linux-kernel - Re: [PATCH v2 3/9] bpf/btf: Add a function to search a member of a struct/union

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Date:   Fri, 21 Jul 2023 23:22:33 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v2 3/9] bpf/btf: Add a function to search a member of a
 struct/union

On Thu, 20 Jul 2023 23:34:42 +0100
Alan Maguire <alan.maguire@...cle.com> wrote:

> On 17/07/2023 16:23, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > Add btf_find_struct_member() API to search a member of a given data structure
> > or union from the member's name.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> A few small things below, but
> 
> Reviewed-by: Alan Maguire <alan.maguire@...cle.com>

Thanks

> 
> > ---
> >  include/linux/btf.h |    3 +++
> >  kernel/bpf/btf.c    |   38 ++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 41 insertions(+)
> > 
> > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > index 98fbbcdd72ec..097fe9b51562 100644
> > --- a/include/linux/btf.h
> > +++ b/include/linux/btf.h
> > @@ -225,6 +225,9 @@ const struct btf_type *btf_find_func_proto(struct btf *btf,
> >  					   const char *func_name);
> >  const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
> >  					   s32 *nr);
> > +const struct btf_member *btf_find_struct_member(struct btf *btf,
> > +						const struct btf_type *type,
> > +						const char *member_name);
> >  
> >  #define for_each_member(i, struct_type, member)			\
> >  	for (i = 0, member = btf_type_member(struct_type);	\
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index e015b52956cb..452ffb0393d6 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -1992,6 +1992,44 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
> >  		return NULL;
> >  }
> >  
> > +/*
> > + * Find a member of data structure/union by name and return it.
> > + * Return NULL if not found, or -EINVAL if parameter is invalid.
> > + */
> > +const struct btf_member *btf_find_struct_member(struct btf *btf,
> > +						const struct btf_type *type,
> > +						const char *member_name)
> > +{
> > +	const struct btf_member *members, *ret;
> > +	const char *name;
> > +	int i, vlen;
> > +
> > +	if (!btf || !member_name || !btf_type_is_struct(type))
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	vlen = btf_type_vlen(type);
> > +	members = (const struct btf_member *)(type + 1);
> > +
> > +	for (i = 0; i < vlen; i++) {
> 
> could use for_each_member() here I think, or perhaps use
> btf_type_member(type) when getting member pointer above.

Thanks! I missed that macro.

> 
> > +		if (!members[i].name_off) {
> > +			/* unnamed union: dig deeper */
> > +			type = btf_type_by_id(btf, members[i].type);
> > +			if (!IS_ERR_OR_NULL(type)) {
> > +				ret = btf_find_struct_member(btf, type,
> > +							     member_name);
> 
> You'll need to skip modifiers before calling btf_find_struct_member()
> here I think; it's possible to have a const anonymous union for example,

Yeah, it is possible. Let me add it.

> so to get to the union you'd need to skip the modifiers first. Otherwise
> you could fail the btf_type_is_struct() test on re-entry.

Indeed. 

Thank you!

> 
> 
> > +				if (!IS_ERR_OR_NULL(ret))
> > +					return ret;
> > +			}
> > +		} else {
> > +			name = btf_name_by_offset(btf, members[i].name_off);
> > +			if (name && !strcmp(member_name, name))
> > +				return &members[i];
> > +		}
> > +	}
> > +
> > +	return NULL;
> > +}
> > +
> >  static u32 btf_resolved_type_id(const struct btf *btf, u32 type_id)
> >  {
> >  	while (type_id < btf->start_id)
> > 
> > 


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