Commit 953d9f5b authored Apr 06, 2023 by Yonghong Song Committed by Alexei Starovoitov Apr 06, 2023

bpf: Improve handling of pattern '<const> <cond_op> <non_const>' in verifier



Currently, the verifier does not handle '<const> <cond_op> <non_const>' well.
For example,
  ...
  10: (79) r1 = *(u64 *)(r10 -16)       ; R1_w=scalar() R10=fp0
  11: (b7) r2 = 0                       ; R2_w=0
  12: (2d) if r2 > r1 goto pc+2
  13: (b7) r0 = 0
  14: (95) exit
  15: (65) if r1 s> 0x1 goto pc+3
  16: (0f) r0 += r1
  ...
At insn 12, verifier decides both true and false branch are possible, but
actually only false branch is possible.

Currently, the verifier already supports patterns '<non_const> <cond_op> <const>.
Add support for patterns '<const> <cond_op> <non_const>' in a similar way.

Also fix selftest 'verifier_bounds_mix_sign_unsign/bounds checks mixing signed and unsigned, variant 10'
due to this change.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230406164505.1046801-1-yhs@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent aec08d67

kernel/bpf/verifier.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -13356,6 +13356,18 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
		src_reg->var_off.value,
		opcode,
		is_jmp32);
		} else if (dst_reg->type == SCALAR_VALUE &&
		is_jmp32 && tnum_is_const(tnum_subreg(dst_reg->var_off))) {
		pred = is_branch_taken(src_reg,
		tnum_subreg(dst_reg->var_off).value,
		flip_opcode(opcode),
		is_jmp32);
		} else if (dst_reg->type == SCALAR_VALUE &&
		!is_jmp32 && tnum_is_const(dst_reg->var_off)) {
		pred = is_branch_taken(src_reg,
		dst_reg->var_off.value,
		flip_opcode(opcode),
		is_jmp32);
		} else if (reg_is_pkt_pointer_any(dst_reg) &&
		reg_is_pkt_pointer_any(src_reg) &&
		!is_jmp32) {

tools/testing/selftests/bpf/progs/verifier_bounds_mix_sign_unsign.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -354,7 +354,7 @@ __naked void signed_and_unsigned_variant_10(void)
		call %[bpf_map_lookup_elem]; \
		if r0 == 0 goto l0_%=; \
		r1 = (u64)(r10 - 16); \
		r2 = 0; \
		r2 = -1; \
		if r2 > r1 goto l1_%=; \
		r0 = 0; \
		exit; \