Re: [PATCH] tools/sched_ext: use btf_vlen() helper in compat.h

[Andrea Righi]

Hi,

On Tue, Jun 30, 2026 at 11:12:46AM +0800, luoliang@xxxxxxxxxx wrote:
> From: luoliang <luoliang@xxxxxxxxxx>
> 
> __COMPAT_read_enum() and __COMPAT_struct_has_field() open-code the
> vlen lookup via the raw BTF_INFO_VLEN(t->info) UAPI macro, whose
> result type is __u32 (t->info is __u32 and 0xffff is promoted to
> unsigned). Comparing it against the int loop counter triggers
> -Wsign-compare in every example scheduler that includes compat.h.
> 
> libbpf already exposes btf_vlen() for exactly this purpose; it
> returns __u16, the natural width of the vlen field, and the usual
> integer promotions turn the 'int < __u16' comparison into a plain
> 'int < int' so the warning goes away without any cast. This matches
> the pattern already used in-kernel (e.g. kernel/bpf/inode.c) and in
> tools/bpf/bpftool. Replace the three open-coded lookups with
> btf_vlen(t). No functional change.

btf_vlen() currently returns __u32, not __u16. This changed in commit
cacd6729c092 ("libbpf: Adjust btf_vlen() to return a __u32"), because BTF vlen
was expanded to 24 bits. We should update the description to match this. And i
should be changed to __u32 for consistency.

With these changes:

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> 
> Signed-off-by: Liang Luo <luoliang@xxxxxxxxxx>
> ---
>  tools/sched_ext/include/scx/compat.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
> index 039854c490d5..df5ca1ce20f6 100644
> --- a/tools/sched_ext/include/scx/compat.h
> +++ b/tools/sched_ext/include/scx/compat.h
> @@ -42,7 +42,7 @@ static inline bool __COMPAT_read_enum(const char *type, const char *name, u64 *v
>  	if (btf_is_enum(t)) {
>  		struct btf_enum *e = btf_enum(t);
>  
> -		for (i = 0; i < BTF_INFO_VLEN(t->info); i++) {
> +		for (i = 0; i < btf_vlen(t); i++) {
>  			n = btf__name_by_offset(__COMPAT_vmlinux_btf, e[i].name_off);
>  			SCX_BUG_ON(!n, "btf__name_by_offset()");
>  			if (!strcmp(n, name)) {
> @@ -53,7 +53,7 @@ static inline bool __COMPAT_read_enum(const char *type, const char *name, u64 *v
>  	} else if (btf_is_enum64(t)) {
>  		struct btf_enum64 *e = btf_enum64(t);
>  
> -		for (i = 0; i < BTF_INFO_VLEN(t->info); i++) {
> +		for (i = 0; i < btf_vlen(t); i++) {
>  			n = btf__name_by_offset(__COMPAT_vmlinux_btf, e[i].name_off);
>  			SCX_BUG_ON(!n, "btf__name_by_offset()");
>  			if (!strcmp(n, name)) {
> @@ -97,7 +97,7 @@ static inline bool __COMPAT_struct_has_field(const char *type, const char *field
>  
>  	m = btf_members(t);
>  
> -	for (i = 0; i < BTF_INFO_VLEN(t->info); i++) {
> +	for (i = 0; i < btf_vlen(t); i++) {
>  		n = btf__name_by_offset(__COMPAT_vmlinux_btf, m[i].name_off);
>  		SCX_BUG_ON(!n, "btf__name_by_offset()");
>  			if (!strcmp(n, field))
> -- 
> 2.43.0
>