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

[Rong Tao <rtoax@foxmail.com>]

On 5/30/25 09:55, Yonghong Song wrote:
>
>
> 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?
Thanks a lot, bpf_probe_read_kernel() works :)
>
>>
>> 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
>>
>>
>