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.
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