commit 4fbb38a3b2cc7bf4340fe9bd9c73622760adc19d
author John Fastabend <john.fastabend@gmail.com> Thu Sep 24 11:45:06 2020 -0700
committer Alexei Starovoitov <ast@kernel.org> Fri Sep 25 16:47:21 2020 -0700
tree 1e4be30e2794b80015074db7f3753054a11940f8
parent 84085f877295b6b12d95262ec4082447214b09be

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>

kernel/bpf/verifier.c

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d4ba29f..b25ba98 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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;
 	}