Re: [PATCH bpf-next v4 07/17] libbpf, bpftool, resolve_btfids: Dump kflagged BTF tags as is

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpf: changing kflag BTF_KIND_TYPE_TAG format breaks C dumps across versions
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commit 02905c14788165a6bc8db8f66ae9bfa227c2d55a
Author: Ihor Solodrai <ihor.solodrai@linux.dev>

libbpf, bpftool, resolve_btfids: Dump kflagged BTF tags as is

This commit changes btf_dump to print kflagged BTF decl and type tags as-is
without wrapping them in __attribute__((...)), updates resolve_btfids to
encode the full __attribute__((address_space(1))) string in TYPE_ATTR_ARENA,
and adds a split-BTF decl tag override in bpftool for older kernel BTF.

> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 5ad5473563034..c16dd93fa6d4c 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -204,7 +204,7 @@ struct btf_links {
>  #define KF_ARENA_ARG2	(1 << 15)
>  #define KF_IMPLICIT_ARGS (1 << 16)
>  #define KF_IMPL_SUFFIX "_impl"
> -#define TYPE_ATTR_ARENA "address_space(1)"
> +#define TYPE_ATTR_ARENA "__attribute__((address_space(1)))"
>  #define PARAM_SUFFIX_ARENA "__arena"
>  #define PARAM_SUFFIX_ARENA_NULLABLE "__arena__nullable"
>  
> diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> index c12c3c248407f..fcd97aa905897 100644
> --- a/tools/lib/bpf/btf_dump.c
> +++ b/tools/lib/bpf/btf_dump.c
> @@ -1023,7 +1023,7 @@ static void btf_dump_emit_decl_tag(struct btf_dump *d, const struct btf_type *t)
>  	const char *name = btf_name_of(d, t->name_off);
>  
>  	if (btf_kflag(t))
> -		btf_dump_printf(d, " __attribute__((%s))", name);
> +		btf_dump_printf(d, " %s", name);
>  	else
>  		btf_dump_printf(d, " __attribute__((btf_decl_tag(\"%s\")))", name);
>  }
> @@ -1602,7 +1602,7 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
>  				break;
>  			name = btf_name_of(d, t->name_off);
>  			if (btf_kflag(t))
> -				btf_dump_printf(d, " __attribute__((%s))", name);
> +				btf_dump_printf(d, " %s", name);
>  			else
>  				btf_dump_printf(d, " __attribute__((btf_type_tag(\"%s\")))", name);
>  			break;

[Severity: Medium]
As reported by bot+bpf-ci@kernel.org on v3 [1], changing how kflagged
BTF_KIND_TYPE_TAG entries are encoded by resolve_btfids and emitted by
btf_dump_emit_type_chain() causes regressions when dumping C headers across
versions.

In tools/bpf/resolve_btfids/main.c, TYPE_ATTR_ARENA is changed to
"__attribute__((address_space(1)))" and passed to btf__add_type_attr() in
arena_tag_ptr(), which encodes the full attribute wrapper into vmlinux BTF for
arena kfuncs. When an existing bpftool built with libbpf 1.6 or later dumps
vmlinux.h from a newly built kernel, its btf_dump_emit_type_chain() wraps the
kflagged type tag in __attribute__((%s)) again, emitting:

  void __attribute__((__attribute__((address_space(1))))) *

This breaks compilation of any BPF program including vmlinux.h unless
BPF_NO_KFUNC_PROTOTYPES is defined.

Conversely, in tools/lib/bpf/btf_dump.c, changing btf_dump_emit_type_chain()
to print kflagged BTF_KIND_TYPE_TAG strings without __attribute__((...))
means that any external caller of btf_dump__dump_type() or
btf_dump__emit_type_decl() outside bpftool (which does not run
btf_c_annotate_type_tags()) will emit bare address_space(1) when dumping
existing v6.16+ kernel BTF or BTF produced by pahole with
--btf_features=attributes, which also fails to compile.

Since btf_dump already allows a kflagged BTF_KIND_DECL_TAG (with
component_idx = -1) in the dump split BTF to override how a referenced
BTF_KIND_TYPE_TAG is printed, could btf_dump_emit_type_chain() and
TYPE_ATTR_ARENA in resolve_btfids be left unchanged, and raw printing be
restricted to kflagged BTF_KIND_DECL_TAG in btf_dump_emit_decl_tag() as
requested by Alexei Starovoitov on v3?

[1] https://lore.kernel.org/r/b8912e062900f51fd8c8f44c44ef43d0788ce6af12582abc0060dd6cd4c05065@mail.kernel.org

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Sashiko AI review · https://sashiko.dev/#/patchset/20261105175845.30357-1-ihor.solodrai@linux.dev?part=7