rlimit: permit setting RLIMIT_NOFILE to RLIM_INFINITY

When a process wants to set the limit of open files to RLIM_INFINITY it
gets EPERM even if it has CAP_SYS_RESOURCE capability.

For example, BIND does:

...
#elif defined(NR_OPEN) && defined(__linux__)
/*
* Some Linux kernels don't accept RLIM_INFINIT; the maximum
* possible value is the NR_OPEN defined in linux/fs.h.
*/
if (resource == isc_resource_openfiles && rlim_value == RLIM_INFINITY) {
rl.rlim_cur = rl.rlim_max = NR_OPEN;
unixresult = setrlimit(unixresource, &rl);
if (unixresult == 0)
return (ISC_R_SUCCESS);
}
#elif ...

If we allow setting RLIMIT_NOFILE to RLIM_INFINITY we increase portability
- you don't have to check if OS is linux and then use different schema for
limits.

The spec says "Specifying RLIM_INFINITY as any resource limit value on a
successful call to setrlimit() shall inhibit enforcement of that resource
limit." and we're presently not doing that.

Cc: Michael Kerrisk <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/sys.c

···
 		return -EINVAL;
 	if (copy_from_user(&new_rlim, rlim, sizeof(*rlim)))
 		return -EFAULT;
-	if (new_rlim.rlim_cur > new_rlim.rlim_max)
-		return -EINVAL;
 	old_rlim = current->signal->rlim + resource;
 	if ((new_rlim.rlim_max > old_rlim->rlim_max) &&
 	    !capable(CAP_SYS_RESOURCE))
 		return -EPERM;
-	if (resource == RLIMIT_NOFILE && new_rlim.rlim_max > sysctl_nr_open)
-		return -EPERM;
+
+	if (resource == RLIMIT_NOFILE) {
+		if (new_rlim.rlim_max == RLIM_INFINITY)
+			new_rlim.rlim_max = sysctl_nr_open;
+		if (new_rlim.rlim_cur == RLIM_INFINITY)
+			new_rlim.rlim_cur = sysctl_nr_open;
+		if (new_rlim.rlim_max > sysctl_nr_open)
+			return -EPERM;
+	}
+
+	if (new_rlim.rlim_cur > new_rlim.rlim_max)
+		return -EINVAL;
 
 	retval = security_task_setrlimit(resource, &new_rlim);
 	if (retval)