[PATCH v2] cap_syslog: don't use WARN_ONCE for CAP_SYS_ADMINdeprecation warning

From: Jonathan Nieder
Date: Mon Aug 08 2011 - 00:23:05 EST

syslog-ng versions before 3.3.0beta1 (2011-05-12) assume that
CAP_SYS_ADMIN is sufficient to access syslog, so ever since CAP_SYSLOG
was introduced (2010-11-25) they have triggered a warning.
v2.6.38-rc5~46 (cap_syslog: accept CAP_SYS_ADMIN for now, 2011-02-10)
improved matters a little by making syslog-ng work again, just keeping
the WARN_ONCE(). But still, this is a warning that writes a stack
trace we don't care about to syslog, sets a taint flag, and alarms
sysadmins when nothing worse has happened than use of an old userspace
with a recent kernel.

Convert the WARN_ONCE to a printk_once to avoid that while continuing
to give userspace developers a hint that this is an unwanted
backward-compatibility feature and won't be around forever.

Reported-by: Ralf Hildebrandt <ralf.hildebrandt@xxxxxxxxxx>
Reported-by: Niels <zorglub_olsen@xxxxxxxxxxx>
Reported-by: PaweÅ Sikora <pluto@xxxxxxxx>
Signed-off-by: Jonathan Nieder <jrnieder@xxxxxxxxx>
Liked-by: Gergely Nagy <algernon@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
Acked-by: James Morris <jmorris@xxxxxxxxx>
Linus Torvalds wrote:

> The problem with this one is that converting away from a warning
> doesn't just get rid of all the ugliness, it gets rid of some of the
> *useful* information in the warning too.
> In particular, when you have a warning like "Attempting to access
> syslog", you'd better tell people *who* is attempting to access
> syslog, so that they can fix it or complain to the right person.

True. I can't imagine people wanting to know much more than the
process's command line and pid, which is basically what the WARN_ONCE
gives (though it also gives registers). Here's an updated patch.

I've carried over the acks from last time; hopefully that's okay.
Thanks for your help.

kernel/printk.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 37dff342..836a2ae0 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -318,8 +318,10 @@ static int check_syslog_permissions(int type, bool from_file)
return 0;
/* For historical reasons, accept CAP_SYS_ADMIN too, with a warning */
if (capable(CAP_SYS_ADMIN)) {
- WARN_ONCE(1, "Attempt to access syslog with CAP_SYS_ADMIN "
- "but no CAP_SYSLOG (deprecated).\n");
+ printk_once(KERN_WARNING "%s (%d): "
+ "Attempt to access syslog with CAP_SYS_ADMIN "
+ "but no CAP_SYSLOG (deprecated).\n",
+ current->comm, task_pid_nr(current));
return 0;
return -EPERM;

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