libbpf: Implement bpf_usdt_arg_size BPF function

Information about USDT argument size is implicitly stored in
__bpf_usdt_arg_spec, but currently it's not accessbile to BPF programs
that use USDT.

Implement bpf_sdt_arg_size() that returns the size of an USDT argument
in bytes.

v1->v2:
  * do not add __bpf_usdt_arg_spec() helper

v1: https://lore.kernel.org/bpf/20250220215904.3362709-1-ihor.solodrai@linux.dev/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20250224235756.2612606-1-ihor.solodrai@linux.dev

diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
index b811f754939f0af5c3017907ff191dde12e1d55b..2a7865c8e3fe3cbc97e033328b5b93db5a1b9d42 100644 (file)
--- a/tools/lib/bpf/usdt.bpf.h
+++ b/tools/lib/bpf/usdt.bpf.h
@@ -108,6 +108,38 @@ int bpf_usdt_arg_cnt(struct pt_regs *ctx)
        return spec->arg_cnt;
 }
 
+/* Returns the size in bytes of the #*arg_num* (zero-indexed) USDT argument.
+ * Returns negative error if argument is not found or arg_num is invalid.
+ */
+static __always_inline
+int bpf_usdt_arg_size(struct pt_regs *ctx, __u64 arg_num)
+{
+       struct __bpf_usdt_arg_spec *arg_spec;
+       struct __bpf_usdt_spec *spec;
+       int spec_id;
+
+       spec_id = __bpf_usdt_spec_id(ctx);
+       if (spec_id < 0)
+               return -ESRCH;
+
+       spec = bpf_map_lookup_elem(&__bpf_usdt_specs, &spec_id);
+       if (!spec)
+               return -ESRCH;
+
+       if (arg_num >= BPF_USDT_MAX_ARG_CNT)
+               return -ENOENT;
+       barrier_var(arg_num);
+       if (arg_num >= spec->arg_cnt)
+               return -ENOENT;
+
+       arg_spec = &spec->args[arg_num];
+
+       /* arg_spec->arg_bitshift = 64 - arg_sz * 8
+        * so: arg_sz = (64 - arg_spec->arg_bitshift) / 8
+        */
+       return (unsigned int)(64 - arg_spec->arg_bitshift) / 8;
+}
+
 /* Fetch USDT argument #*arg_num* (zero-indexed) and put its value into *res.
  * Returns 0 on success; negative error, otherwise.
  * On error *res is guaranteed to be set to zero.