Re: [PATCH v2 bpf-next 1/8] bpf: Extend BTF UAPI vlen, kinds to use unused bits

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [High] UAPI C preprocessor breakage: Converting `BTF_MAX_*` constants to self-referential macros breaks existing `#if` evaluations in userspace.
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commit ba6ded5102d9dc62d6101147f738a8a12d499303
Author: Alan Maguire <alan.maguire@oracle.com>

bpf: Extend BTF UAPI vlen, kinds to use unused bits

This commit extends BTF maximum vlen from 16 to 24 bits and kinds from
5 to 7 bits to support upcoming BTF location information. It converts the
BTF_MAX_* macros to an enum so they are encoded in kernel BTF, leaving
self-referential macros for backward compatibility.

Link: https://lore.kernel.org/bpf/CAEf4BzZx=X6vGqcA8SPU6D+v6k+TR=ZewebXMuXtpmML058piw@mail.gmail.com/

> diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
> --- a/include/uapi/linux/btf.h
> +++ b/include/uapi/linux/btf.h
[ ... ]
> +enum btf_max {
> +	/* Max possible kind */
> +	BTF_MAX_KIND =		0x0000007f,
> +	/* Max # of type identifier */
> +	BTF_MAX_TYPE =		0x000fffff,
> +	/* Max offset into the string section */
> +	BTF_MAX_NAME_OFFSET =	0x00ffffff,
> +	/* Max # of struct/union/enum members or func args */
> +	BTF_MAX_VLEN =		0x00ffffff,
> +};
> +
> +/*
> + * Macros needed for backwards compatibility; enum above ensures values
> + * make it into BTF.
> + */
> +#define BTF_MAX_TYPE		BTF_MAX_TYPE
> +#define BTF_MAX_NAME_OFFSET	BTF_MAX_NAME_OFFSET
> +#define BTF_MAX_VLEN		BTF_MAX_VLEN

Will these self-referential macros break existing C preprocessor logic in
userspace?

While #ifdef BTF_MAX_VLEN will still evaluate to true, the preprocessor
does not have visibility into enum values. When evaluating #if directives
like #if BTF_MAX_VLEN == 0xffff, the macro expands to the identifier
BTF_MAX_VLEN, which is treated as an undefined symbol and evaluates to 0.

This means the check silently evaluates to false, which might break
build-time feature detection for userspace programs depending on this header.

Could these macros be defined to their exact integer literals after the enum
block instead?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260416143904.1220662-1-alan.maguire@oracle.com?part=1