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bpf: Introduce pseudo_btf_id

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Pseudo_btf_id is a type of ld_imm insn that associates a btf_id to a
ksym so that further dereferences on the ksym can use the BTF info
to validate accesses. Internally, when seeing a pseudo_btf_id ld insn,
the verifier reads the btf_id stored in the insn[0]'s imm field and
marks the dst_reg as PTR_TO_BTF_ID. The btf_id points to a VAR_KIND,
which is encoded in btf_vminux by pahole. If the VAR is not of a struct
type, the dst reg will be marked as PTR_TO_MEM instead of PTR_TO_BTF_ID
and the mem_size is resolved to the size of the VAR's type.

>From the VAR btf_id, the verifier can also read the address of the
ksym's corresponding kernel var from kallsyms and use that to fill
dst_reg.

Therefore, the proper functionality of pseudo_btf_id depends on (1)
kallsyms and (2) the encoding of kernel global VARs in pahole, which
should be available since pahole v1.18.

Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200929235049.2533242-2-haoluo@google.com
This commit is contained in:
Hao Luo 2020-09-29 16:50:44 -07:00 • committed by Alexei Starovoitov
parent 440c5752a3
commit 4976b718c3
6 changed files with 186 additions and 44 deletions
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15
include/linux/btf.h
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@ -145,6 +145,21 @@ static inline bool btf_type_is_func_proto(const struct btf_type *t)
return BTF_INFO_KIND(t->info) == BTF_KIND_FUNC_PROTO;
}
static inline bool btf_type_is_var(const struct btf_type *t)
{
return BTF_INFO_KIND(t->info) == BTF_KIND_VAR;
}
/* union is only a special case of struct:
* all its offsetof(member) == 0
*/
static inline bool btf_type_is_struct(const struct btf_type *t)
{
u8 kind = BTF_INFO_KIND(t->info);
return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
}
static inline u16 btf_type_vlen(const struct btf_type *t)
{
return BTF_INFO_VLEN(t->info);