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 <[e-mail]>
Acked-by: Dave Marchevsky <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 				       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

···
 	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;						\