On Thu, Jun 25, 2020 at 4:49 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Adding d_path helper function that returns full path
> for give 'struct path' object, which needs to be the
> kernel BTF 'path' object.
>
> The helper calls directly d_path function.
>
> Updating also bpf.h tools uapi header and adding
> 'path' to bpf_helpers_doc.py script.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> include/uapi/linux/bpf.h | 14 +++++++++-
> kernel/trace/bpf_trace.c | 47 ++++++++++++++++++++++++++++++++++
> scripts/bpf_helpers_doc.py | 2 ++
> tools/include/uapi/linux/bpf.h | 14 +++++++++-
> 4 files changed, 75 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 0cb8ec948816..23274c81f244 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3285,6 +3285,17 @@ union bpf_attr {
> * Dynamically cast a *sk* pointer to a *udp6_sock* pointer.
> * Return
> * *sk* if casting is valid, or NULL otherwise.
> + *
> + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> + * Description
> + * Return full path for given 'struct path' object, which
> + * needs to be the kernel BTF 'path' object. The path is
> + * returned in buffer provided 'buf' of size 'sz'.
> + *
> + * Return
> + * length of returned string on success, or a negative
> + * error in case of failure
It's important to note whether string is always zero-terminated (I'm
guessing it is, right?).
> + *
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3427,7 +3438,8 @@ union bpf_attr {
> FN(skc_to_tcp_sock), \
> FN(skc_to_tcp_timewait_sock), \
> FN(skc_to_tcp_request_sock), \
> - FN(skc_to_udp6_sock),
> + FN(skc_to_udp6_sock), \
> + FN(d_path),
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> * function eBPF program intends to call
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b124d468688c..6f31e21565b6 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1060,6 +1060,51 @@ static const struct bpf_func_proto bpf_send_signal_thread_proto = {
> .arg1_type = ARG_ANYTHING,
> };
>
> +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> +{
> + char *p = d_path(path, buf, sz - 1);
> + int len;
> +
> + if (IS_ERR(p)) {
> + len = PTR_ERR(p);
> + } else {
> + len = strlen(p);
> + if (len && p != buf) {
> + memmove(buf, p, len);
> + buf[len] = 0;
if len above is zero, you won't zero-terminate it, so probably better
to move buf[len] = 0 out of if to do unconditionally
> + }
> + }
> +
> + return len;
> +}
> +
> +BTF_SET_START(btf_whitelist_d_path)
> +BTF_ID(func, vfs_truncate)
> +BTF_ID(func, vfs_fallocate)
> +BTF_ID(func, dentry_open)
> +BTF_ID(func, vfs_getattr)
> +BTF_ID(func, filp_close)
> +BTF_SET_END(btf_whitelist_d_path)
> +
> +static bool bpf_d_path_allowed(const struct bpf_prog *prog)
> +{
> + return btf_id_set_contains(&btf_whitelist_d_path, prog->aux->attach_btf_id);
> +}
> +
This looks pretty great and clean, considering what's happening under
the covers. Nice work, thanks a lot!
> +BTF_ID_LIST(bpf_d_path_btf_ids)
> +BTF_ID(struct, path)
this is a bit more confusing to read and error-prone, but I couldn't
come up with any better way to do this... Still better than
alternatives.
> +
> +static const struct bpf_func_proto bpf_d_path_proto = {
> + .func = bpf_d_path,
> + .gpl_only = true,
Does it have to be GPL-only? What's the criteria? Sorry if this was
brought up previously.
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_PTR_TO_BTF_ID,
> + .arg2_type = ARG_PTR_TO_MEM,
> + .arg3_type = ARG_CONST_SIZE,
> + .btf_id = bpf_d_path_btf_ids,
> + .allowed = bpf_d_path_allowed,
> +};
> +
[...]
