Re: Is zero a valid value for the pid member of the AUDIT_SIGNAL_INFO message?
On Tuesday, March 11, 2014 11:32:01 PM Richard Guy Briggs wrote:
On 14/03/11, Eric Paris wrote:
> On Tue, 2014-03-11 at 18:15 -0400, Richard Guy Briggs wrote:
> > Is zero a valid value for the pid member of the AUDIT_SIGNAL_INFO
> > message?
>
> No...
>
> Given that userspace requests AUDIT_SIGNAL_INFO after it gets a signal,
> and that audit_sig_{uid,pid,...} get filled in when some task sent
> auditd that signal, the idea that the pid would be 0 doesn't make
> sense... (unless auditd requests AUDIT_SIGNAL_INFO without getting a
> signal, but that's just dumb)
The reason I ask is that it is initialized to -1, which I assume is no
more valid than zero in your interpretation above.
pid=-1 has a special meaning for signals. But in terms of seeing it in a
sigaction handler for siginfo, not possible. So its a good init value. If you
look at sigaction(2), there is a si_code that indicates why the signal was
sent. One of them is SI_KERNEL. So, its possible that the kernel decides to
send a signal on certain occasions.
I looked at converting audit_sig_pid from pid_t to struct pid *, but
then get_pid() would also be needed to protect that reference. A
put_pid() would need to be done once it is no longer needed, which could
be immediately after it is read in the AUDIT_SIGNAL_INFO message
preparation, assuming it would never need to be read again. If this
isn't the case, put_pid() could be called when audit_pid is nulled, but
if that message never comes, that struct pid is stuck with a stale
refcount. (That isn't an issue if it is init or systemd, but it is
still wrong.)
This looks more and more like overkill and should probably leave
audit_sig_pid as pid_t.
The code has been working good for a long time. I am wondering if the original
intent was to make it general in case we decided to add more signals that we
are interested in.
-Steve