linux-kernel - Re: [PATCH v4 bpf-next 1/5] bpf: add in-kernel split BTF support

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Message-ID: <695E976D-DECA-4BE1-BFB0-771878B9CFCD@fb.com>
Date:   Tue, 10 Nov 2020 17:49:53 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii@...nel.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Alexei Starovoitov" <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4 bpf-next 1/5] bpf: add in-kernel split BTF support



> On Nov 9, 2020, at 5:19 PM, Andrii Nakryiko <andrii@...nel.org> wrote:
> 
> Adjust in-kernel BTF implementation to support a split BTF mode of operation.
> Changes are mostly mirroring libbpf split BTF changes, with the exception of
> start_id being 0 for in-kernel implementation due to simpler read-only mode.
> 
> Otherwise, for split BTF logic, most of the logic of jumping to base BTF,
> where necessary, is encapsulated in few helper functions. Type numbering and
> string offset in a split BTF are logically continuing where base BTF ends, so
> most of the high-level logic is kept without changes.
> 
> Type verification and size resolution is only doing an added resolution of new
> split BTF types and relies on already cached size and type resolution results
> in the base BTF.
> 
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
> kernel/bpf/btf.c | 171 +++++++++++++++++++++++++++++++++--------------
> 1 file changed, 119 insertions(+), 52 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 6324de8c59f7..727c1c27053f 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -203,12 +203,17 @@ struct btf {
> 	const char *strings;
> 	void *nohdr_data;
> 	struct btf_header hdr;
> -	u32 nr_types;
> +	u32 nr_types; /* includes VOID for base BTF */
> 	u32 types_size;
> 	u32 data_size;
> 	refcount_t refcnt;
> 	u32 id;
> 	struct rcu_head rcu;
> +
> +	/* split BTF support */
> +	struct btf *base_btf;
> +	u32 start_id; /* first type ID in this BTF (0 for base BTF) */
> +	u32 start_str_off; /* first string offset (0 for base BTF) */
> };
> 
> enum verifier_phase {
> @@ -449,14 +454,27 @@ static bool btf_type_is_datasec(const struct btf_type *t)
> 	return BTF_INFO_KIND(t->info) == BTF_KIND_DATASEC;
> }
> 
> +static u32 btf_nr_types_total(const struct btf *btf)
> +{
> +	u32 total = 0;
> +
> +	while (btf) {
> +		total += btf->nr_types;
> +		btf = btf->base_btf;
> +	}
> +
> +	return total;
> +}
> +
> s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
> {
> 	const struct btf_type *t;
> 	const char *tname;
> -	u32 i;
> +	u32 i, total;
> 
> -	for (i = 1; i <= btf->nr_types; i++) {
> -		t = btf->types[i];
> +	total = btf_nr_types_total(btf);
> +	for (i = 1; i < total; i++) {
> +		t = btf_type_by_id(btf, i);
> 		if (BTF_INFO_KIND(t->info) != kind)
> 			continue;
> 
> @@ -599,8 +617,14 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
> 
> static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
> {
> -	return BTF_STR_OFFSET_VALID(offset) &&
> -		offset < btf->hdr.str_len;
> +	if (!BTF_STR_OFFSET_VALID(offset))
> +		return false;
> +
> +	while (offset < btf->start_str_off)
> +		btf = btf->base_btf;

Do we need "if (!btf) return false;" in the while loop? (and some other loops below)

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