When doing BPF filter program conversion, a common way
to zero a register in single instruction is:
     xor r7,r7

The BPF validator would not allow this because the value of
r7 was undefined. But after this operation it always zero so
allow it as a special case.

Cc: Konstantin Ananyev <redacted>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/bpf/bpf_validate.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/bpf/bpf_validate.c b/lib/bpf/bpf_validate.c
index 7b1291b382e9..7647a7454dc2 100644
--- a/lib/bpf/bpf_validate.c
+++ b/lib/bpf/bpf_validate.c

@@ -661,8 +661,12 @@ eval_alu(struct bpf_verifier *bvf, const struct ebpf_insn *ins)
 
 	op = BPF_OP(ins->code);
 
-	err = eval_defined((op != EBPF_MOV) ? rd : NULL,
-			(op != BPF_NEG) ? &rs : NULL);
+	/* Allow self-xor as way to zero register */
+	if (op == BPF_XOR && ins->src_reg == ins->dst_reg)
+		err = NULL;
+	else
+		err = eval_defined((op != EBPF_MOV) ? rd : NULL,
+				   (op != BPF_NEG) ? &rs : NULL);
 	if (err != NULL)
 		return err;
 

-- 
2.30.2