On Wed, May 6, 2020 at 6:30 AM Jiri Olsa [off-list ref] 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@kernel.org>
---
 include/uapi/linux/bpf.h       | 14 +++++++++++++-
 kernel/trace/bpf_trace.c       | 31 +++++++++++++++++++++++++++++++
 scripts/bpf_helpers_doc.py     |  2 ++
 tools/include/uapi/linux/bpf.h | 14 +++++++++++++-
 4 files changed, 59 insertions(+), 2 deletions(-)

[...]

+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;
+               }
+       }
+
+       return len;
+}
+
+static u32 bpf_d_path_btf_ids[3];

Using shorter than 5 element array is "unconventional", but seems like
btf_distill_func_proto will never access elements that are not
ARG_PTR_TO_BTF_ID, so it's fine. But than again, if we are saving
space, why not just 1-element array? :)

+static const struct bpf_func_proto bpf_d_path_proto = {
+       .func           = bpf_d_path,
+       .gpl_only       = true,
+       .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,
+};
+

[...]

Please specify if it's always zero-terminated string (especially on truncation).