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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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34
tools/include/uapi/linux/bpf.h
View file

@ -356,18 +356,36 @@ enum bpf_link_type {
#define BPF_F_SLEEPABLE (1U << 4)
/* When BPF ldimm64's insn[0].src_reg != 0 then this can have
* two extensions:
* the following extensions:
*
* insn[0].src_reg: BPF_PSEUDO_MAP_FD BPF_PSEUDO_MAP_VALUE
* insn[0].imm: map fd map fd
* insn[1].imm: 0 offset into value
* insn[0].off: 0 0
* insn[1].off: 0 0
* ldimm64 rewrite: address of map address of map[0]+offset
* verifier type: CONST_PTR_TO_MAP PTR_TO_MAP_VALUE
* insn[0].src_reg: BPF_PSEUDO_MAP_FD
* insn[0].imm: map fd
* insn[1].imm: 0
* insn[0].off: 0
* insn[1].off: 0
* ldimm64 rewrite: address of map
* verifier type: CONST_PTR_TO_MAP
*/
#define BPF_PSEUDO_MAP_FD 1
/* insn[0].src_reg: BPF_PSEUDO_MAP_VALUE
* insn[0].imm: map fd
* insn[1].imm: offset into value
* insn[0].off: 0
* insn[1].off: 0
* ldimm64 rewrite: address of map[0]+offset
* verifier type: PTR_TO_MAP_VALUE
*/
#define BPF_PSEUDO_MAP_VALUE 2
/* insn[0].src_reg: BPF_PSEUDO_BTF_ID
* insn[0].imm: kernel btd id of VAR
* insn[1].imm: 0
* insn[0].off: 0
* insn[1].off: 0
* ldimm64 rewrite: address of the kernel variable
* verifier type: PTR_TO_BTF_ID or PTR_TO_MEM, depending on whether the var
* is struct/union.
*/
#define BPF_PSEUDO_BTF_ID 3
/* when bpf_call->src_reg == BPF_PSEUDO_CALL, bpf_call->imm == pc-relative
* offset to another bpf function