[bpf-next,v1,0/3] add bpf_file_d_path helper and selftests

Message

Lin Yikai July 18, 2024, 11:51 a.m. UTC

v1:
 - patch 2:
   - [1/2] add bpf_file_d_path helper
   - [2/2] add selftest to it

Hi, we are looking to add the "bpf_file_d_path" helper, 
used to retrieve the path from a struct file object.
	bpf_file_d_path(void *file, char *dst, u32 size);
	
It's worth noting that the "file" parameter is defined as "void*" type.

* Our problems *
Previously, we encountered issues 
on some user-space operating systems(OS):

1.Difficulty using vmlinux.h
(1) The OS lacks support for bpftool.
We can not use:
"bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h".
Bpftool need a separate complex cross-compilation environment to build.

(2) Many duplicate definitions between OS and vmlinux.h.

(3) The vmlinux.h size is large (2.8MB on arm64/Android), 
causing increased ebpf prog size and user space consumption.

2.The "struct file" has many internal variables and definitions,
and maybe change along with Linux version iterations,
making it hard to copy it to OS.


* Benefits of this commit *
1.There is no need to include vmlinux.h or redefine "struct file".

For example, with bpf on kprobe, 
we can directly pass param "(void*)PT_REGS_PARM1(pt_regs)"
to "bpf_file_d_path" helper in order to retrieve the path.


Appreciate your review and assistance. Thank you.
Yikai


Lin Yikai (2):
  bpf: Add bpf_file_d_path helper
  selftests/bpf:Adding test for bpf_file_d_path helper

 include/uapi/linux/bpf.h                      |  20 +++
 kernel/trace/bpf_trace.c                      |  34 ++++++
 tools/include/uapi/linux/bpf.h                |  20 +++
 .../selftests/bpf/prog_tests/file_d_path.c    | 115 ++++++++++++++++++
 .../selftests/bpf/progs/test_file_d_path.c    |  32 +++++
 5 files changed, 221 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/file_d_path.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_file_d_path.c

Comments

Martin KaFai Lau July 18, 2024, 5:16 p.m. UTC | #1

On 7/18/24 4:51 AM, Lin Yikai wrote:
> v1:
>   - patch 2:
>     - [1/2] add bpf_file_d_path helper
>     - [2/2] add selftest to it
> 
> Hi, we are looking to add the "bpf_file_d_path" helper,
> used to retrieve the path from a struct file object.
> 	bpf_file_d_path(void *file, char *dst, u32 size);
> 	
> It's worth noting that the "file" parameter is defined as "void*" type.
> 
> * Our problems *
> Previously, we encountered issues
> on some user-space operating systems(OS):
> 
> 1.Difficulty using vmlinux.h
> (1) The OS lacks support for bpftool.
> We can not use:
> "bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h".
> Bpftool need a separate complex cross-compilation environment to build.
> 
> (2) Many duplicate definitions between OS and vmlinux.h.
> 
> (3) The vmlinux.h size is large (2.8MB on arm64/Android),
> causing increased ebpf prog size and user space consumption.

The compiled bpf prog size is increased by 2.8MB because it included vmlinux.h?

> 
> 2.The "struct file" has many internal variables and definitions,
> and maybe change along with Linux version iterations,
> making it hard to copy it to OS.

If vmlinux.h is not convenience in your use case, you can try to define "struct 
file" with __attribute__((preserve_access_index)) and the libbpf will adjust the 
bpf prog against the running kernel.

There was a discussion a year ago about bpf helpers freeze. No new helper can be 
added since then. The same goes for this one.

pw-bot: cr