[PATCH 1/2] coredump: use from_kuid/kgid_munged when formatting corename

Nicolas Iooss nicolas.iooss_linux at m4x.org
Wed May 6 12:18:12 UTC 2015

On 05/06/2015 03:13 AM, Eric W. Biederman wrote:
> Nicolas Iooss <nicolas.iooss_linux at m4x.org> writes:
>> When adding __printf attribute to cn_printf, gcc reports some issues:
>>   fs/coredump.c:213:5: warning: format '%d' expects argument of type
>>   'int', but argument 3 has type 'kuid_t' [-Wformat=]
>>        err = cn_printf(cn, "%d", cred->uid);
>>        ^
>>   fs/coredump.c:217:5: warning: format '%d' expects argument of type
>>   'int', but argument 3 has type 'kgid_t' [-Wformat=]
>>        err = cn_printf(cn, "%d", cred->gid);
>>        ^
>> These warnings come from the fact that the value of uid/gid needs to be
>> extracted from the kuid_t/kgid_t structure before being used as an
>> integer.  More precisely, cred->uid and cred->gid need to be converted
>> to either user-namespace uid/gid or to init_user_ns uid/gid.
>> As Documentation/sysctl/kernel.txt does not specify which user namespace
>> is used to translate %u and %g in core_pattern, but lowercase %p and %i
>> are used to format pid/tid in the current process namespace, it seems
>> intuitive that lowercase %u and %g use the current user namespace.
> I love the logic of lower vs upper case letters in the selection of how
> an identifier should be used.  Unfortunately I can't support it.
> Converting to anything other than init_user_ns will actually be an ABI
> break.  Which in practice should trump everything else.

I agree.  Initially I thought that my patch introduced only a minor
change to make the ABI more consistent, but if you think this change is
actually large enough to be considered a "not-wanted ABI break", I will
follow your decision.

> Further only the global root user can set this value, which largely
> implies that the program setting the core dump patter will expect the
> values to be in the initial user namespace.

Ok, I missed this.  The main source of my confusion is that the coredump
uses the current mount namespace to create files (I tested using a
Docker container with core_pattern set to something in /tmp) and the
global process/mount namespaces when using pipes (tested with

> In practice your patch allows any user to put any uid they want on core
> files which seems to make the uid parameter useless.
> So for all of the reasons above I think we need to print uids and gids
> in the initial user namespace unless explicitly requested to do so.

So be it.  I'll update my patches and send them again.  In order to make
things clearer, I also would like to modify the documentation of %u and
%g, with something like:

--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -197,8 +197,8 @@
 	%P	global pid (init PID namespace)
 	%i	tid
 	%I	global tid (init PID namespace)
-	%u	uid
-	%g	gid
+	%u	uid (in init user namespace)
+	%g	gid (in init user namespace)
 	%d	dump mode, matches PR_SET_DUMPABLE and
 	%s	signal number

Would such a change be accepted, and if yes is it better to put it in
its own commit or in the same as the one modifying the code?



More information about the Containers mailing list