Date:   Sat, 15 Dec 2018 23:37:56 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, netdev@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...com>, kernel-team@...com,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v2 2/8] bpf: btf: fix struct/union/fwd types with
 kind_flag

On 12/15/2018 12:34 AM, Yonghong Song wrote:
> This patch fixed two issues with BTF. One is related to
> struct/union bitfield encoding and the other is related to
> forward type.
> 
> Issue #1 and solution:
> ======================
> 
> Current btf encoding of bitfield follows what pahole generates.
> For each bitfield, pahole will duplicate the type chain and
> put the bitfield size at the final int or enum type.
> Since the BTF enum type cannot encode bit size,
> pahole workarounds the issue by generating
> an int type whenever the enum bit size is not 32.
> 
> For example,
>   -bash-4.4$ cat t.c
>   typedef int ___int;
>   enum A { A1, A2, A3 };
>   struct t {
>     int a[5];
>     ___int b:4;
>     volatile enum A c:4;
>   } g;
>   -bash-4.4$ gcc -c -O2 -g t.c
> The current kernel supports the following BTF encoding:
>   $ pahole -JV t.o
>   [1] TYPEDEF ___int type_id=2
>   [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>   [3] ENUM A size=4 vlen=3
>         A1 val=0
>         A2 val=1
>         A3 val=2
>   [4] STRUCT t size=24 vlen=3
>         a type_id=5 bits_offset=0
>         b type_id=9 bits_offset=160
>         c type_id=11 bits_offset=164
>   [5] ARRAY (anon) type_id=2 index_type_id=2 nr_elems=5
>   [6] INT sizetype size=8 bit_offset=0 nr_bits=64 encoding=(none)
>   [7] VOLATILE (anon) type_id=3
>   [8] INT int size=1 bit_offset=0 nr_bits=4 encoding=(none)
>   [9] TYPEDEF ___int type_id=8
>   [10] INT (anon) size=1 bit_offset=0 nr_bits=4 encoding=SIGNED
>   [11] VOLATILE (anon) type_id=10
> 
> Two issues are in the above:
>   . by changing enum type to int, we lost the original
>     type information and this will not be ideal later
>     when we try to convert BTF to a header file.
>   . the type duplication for bitfields will cause
>     BTF bloat. Duplicated types cannot be deduplicated
>     later if the bitfield size is different.
> 
> To fix this issue, this patch implemented a compatible
> change for BTF struct type encoding:
>   . the bit 31 of struct_type->info, previously reserved,
>     now is used to indicate whether bitfield_size is
>     encoded in btf_member or not.
>   . if bit 31 of struct_type->info is set,
>     btf_member->offset will encode like:
>       bit 0 - 23: bit offset
>       bit 24 - 31: bitfield size
>     if bit 31 is not set, the old behavior is preserved:
>       bit 0 - 31: bit offset
> 
> So if the struct contains a bit field, the maximum bit offset
> will be reduced to (2^24 - 1) instead of MAX_UINT. The maximum
> bitfield size will be 256 which is enough for today as maximum
> bitfield in compiler can be 128 where int128 type is supported.

Looks good in general, just small nit below.

> This kernel patch intends to support the new BTF encoding:
>   $ pahole -JV t.o
>   [1] TYPEDEF ___int type_id=2
>   [2] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
>   [3] ENUM A size=4 vlen=3
>         A1 val=0
>         A2 val=1
>         A3 val=2
>   [4] STRUCT t kind_flag=1 size=24 vlen=3
>         a type_id=5 bitfield_size=0 bits_offset=0
>         b type_id=1 bitfield_size=4 bits_offset=160
>         c type_id=7 bitfield_size=4 bits_offset=164
>   [5] ARRAY (anon) type_id=2 index_type_id=2 nr_elems=5
>   [6] INT sizetype size=8 bit_offset=0 nr_bits=64 encoding=(none)
>   [7] VOLATILE (anon) type_id=3
[...]
> +static int btf_int_check_kflag_member(struct btf_verifier_env *env,
> +				      const struct btf_type *struct_type,
> +				      const struct btf_member *member,
> +				      const struct btf_type *member_type)
> +{
> +	u32 struct_bits_off, nr_bits, nr_int_data_bits, bytes_offset;
> +	u32 int_data = btf_type_int(member_type);
> +	u32 struct_size = struct_type->size;
> +	u32 nr_copy_bits;
> +
> +	/* a regular int type is required for the kflag int member */
> +	if (!btf_type_int_is_regular(member_type)) {
> +		btf_verifier_log_member(env, struct_type, member,
> +					"Invalid member base type");
> +		return -EINVAL;
> +	}
> +
> +	/* check sanity of bitfield size */
> +	nr_bits = BTF_MEMBER_BITFIELD_SIZE(member->offset);
> +	struct_bits_off = BTF_MEMBER_BIT_OFFSET(member->offset);
> +	nr_int_data_bits = BTF_INT_BITS(int_data);
> +	if (!nr_bits) {
> +		/* Not a bitfield member, member offset must be at byte
> +		 * boundary.
> +		 */
> +		if (BITS_PER_BYTE_MASKED(struct_bits_off)) {
> +			btf_verifier_log_member(env, struct_type, member,
> +						"Invalid member offset");
> +			return -EINVAL;
> +		}
> +
> +		nr_bits = nr_int_data_bits;
> +	} else if (nr_bits > nr_int_data_bits) {

Should the test here not include the bit offset as well aka nr_copy_bits?
Thus test would be e.g. (nr_copy_bits > nr_int_data_bits || nr_copy_bits >
BITS_PER_U64) ...

Wrt to future 256 bit support, doesn't UAPI on BTF_INT_BITS() only support
up to max 255 bits?

> +		btf_verifier_log_member(env, struct_type, member,
> +					"Invalid member bitfield_size");
> +		return -EINVAL;
> +	}
> +
> +	bytes_offset = BITS_ROUNDDOWN_BYTES(struct_bits_off);
> +	nr_copy_bits = nr_bits + BITS_PER_BYTE_MASKED(struct_bits_off);
> +	if (nr_copy_bits > BITS_PER_U64) {
> +		btf_verifier_log_member(env, struct_type, member,
> +					"nr_copy_bits exceeds 64");
> +		return -EINVAL;
> +	}
> +
> +	if (struct_size < bytes_offset ||
> +	    struct_size - bytes_offset < BITS_ROUNDUP_BYTES(nr_copy_bits)) {
> +		btf_verifier_log_member(env, struct_type, member,
> +					"Member exceeds struct_size");
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}

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