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SubjectRe: in_group_p test for another process?
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Some time ago, I asked...

Markku Savela <msa@moth.iki.fi> writes:
> If I have a PID and some GID at hand, how do I find whether the OHTER
> process identified by PID has the GID in its supplementary set of
> groups?
>
> I can find the "in_group_p" test, but that does it only for current process.
>
> Do I have to write my own function that uses the "tasklist_lock",
> finds task by "find_task_by_pid", and then calls
> "groups_search(task->group_info, gid)"?
>
> Can I do this in a module, or do I have to patch the "kernel/sys.c"
> myself for a privately modified version of the kernel?

Well, I've wrote a modification to the kernel/sys.c and it seems to
work for me. As I'm not really a hard core kernel expert, I'm not
quite sure the solution is bullet proof. Thus, here is my added
function that I use in my privately modified kernel for comments, if
any:


--- include/linux/sched-orig.h 2008-05-01 16:53:02.000000000 +0300
+++ include/linux/sched.h 2008-05-30 11:18:35.000000000 +0300
@@ -1582,6 +1582,7 @@

extern int in_group_p(gid_t);
extern int in_egroup_p(gid_t);
+extern int pid_in_egroup_p(gid_t, pid_t);

extern void proc_caches_init(void);
extern void flush_signals(struct task_struct *);
--- kernel/sys-orig.c 2008-02-11 07:51:11.000000000 +0200
+++ kernel/sys.c 2008-05-30 11:17:52.000000000 +0300
@@ -1338,6 +1338,29 @@

EXPORT_SYMBOL(in_egroup_p);

+int pid_in_egroup_p(gid_t grp, pid_t pid)
+{
+ int retval = 1;
+ struct task_struct *p;
+
+ if (pid <= 0)
+ return 0;
+
+ read_lock(&tasklist_lock);
+ p = find_task_by_vpid(pid);
+ if (p) {
+ task_lock(p);
+ if (grp != p->egid)
+ retval = groups_search(p->group_info, grp);
+ task_unlock(p);
+ }
+ else
+ retval = 0;
+ read_unlock(&tasklist_lock);
+ return retval;
+}
+EXPORT_SYMBOL(pid_in_egroup_p);
+
DECLARE_RWSEM(uts_sem);

EXPORT_SYMBOL(uts_sem);

--
Markku Savela


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