /proc

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

On Tue, 17 Nov 2009 18:06:29 +0100
Bernd Petrovitsch <bernd@firmix.at> wrote:

> Hi all!
> 
> Is there a specific reason that open can not open files (at
> least /proc/noatime and /proc/cpuinfo) under /proc with NO_ATIME as the
> following program shows:
> ----  snip  ----
> {12}cat noatime.c
> 
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <string.h>
> #include <errno.h>
> #include <fcntl.h>
> 
> int main(void)
> {
> 	int fd = open("/proc/uptime", O_RDONLY|O_NOATIME);
> 	if (fd == -1) {
> 		printf("fd=%d, errno=%s\n", fd, strerror(errno));
> 	} else {
> 		printf("fd=%d\n", fd);
> 	}
> 	return 0;
> }
> ----  snip  ----
> When I compile and run it, it prints
> ----  snip  ----
> {13}./noatime 
> fd=-1, errno=Operation not permitted
> ----  snip  ----
> Removing the "NO_ATIME" makes it work (of course).
> 
> I can also set the "noatime" mount flag on a remount and it shows up
> in /proc/mounts but it makes for the above no difference.
> 

I guess you're hitting the check in may_open():

	/* O_NOATIME can only be set by the owner or superuser */
	if (flag & O_NOATIME)
		if (!is_owner_or_cap(inode)) {
			error = -EPERM;
			goto err_out;
		}

This code was added in 2004 and neither the changelog nor the code
comment explain _why_ this was done (bad).  It might be recorded in the
contemporary email discussion.

I assume it was done this way under the assumption that people might
want to use atime to determine if other users have been peeking at
their junk.  Avoid permitting junk-peekers to conceal their tracks.