Re: [PATCH v14 02/14] tracing/probes: Fix BTF kflag check for anonymous struct member access

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Sun, 30 Aug 2026 14:38:59 +0000
sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Unnamed bitfields break nested anonymous struct resolution in btf_find_struct_member().
> --
> 
> commit 20a606aa6d4e155be5a531c5d52d78c34205c0b4
> Author: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> tracing/probes: Fix BTF kflag check for anonymous struct member access
> 
> This commit fixes how BTF kflags are checked when traversing nested
> anonymous structures and unions in btf_find_struct_member() by using
> the actual containing type.
> 
> > diff --git a/kernel/trace/trace_btf.c b/kernel/trace/trace_btf.c
> > --- a/kernel/trace/trace_btf.c
> > +++ b/kernel/trace/trace_btf.c
> 
> [ ... ]
> 
> > @@ -91,17 +92,23 @@ const struct btf_member *btf_find_struct_member(struct btf *btf,
> >  	for_each_member(i, type, member) {
> >  		if (!member->name_off) {
> >  			/* Anonymous union/struct: push it for later use */
> > +			u32 m_off = btf_type_kflag(type) ?
> > +				BTF_MEMBER_BIT_OFFSET(member->offset) :
> > +				member->offset;
> > +
> >  			if (btf_type_skip_modifiers(btf, member->type, &tid) &&
> >  			    top < BTF_ANON_STACK_MAX) {
> >  				anon_stack[top].tid = tid;
> >  				anon_stack[top++].offset =
> > -					cur_offset + member->offset;
> > +					cur_offset + m_off;
> >  			}
> >  		} else {
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but is it possible for unnamed
> bitfields to break nested anonymous struct resolution here?
> 

Oops, unnamed bitfield.... I didn't expected that.

> Since unnamed bitfields (like int : 5) also have name_off == 0, they get
> pushed onto anon_stack alongside actual anonymous structs.

OK, I got it. This function only expects anonymous struct/union, not
anonymous bitfield.

> 
> Because anon_stack is processed in LIFO order, an unnamed bitfield defined
> after an anonymous struct will be popped first. When it is popped at the top
> of btf_find_struct_member():
> 
> retry:
> 	if (!btf_type_is_struct(type)) {
> 		member = ERR_PTR(-EINVAL);
> 		goto out;
> 	}
> 
> Does this cause the scalar type check to fail and abort the entire search,
> preventing the earlier anonymous struct from ever being searched?

Yeah, this is wrong. it should check this is the last anon_stack entry
and retry. Maybe we should make it a loop.

Thanks!

> 
> >  			name = btf_name_by_offset(btf, member->name_off);
> >  			if (name && !strcmp(member_name, name)) {
> >  				if (anon_offset)
> >  					*anon_offset = cur_offset;
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/178810001186.64882.2161016469449127450.stgit@devnote2?part=2


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>