netdev - Re: [PATCH bpf-next v5 6/9] bpftool: Implement relocations recording for BTFGen

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Date:   Fri, 4 Feb 2022 03:20:22 -0300
From:   Rafael David Tinoco <rafaeldtinoco@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Mauricio Vásquez Bernal <mauricio@...volk.io>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        Lorenzo Fontana <lorenzo.fontana@...stic.co>,
        Leonardo Di Donato <leonardo.didonato@...stic.co>
Subject: Re: [PATCH bpf-next v5 6/9] bpftool: Implement relocations recording
 for BTFGen

>>> As in, do you substitute forward declarations for types that are
>>> never directly used? If not, that's going to be very suboptimal for
>>> something like task_struct and any other type that's part of a big
>>> cluster of types.

>> We decided to include the whole types and all direct and indirect
>> types referenced from a structure field for type-based relocations.
>> Our reasoning is that we don't know if the matching algorithm of
>> libbpf could be changed to require more information in the future and
>> type-based relocations are few compared to field based relocations.

> It will depend on application and which type is used in relocation.
> task_struct reaches tons of types and will add a very noticeable size
> to minimized BTF, for no good reason, IMO. If we discover that we do
> need those types, we'll update bpftool to generate more.

Just to see if I understood this part correctly. IIRC, we started type
based relocations support in btfgen because of this particular case:

	union kernfs_node_id {
	    struct {
	        u32 ino;
	        u32 generation;
	    };
	    u64 id;
	};

	struct kernfs_node___older_v55 {
	    const char *name;
	    union kernfs_node_id id;
	};

	struct kernfs_node___rh8 {
	    const char *name;
	    union {
	        u64 id;
	        struct {
	            union kernfs_node_id id;
	        } rh_kabi_hidden_172;
	        union { };
	    };
	};

So we have 3 situations:

(struct kernfs_node *)->id as u64

	[29] STRUCT 'kernfs_node' size=128 vlen=1
	        'id' type_id=42 bits_offset=832
	[42] TYPEDEF 'u64' type_id=10

(struct kernfs_node___older_v55 *)->id as u64 (union kernfs_node_id)->id

	[79] STRUCT 'kernfs_node' size=128 vlen=1
	        'id' type_id=69 bits_offset=832
	[69] UNION 'kernfs_node_id' size=8 vlen=2
	        '(anon)' type_id=132 bits_offset=0
	        'id' type_id=40 bits_offset=0
	[40] TYPEDEF 'u64' type_id=12

(struct kernfs_node___rh8 *)->id = (anon union)->id

	[56] STRUCT 'kernfs_node' size=128 vlen=1
	        '(anon)' type_id=24 bits_offset=832
	[24] UNION '(anon)' size=8 vlen=1
	        'id' type_id=40 bits_offset=0
	[40] TYPEDEF 'u64' type_id=11

We're finding needed BTF types, that should be added to generated BTF,
based on fields/members of CORE relo info. How we would know we had to
add the anon union of the last case if it does not exist in the local
BTF ? What is your suggestion ?

Thanks!

-rafaeldtinoco