Commit 4fbb38a3 authored Sep 24, 2020 by John Fastabend Committed by Alexei Starovoitov Sep 25, 2020

bpf, verifier: Remove redundant var_off.value ops in scalar known reg cases



In BPF_AND and BPF_OR alu cases we have this pattern when the src and dst
tnum is a constant.

 1 dst_reg->var_off = tnum_[op](dst_reg->var_off, src_reg.var_off)
 2 scalar32_min_max_[op]
 3       if (known) return
 4 scalar_min_max_[op]
 5       if (known)
 6          __mark_reg_known(dst_reg,
                   dst_reg->var_off.value [op] src_reg.var_off.value)

The result is in 1 we calculate the var_off value and store it in the
dst_reg. Then in 6 we duplicate this logic doing the op again on the
value.

The duplication comes from the the tnum_[op] handlers because they have
already done the value calcuation. For example this is tnum_and().

 struct tnum tnum_and(struct tnum a, struct tnum b)
 {
	u64 alpha, beta, v;

	alpha = a.value | a.mask;
	beta = b.value | b.mask;
	v = a.value & b.value;
	return TNUM(v, alpha & beta & ~v);
 }

So lets remove the redundant op calculation. Its confusing for readers
and unnecessary. Its also not harmful because those ops have the
property, r1 & r1 = r1 and r1 | r1 = r1.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 84085f87

kernel/bpf/verifier.c

+2 −4

Original line number	Diff line number	Diff line
		@@ -5849,8 +5849,7 @@ static void scalar_min_max_and(struct bpf_reg_state *dst_reg,
		u64 umax_val = src_reg->umax_value;

		if (src_known && dst_known) {
		__mark_reg_known(dst_reg, dst_reg->var_off.value &
		src_reg->var_off.value);
		__mark_reg_known(dst_reg, dst_reg->var_off.value);
		return;
		}

		@@ -5920,8 +5919,7 @@ static void scalar_min_max_or(struct bpf_reg_state *dst_reg,
		u64 umin_val = src_reg->umin_value;

		if (src_known && dst_known) {
		__mark_reg_known(dst_reg, dst_reg->var_off.value \|
		src_reg->var_off.value);
		__mark_reg_known(dst_reg, dst_reg->var_off.value);
		return;
		}

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