I did a quick prototyping and the above interface seems working fine.

The kernel change:
===============

[yhs@localhost bpf-next]$ git diff

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 97446bbe2ca5..669b7383fddb 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -1976,7 +1976,8 @@ union bpf_attr {
        FN(fib_lookup),                 \
        FN(sock_hash_update),           \
        FN(msg_redirect_hash),          \
-       FN(sk_redirect_hash),
+       FN(sk_redirect_hash),           \
+       FN(get_current_cgroup_id),

 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index ce2cbbff27e4..e11e3298f911 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -493,6 +493,21 @@ static const struct bpf_func_proto

bpf_current_task_under_cgroup_proto = {
        .arg2_type      = ARG_ANYTHING,
 };

+BPF_CALL_0(bpf_get_current_cgroup_id)
+{
+       struct cgroup *cgrp = task_dfl_cgroup(current);
+       if (!cgrp)
+               return -EINVAL;
+
+       return cgrp->kn->id.id;
+}
+
+static const struct bpf_func_proto bpf_get_current_cgroup_id_proto = {
+       .func           = bpf_get_current_cgroup_id,
+       .gpl_only       = false,
+       .ret_type       = RET_INTEGER,
+};
+
 BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
           const void *, unsafe_ptr)
 {

@@ -563,6 +578,8 @@ tracing_func_proto(enum bpf_func_id func_id, const

struct bpf_prog *prog)
                return &bpf_get_prandom_u32_proto;
        case BPF_FUNC_probe_read_str:
                return &bpf_probe_read_str_proto;
+       case BPF_FUNC_get_current_cgroup_id:
+               return &bpf_get_current_cgroup_id_proto;
        default:
                return NULL;
        }

The following program can be used to print out a cgroup id given a cgroup path.
[yhs@localhost cg]$ cat get_cgroup_id.c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char **argv)
{
    int dirfd, err, flags, mount_id, fhsize;
    struct file_handle *fhp;
    char *pathname;

    if (argc != 2) {
        printf("usage: %s <cgroup_path>\n", argv[0]);
        return 1;
    }

    pathname = argv[1];
    dirfd = AT_FDCWD;
    flags = 0;

    fhsize = sizeof(*fhp);
    fhp = malloc(fhsize);
    if (!fhp)
        return 1;

    err = name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags);
    if (err >= 0) {
        printf("error\n");
        return 1;
    }

    fhsize = sizeof(struct file_handle) + fhp->handle_bytes;
    fhp = realloc(fhp, fhsize);
    if (!fhp)
        return 1;

    err = name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags);
    if (err < 0)
        perror("name_to_handle_at");
    else {
        int i;

        printf("dir = %s, mount_id = %d\n", pathname, mount_id);
        printf("handle_bytes = %d, handle_type = %d\n", fhp->handle_bytes,
            fhp->handle_type);
        if (fhp->handle_bytes != 8)
            return 1;

        printf("cgroup_id = 0x%llx\n", *(unsigned long long *)fhp->f_handle);
    }

    return 0;
}
[yhs@localhost cg]$

Given a cgroup path, the user can get cgroup_id and use it in their bpf
program for filtering purpose.

I run a simple program t.c
   int main() { while(1) sleep(1); return 0; }
in the cgroup v2 directory /home/yhs/tmp/yhs
   none on /home/yhs/tmp type cgroup2 (rw,relatime,seclabel)

$ ./get_cgroup_id /home/yhs/tmp/yhs
dir = /home/yhs/tmp/yhs, mount_id = 124
handle_bytes = 8, handle_type = 1
cgroup_id = 0x1000006b2

// the below command to get cgroup_id from the kernel for the
// process compiled with t.c and ran under /home/yhs/tmp/yhs:
$ sudo ./trace.py -p 4067 '__x64_sys_nanosleep "cgid = %llx", $cgid'
PID     TID     COMM            FUNC             -
4067    4067    a.out           __x64_sys_nanosleep cgid = 1000006b2
4067    4067    a.out           __x64_sys_nanosleep cgid = 1000006b2
4067    4067    a.out           __x64_sys_nanosleep cgid = 1000006b2
^C[yhs@localhost tools]$

The kernel and user space cgid matches. Will provide a
formal patch later.




On Mon, May 21, 2018 at 5:24 PM, Y Song [off-list ref] wrote:

On Mon, May 21, 2018 at 9:26 AM, Alexei Starovoitov
[off-list ref] wrote:

quoted

On Sun, May 13, 2018 at 07:33:18PM +0200, Alban Crequy wrote:

quoted

+BPF_CALL_2(bpf_get_current_cgroup_ino, u32, hierarchy, u64, flags)
+{
+     // TODO: pick the correct hierarchy instead of the mem controller
+     struct cgroup *cgrp = task_cgroup(current, memory_cgrp_id);
+
+     if (unlikely(!cgrp))
+             return -EINVAL;
+     if (unlikely(hierarchy))
+             return -EINVAL;
+     if (unlikely(flags))
+             return -EINVAL;
+
+     return cgrp->kn->id.ino;

ino only is not enough to identify cgroup. It needs generation number too.
I don't quite see how hierarchy and flags can be used in the future.
Also why limit it to memcg?

How about something like this instead:

BPF_CALL_2(bpf_get_current_cgroup_id)
{
        struct cgroup *cgrp = task_dfl_cgroup(current);

        return cgrp->kn->id.id;
}
The user space can use fhandle api to get the same 64-bit id.

I think this should work. This will also be useful to bcc as user
space can encode desired id
in the bpf program and compared that id to the current cgroup id, so we can have
cgroup level tracing (esp. stat collection) support. To cope with
cgroup hierarchy, user can use
cgroup-array based approach or explicitly compare against multiple cgroup id's.