Re: [PATCH dwarves v9 3/3] pahole: Inject kfunc decl tags into BTF

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Apr 29, 2024 at 04:46:00PM -0600, Daniel Xu wrote:
> This commit teaches pahole to parse symbols in .BTF_ids section in
> vmlinux and discover exported kfuncs. Pahole then takes the list of
> kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
> 
> Example of encoding:
> 
>         $ bpftool btf dump file .tmp_vmlinux.btf | rg "DECL_TAG 'bpf_kfunc'" | wc -l
>         121
> 
>         $ bpftool btf dump file .tmp_vmlinux.btf | rg 56337
>         [56337] FUNC 'bpf_ct_change_timeout' type_id=56336 linkage=static
>         [127861] DECL_TAG 'bpf_kfunc' type_id=56337 component_idx=-1
> 
> This enables downstream users and tools to dynamically discover which
> kfuncs are available on a system by parsing vmlinux or module BTF, both
> available in /sys/kernel/btf.
> 
> This feature is enabled with --btf_features=decl_tag,decl_tag_kfuncs.
> 
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> Tested-by: Jiri Olsa <jolsa@kernel.org>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> Tested-by: Alan Maguire <alan.maguire@oracle.com>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>

Here I needed this to avoid an strdup possibly followed by a free and
then checking the strdup result, please Ack/revalidate tags. I'm
dropping them as there are changes.

- Arnaldo

diff --git a/btf_encoder.c b/btf_encoder.c
index e9d82e0af0e178fd..c2df2bc7a374447b 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -1434,11 +1434,13 @@ static char *get_func_name(const char *sym)
 		return NULL;
 
 	/* Strip prefix and handle malformed input such as  __BTF_ID__func___ */
-	func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
-	if (!strstr(func, "__")) {
-                free(func);
+	const char *func_sans_prefix = sym + sizeof(BTF_ID_FUNC_PFX) - 1;
+	if (!strstr(func_sans_prefix, "__"))
                 return NULL;
-        }
+
+	func = strdup(func_sans_prefix);
+	if (!func)
+		return NULL;
 
 	/* Strip suffix */
 	end = strrchr(func, '_');