Re: [PATCH bpf-next v2] bpf: Allow type tag BTF records to succeed other modifier records

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Tue Jun 16, 2026 at 2:42 AM EDT, bot+bpf-ci wrote:
>>     bpf: Allow type tag BTF records to succeed other modifier records
>>
>>     As of recently, Clang is able to attach type tag records to modifier BTF
>>     records. This is useful for using typedefs that encompass a base type
>>     and a type tag, e.g.:
>>
>>     typedef struct rbtree __arena rbtree_t;
>>
>>     Modify btf_check_type_tags() so that it allows this sequence of records.
>>     The function now only checks for record loops in BTF modifier record
>>     chains. Rename to btf_check_modifier_chain_length to reflect this.
>>
>>     Also expand the BTF modifier traversal code to take into account that
>>     type record can be interleaved with other modifier records. In effect
>>     this means traversing all modifiers to collect the type tags.
>>
>>     Also modify existing selftests to now accept modifier records (const,
>>     typedef) that point to type tag records.
>>
>>     Cc: Yonghong Song <yonghong.song@linux.dev>
>>     Cc: Vineet Gupta <vineet.gupta@linux.dev>
>>     Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
>
> This looks like it relaxes a constraint that was added earlier.  It
> removes the "Type tags don't precede modifiers" check (the in_tags logic
> in btf_check_type_tags()) that rejects valid BTF now produced by Clang.
>
> That check was introduced by commit eb596b090558 ("bpf: Ensure type tags
> precede modifiers in BTF").  Should this carry:
>
>   Fixes: eb596b090558 ("bpf: Ensure type tags precede modifiers in BTF")
>

This is not a fix. Circumstances have just changed and we must now
account for interleaving modifiers and type tags. The previous behavior
was fine, and the new behavior is fine too.

>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27598443532