[PATCH v5 bpf-next 14/23] bpf: generalize is_branch_taken to handle all conditional jumps in one place

Andrii Nakryiko
2023-10-27T18:16:57Z

Make is_branch_taken() a single entry point for branch pruning decision
making, handling both pointer vs pointer, pointer vs scalar, and scalar
vs scalar cases in one place. This also nicely cleans up check_cond_jmp_op().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 49 ++++++++++++++++++++++---------------------
1 file changed, 25 insertions(+), 24 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 25b5234ebda3..fedd6d0e76e5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14169,6 +14169,19 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate,
}));
}

+/* check if register is a constant scalar value */
+static bool is_reg_const(struct bpf_reg_state *reg, bool subreg32)
+{
+ return reg->type == SCALAR_VALUE &&
+ tnum_is_const(subreg32 ? tnum_subreg(reg->var_off) : reg->var_off);
+}
+
+/* assuming is_reg_const() is true, return constant value of a register */
+static u64 reg_const_value(struct bpf_reg_state *reg, bool subreg32)
+{
+ return subreg32 ? tnum_subreg(reg->var_off).value : reg->var_off.value;
+}
+
/*
* <reg1> <op> <reg2>, currently assuming reg2 is a constant
*/
@@ -14410,12 +14423,20 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
u8 opcode, bool is_jmp32)
{
- struct tnum reg2_tnum = is_jmp32 ? tnum_subreg(reg2->var_off) : reg2->var_off;
u64 val;

- if (!tnum_is_const(reg2_tnum))
+ if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
+ return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
+
+ /* try to make sure reg2 is a constant SCALAR_VALUE */
+ if (!is_reg_const(reg2, is_jmp32)) {
+ opcode = flip_opcode(opcode);
+ swap(reg1, reg2);
+ }
+ /* for now we expect reg2 to be a constant to make any useful decisions */
+ if (!is_reg_const(reg2, is_jmp32))
return -1;
- val = reg2_tnum.value;
+ val = reg_const_value(reg2, is_jmp32);

if (__is_pointer_value(false, reg1)) {
if (!reg_not_null(reg1))
@@ -14896,27 +14917,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
}

is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
-
- if (BPF_SRC(insn->code) == BPF_K) {
- pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
- } else if (src_reg->type == SCALAR_VALUE &&
- is_jmp32 && tnum_is_const(tnum_subreg(src_reg->var_off))) {
- pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
- } else if (src_reg->type == SCALAR_VALUE &&
- !is_jmp32 && tnum_is_const(src_reg->var_off)) {
- pred = is_branch_taken(dst_reg, src_reg, 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, dst_reg, 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, flip_opcode(opcode), is_jmp32);
- } else if (reg_is_pkt_pointer_any(dst_reg) &&
- reg_is_pkt_pointer_any(src_reg) &&
- !is_jmp32) {
- pred = is_pkt_ptr_branch_taken(dst_reg, src_reg, opcode);
- }
-
+ pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
if (pred >= 0) {
/* If we get here with a dst_reg pointer type it is because
* above is_branch_taken() special cased the 0 comparison.
--
2.34.1