Re: [PATCH bpf-next v3 03/10] bpf: btf: Introduce helpers for dynamic BTF set registration

On Wed, Sep 15, 2021 at 11:06 AM Kumar Kartikeya Dwivedi
[off-list ref] wrote:

On Wed, Sep 15, 2021 at 09:48:35PM IST, Alexei Starovoitov wrote:

quoted

On Wed, Sep 15, 2021 at 10:39:36AM +0530, Kumar Kartikeya Dwivedi wrote:

quoted

This adds helpers for registering btf_id_set from modules and the
check_kfunc_call callback that can be used to look them up.

With in kernel sets, the way this is supposed to work is, in kernel
callback looks up within the in-kernel kfunc whitelist, and then defers
to the dynamic BTF set lookup if it doesn't find the BTF id. If there is
no in-kernel BTF id set, this callback can be used directly.

Also fix includes for btf.h and bpfptr.h so that they can included in
isolation. This is in preparation for their usage in tcp_bbr, tcp_cubic
and tcp_dctcp modules in the next patch.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpfptr.h |  1 +
 include/linux/btf.h    | 32 ++++++++++++++++++++++++++
 kernel/bpf/btf.c       | 51 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+)

diff --git a/include/linux/bpfptr.h b/include/linux/bpfptr.h
index 546e27fc6d46..46e1757d06a3 100644
--- a/include/linux/bpfptr.h
+++ b/include/linux/bpfptr.h

@@ -3,6 +3,7 @@
 #ifndef _LINUX_BPFPTR_H
 #define _LINUX_BPFPTR_H

+#include <linux/mm.h>

Could you explain what this is for?

When e.g. tcp_bbr.c includes btf.h and btf_ids.h without this, it leads to this
error.

                 from net/ipv4/tcp_bbr.c:59:
./include/linux/bpfptr.h: In function ‘kvmemdup_bpfptr’:
./include/linux/bpfptr.h:67:19: error: implicit declaration of function ‘kvmalloc’;
 did you mean ‘kmalloc’? [-Werror=implicit-function-declaration]
   67 |         void *p = kvmalloc(len, GFP_USER | __GFP_NOWARN);
      |                   ^~~~~~~~
      |                   kmalloc
./include/linux/bpfptr.h:67:19: warning: initialization of ‘void *’ from ‘int’
        makes pointer from integer without a cast [-Wint-conversion]
./include/linux/bpfptr.h:72:17: error: implicit declaration of function ‘kvfree’;
        did you mean ‘kfree’? [-Werror=implicit-function-declaration]
   72 |                 kvfree(p);
      |                 ^~~~~~
      |                 kfree

Interesting.
It's because of kvmalloc in kvmemdup_bpfptr.
Which is used in ___bpf_copy_key.
Which is used in map_update_elem.
And afair all maps enforce key_size < KMALLOC_MAX_SIZE.
Not sure why kvmalloc was there.
If it was kmalloc instead then
#include <linux/slab.h>
in linux/sockptr.h that is included by linux/bpfptr.h
would have been enough.
A food for thought.