Re: [PATCH bpf-next] bpf: clean up btf_scan_decl_tags()

[Matt Bobrowski <mattbobrowski@google.com>]

On Thu, Jun 04, 2026 at 11:59:26AM -0400, Emil Tsalapatis wrote:
> On Wed Jun 3, 2026 at 4:18 PM EDT, Matt Bobrowski wrote:
> > Refactor the newly introduced btf_scan_decl_tags() to improve
> > readability and maintainability. The current implementation uses a
> > manual if-else chain and a magic number offset to strip the "arg:"
> > prefix from declaration tags.
> >
> > Replace the if-else logic with a table-driven approach using a static
> > const array. This separates the tag data from the scanning logic, making
> > the helper more extensible for future tags. Additionally, replace the
> > magic number '4' with a sizeof-based calculation on the prefix string to
> > ensure the offset remains synchronized with the search key.
> >
> > Finally, optimize the loop by moving the is_global check to the top of
> > the block. This allows the verifier to fail-fast on static subprograms
> > without performing unnecessary BTF string and type lookups.
> >
> > Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> 
> We can also modify the new btf_scan_type_tags() to conform to the same
> pattern. It currently recognizes a single type tag, so if anything it
> would be more code, but we don't want it to slowly end up looking like
> the if-else chain we're replacing below.

OK, I'll also take a look at btf_scan_type_tags() and if so adapt it
and send a patch through.

> > ---
> >  kernel/bpf/btf.c | 46 ++++++++++++++++++++++++++++++----------------
> >  1 file changed, 30 insertions(+), 16 deletions(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 68921d9172b5..55aa3ba1b1e0 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -7808,14 +7808,28 @@ static int btf_scan_decl_tags(struct bpf_verifier_env *env,
> >  			      u32 arg_idx, bool is_global, u32 *tags)
> >  {
> >  	int id = btf_named_start_id(btf, false) - 1;
> > +	const char tag_key[] = "arg:";
> > +	static const struct {
> > +		const char *tag_value;
> > +		enum btf_arg_tag arg_tag;
> > +	} tag_values[] = {
> > +		{ "ctx", ARG_TAG_CTX },
> > +		{ "trusted", ARG_TAG_TRUSTED },
> > +		{ "untrusted", ARG_TAG_UNTRUSTED },
> > +		{ "nonnull", ARG_TAG_NONNULL },
> > +		{ "nullable", ARG_TAG_NULLABLE },
> > +		{ "arena", ARG_TAG_ARENA },
> > +	};
> >  
> >  	/*
> >  	 * The 'arg:<tag>' decl_tag takes precedence over the derivation
> >  	 * of the register type from the BTF type itself.
> >  	 */
> > -	while ((id = btf_find_next_decl_tag(btf, fn_t, arg_idx, "arg:", id)) > 0) {
> > -		const struct btf_type *tag_t = btf_type_by_id(btf, id);
> > -		const char *tag = __btf_name_by_offset(btf, tag_t->name_off) + 4;
> > +	while ((id = btf_find_next_decl_tag(btf, fn_t, arg_idx, tag_key, id)) > 0) {
> > +		const struct btf_type *tag_t;
> > +		const char *tag;
> > +		int i;
> > +		bool found;
> >  
> >  		/* disallow arg tags in static subprogs */
> >  		if (!is_global) {
> > @@ -7825,19 +7839,19 @@ static int btf_scan_decl_tags(struct bpf_verifier_env *env,
> >  			return -EOPNOTSUPP;
> >  		}
> >  
> > -		if (strcmp(tag, "ctx") == 0) {
> > -			*tags |= ARG_TAG_CTX;
> > -		} else if (strcmp(tag, "trusted") == 0) {
> > -			*tags |= ARG_TAG_TRUSTED;
> > -		} else if (strcmp(tag, "untrusted") == 0) {
> > -			*tags |= ARG_TAG_UNTRUSTED;
> > -		} else if (strcmp(tag, "nonnull") == 0) {
> > -			*tags |= ARG_TAG_NONNULL;
> > -		} else if (strcmp(tag, "nullable") == 0) {
> > -			*tags |= ARG_TAG_NULLABLE;
> > -		} else if (strcmp(tag, "arena") == 0) {
> > -			*tags |= ARG_TAG_ARENA;
> > -		} else {
> > +		tag_t = btf_type_by_id(btf, id);
> > +		tag = __btf_name_by_offset(btf, tag_t->name_off) + (sizeof(tag_key) - 1);
> > +
> > +		found = false;
> > +		for (i = 0; i < ARRAY_SIZE(tag_values); ++i) {
> > +			if (!strcmp(tag, tag_values[i].tag_value)) {
> > +				*tags |= tag_values[i].arg_tag;
> > +				found = true;
> > +				break;
> > +			}
> > +		}
> > +
> > +		if (!found) {
> >  			bpf_log(&env->log, "arg#%d has unsupported set of tags\n", arg_idx);
> >  			return -EOPNOTSUPP;
> >  		}
>