nfp: bpf: allow stack accesses via modified stack registers

As long as the verifier tells us the stack offset exactly we
can render the LMEM reads quite easily.  Simply make sure that
the offset is constant for a given instruction and add it to
the instruction's offset.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 17 insertions, 7 deletions

diff --git a/drivers/net/ethernet/netronome/nfp/bpf/verifier.c b/drivers/net/ethernet/netronome/nfp/bpf/verifier.c
index 376d9938b823..633db3e1a11e 100644
--- a/drivers/net/ethernet/netronome/nfp/bpf/verifier.c
+++ b/drivers/net/ethernet/netronome/nfp/bpf/verifier.c
@@ -111,19 +111,29 @@ nfp_bpf_check_exit(struct nfp_prog *nfp_prog,
 	return 0;
 }
 
-static int nfp_bpf_check_stack_access(const struct bpf_reg_state *reg)
+static int
+nfp_bpf_check_stack_access(struct nfp_insn_meta *meta,
+			   const struct bpf_reg_state *reg)
 {
+	s32 old_off, new_off;
+
 	if (!tnum_is_const(reg->var_off)) {
 		pr_info("variable ptr stack access\n");
 		return -EINVAL;
 	}
 
-	if (reg->var_off.value || reg->off) {
-		pr_info("stack access via modified register\n");
-		return -EINVAL;
-	}
+	if (meta->ptr.type == NOT_INIT)
+		return 0;
 
-	return 0;
+	old_off = meta->ptr.off + meta->ptr.var_off.value;
+	new_off = reg->off + reg->var_off.value;
+
+	if (old_off == new_off)
+		return 0;
+
+	pr_info("stack access changed location was:%d is:%d\n",
+		old_off, new_off);
+	return -EINVAL;
 }
 
 static int
@@ -141,7 +151,7 @@ nfp_bpf_check_ptr(struct nfp_prog *nfp_prog, struct nfp_insn_meta *meta,
 	}
 
 	if (reg->type == PTR_TO_STACK) {
-		err = nfp_bpf_check_stack_access(reg);
+		err = nfp_bpf_check_stack_access(meta, reg);
 		if (err)
 			return err;
 	}