proc: Add complete process group list

If a process is in more than NGROUPS_SMALL (32) groups, it's not possible
for any other user space process to determine the list of groups it is in using /proc/<pid>/status.

Increasing the list of groups listed by /proc/<pid>/status would lead to
very long lines that file, and possible misbehavior of userspace programs that parse /proc/<pid>/status, so instead I have opted to create a new file /proc/<pid>/groups, which contains the list of supplementary groups for each pid.

The new file /proc/<pid>/groups consists of a single group id per line,
with each line being 11 characters long. This should be enough space for 16bit or 32bit group ids.

This feature might be useful for a server listening on a unix domain pipe
to determine the list of groups that a client process is in from its pid.

---
 fs/proc/base.c |   55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 55 insertions(+), 0 deletions(-)

index acb7ef8..689362c 100644
@@ -82,6 +82,8 @@
 #include <linux/pid_namespace.h>
 #include <linux/fs_struct.h>
 #include <linux/slab.h>
+#include <linux/cred.h>
+
 #include "internal.h"
 
 /* NOTE:

@@ -1325,6 +1327,57 @@ static const struct file_operations proc_pid_set_comm_operations = {
 	.release	= single_release,
 };
 
+/* supplementary groups, one per line */
+static int groups_proc_show(struct seq_file *m, void *v)
+{
+	struct inode *inode = m->private;
+	struct group_info *group_info;
+	struct task_struct *task;
+	const struct cred *cred;
+	struct pid *pid;
+	unsigned int g;
+
+	pid = proc_pid(inode);
+	task = get_pid_task(pid, PIDTYPE_PID);
+	if (!task)
+		return -ESRCH;
+
+	cred = get_task_cred(task);
+	group_info = cred->group_info;
+
+	/*
+	 * Groups may be 16bit or 32bit.
+	 * Try to be easily machine and human readable.
+	 */
+	for (g = 0; g < group_info->ngroups; g++)
+		seq_printf(m, "%-10u\n", GROUP_AT(group_info, g));
+
+	put_cred(cred);
+	put_task_struct(task);
+
+	return 0;
+}
+
+static int groups_proc_open(struct inode *inode, struct file *filp)
+{
+	int ret;
+
+	ret = single_open(filp, groups_proc_show, NULL);
+	if (!ret) {
+		struct seq_file *m = filp->private_data;
+
+		m->private = inode;
+	}
+	return ret;
+}
+
+static const struct file_operations proc_groups_operations = {
+	.open		= groups_proc_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 /*
  * We added or removed a vma mapping the executable. The vmas are only mapped
  * during exec and are not mapped with the mmap system call.

@@ -2586,6 +2639,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 	INF("cmdline",    S_IRUGO, proc_pid_cmdline),
 	ONE("stat",       S_IRUGO, proc_tgid_stat),
 	ONE("statm",      S_IRUGO, proc_pid_statm),
+	REG("groups",     S_IRUSR, proc_groups_operations),
 	REG("maps",       S_IRUGO, proc_maps_operations),
 #ifdef CONFIG_NUMA
 	REG("numa_maps",  S_IRUGO, proc_numa_maps_operations),

@@ -2921,6 +2975,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	INF("cmdline",   S_IRUGO, proc_pid_cmdline),
 	ONE("stat",      S_IRUGO, proc_tid_stat),
 	ONE("statm",     S_IRUGO, proc_pid_statm),
+	REG("groups",    S_IRUSR, proc_groups_operations),
 	REG("maps",      S_IRUGO, proc_maps_operations),
 #ifdef CONFIG_NUMA
 	REG("numa_maps", S_IRUGO, proc_numa_maps_operations),

1.5.6.5

As usual, I can never comment whether we need this or not. Just a minor nit about the code,

	group_info = NULL;

	rcu_read_lock();
	task = pid_task(proc_pid(inode), PIDTYPE_PID);
	if (task) {
		group_info = _task_cred(task)->group_info;
		get_group_info(group_info);
	}
	rcu_read_unlock();

	if (!group_info)
		retrn ESRCH;

"might be"?

It would be useful to hear a bit more about usage scenarios, why this is needed, etc - some hard info which would justify permanent extension of the kernel->userspace API. How does this get used, why is it needed, what are the alternatives, etc.

I don't recall having heard of anyone using the groups fields in /proc/pid/status before.

Well, "would be" :-)

This will be used in a device with groups permissions checked in userspace.

Say you have a process called "telephony-server", and it talks to a number of client processes with different privilege levels via a unix domain socket.

telephony-server might be able do things which should have different privilege levels, like send SMS messages, make phone calls, download firmware to a 3G modem, etc. The client processes would be members of groups reflecting each privilege. Depending on the number of similar servers in the system, and how fine-grained the privileges are, there might be lots of groups (>32).

telephony-server should be able to allow or deny requests depending on whether an application is a member of the correct group or not.

unix sockets can pass credentials, but currently I can only see struct ucred (pid, uid and gid) being passed. Using the pid, /proc/pid/status can be read for a list of groups, but it only lists up to 32 groups.

Ways I can see to get the groups for a unix socket peer from it's pid all mostly require some kernel modification:

* modify kernel to list all groups in /proc/<pid>/status
- very long lines become possible in status file
- no way to know whether you're using an old kernel with 32 group limit or new kernel and pid only has 32 groups

* modify kernel to add /proc/<pid>/groups
- more kernel-userland interface

* implement LOCAL_CREDS for unix domain sockets in Linux
- work

* limit number of groups to 32
- limit is imposed by /proc code

* create multiple unix domian sockets per privilege with group r/w only
- seems like trouble

What do you think?

thanks,

Mike

Is this necessary? Why?
Who need 32 groups limitation?

My personal opinion (aka my personal prefer) is,
- If fixing /proc/<pid>/status is zero downside, it should do.
- If fixing /proc/</pid>status is some downside (e.g. performance down), /proc/<pid>groups is better

because, 99% user don't use >32groups.

And, personally I dislike following three ;)

Just dumb question.

Why don't you fix /proc/<pid>/status? Can we share your worry? I haven't review your patch carefully yet. but your groups_proc_show() seems don't have heavy weight lock.

note: I'm not against your plan. it's just curiosity.