Re: [PATCH bpf-next 1/2] bpf: Add bpf_task_cwd_from_pid() kfunc

[Yonghong Song <yonghong.song@linux.dev>]

On 5/29/25 6:28 PM, Rong Tao wrote:
>
> On 5/29/25 13:44, Alexei Starovoitov wrote:
>> On Wed, May 28, 2025 at 8:37 PM Rong Tao <rtoax@foxmail.com> wrote:
>>> From: Rong Tao <rongtao@cestc.cn>
>>>
>>> It is a bit troublesome to get cwd based on pid in bpf program, such as
>>> bpftrace example [1].
>>>
>>> This patch therefore adds a new bpf_task_cwd_from_pid() kfunc which
>>> allows BPF programs to get cwd from a pid.
>>>
>>> [1] https://github.com/bpftrace/bpftrace/issues/3314
>> Yes. This is cumbersome, but adding a very specific kfunc
>> to the kernel is not a solution.
>> This is tracing, no need for precise cwd. probe_read_kernel
>> can do the job. bpftrace needs to have better C interop.
>> Once that happens any kind of tracing extraction will be
>> easy to write in C. Like this bpf_task_cwd_from_pid()
>> can already be written as C bpf program.
> Thanks for your reply, Yesterday I tried many ways to implement
> the solution of getting cwd from pid/task, but all failed. The basic
> idea is to rewrite the d_path() code, but in the bpf program, there
> will be various memory security access problems, even if enough
>  `if (!ptr)` are added, the program cannot be loaded successfully.
>
> https://github.com/Rtoax/bcc/commit/2ba7a2389fc1183264e5195ff26561d93038886c 
>
>
>     bcc/tools$ sudo ./opensnoop.py -F
>
>     ; if (dentry == vfsmnt->mnt_root || dentry == dentry->d_parent) { 
> @ main.c:174
>     109: (79) r2 = *(u64 *)(r7 +0)
>     R7 invalid mem access 'scalar'

I think you can use bpf_probe_read_kernel() helper to get r2?

>
> At the same time, bpf_d_path cannot be used because it can only be
> applied to functions in btf_allowlist_d_path. Currently, it is
> impossible to get cwd from pid/task in user mode. Any suggestions?
>
> In addition, I fully tested this patch yesterday and it performed well.
>
> Rong Tao
>
>