On 7/18/22 7:36 AM, Benjamin Tissoires wrote:

On Sat, Jul 16, 2022 at 6:29 AM Yonghong Song [off-list ref] wrote:

quoted

On 7/12/22 7:58 AM, Benjamin Tissoires wrote:

quoted

When a kfunc is not returning a pointer to a struct but to a plain type,
we can consider it is a valid allocated memory assuming that:
- one of the arguments is either called rdonly_buf_size or
    rdwr_buf_size
- and this argument is a const from the caller point of view

We can then use this parameter as the size of the allocated memory.

The memory is either read-only or read-write based on the name
of the size parameter.

If I understand correctly, this permits a kfunc like
     int *kfunc(..., int rdonly_buf_size);
     ...
     int *p = kfunc(..., 20);
so the 'p' points to a memory buffer with size 20.

Yes, exactly.

quoted

This looks like a strange interface although probably there
is a valid reason for this as I didn't participated in
earlier discussions.

Well, the point is I need to be able to access a memory region that
was allocated dynamically. For drivers, the incoming data can not
usually be bound to a static value, and so we can not have the data
statically defined in the matching struct.
So this allows defining a kfunc to return any memory properly
allocated and owned by the device.

Okay, thanks for explanation.

quoted

quoted

Signed-off-by: Benjamin Tissoires <redacted>

---

changes in v6:
- code review from Kartikeya:
    - remove comment change that had no reasons to be
    - remove handling of PTR_TO_MEM with kfunc releases
    - introduce struct bpf_kfunc_arg_meta
    - do rdonly/rdwr_buf_size check in btf_check_kfunc_arg_match
    - reverted most of the changes in verifier.c
    - make sure kfunc acquire is using a struct pointer, not just a plain
      pointer
    - also forward ref_obj_id to PTR_TO_MEM in kfunc to not use after free
      the allocated memory

changes in v5:
- updated PTR_TO_MEM comment in btf.c to match upstream
- make it read-only or read-write based on the name of size

new in v4
---
   include/linux/bpf.h   | 10 ++++++-
   include/linux/btf.h   | 12 ++++++++
   kernel/bpf/btf.c      | 67 ++++++++++++++++++++++++++++++++++++++++---
   kernel/bpf/verifier.c | 49 +++++++++++++++++++++++--------
   4 files changed, 121 insertions(+), 17 deletions(-)

[...]