bpf,test: Fix verifier issues in IPv6 BPF tests when running locally

When running the `TestBPF/ipv6_test.o` test locally with clang v13 and
kernel v5.17 it resulted in the following verifier error:

```
171: (b7) r2 = 1                      ; R2_w=inv1
; authhdr->seq = 1;
172: (63) *(u32 *)(r1 +8) = r2
invalid access to packet, off=8 size=4, R1(id=6,off=8,r=2)
R1 offset is outside of the packet
```

Tracked this down to `ipv6_with_hop_auth_tcp_pktgen`. It gets a pointer
from `pktgen__append_ipv6_extension_header`. All expected bounds checks
were in place. It even has a function called `ctx_data_valid` with some
inline assembly which is supposed to perform additional bounds checks.

Was able to fix this by replacing `ctx_data_valid` with a normal check
and adding a bounds check to `ipv6_with_hop_auth_tcp_pktgen`. This
bounds check seems to be necessary because bounds checks do not seem
to apply to copies.

The verifier log reveals the following:

```
85: (69) r2 = *(u16 *)(r10 -40)       ; R2_w=invP14 R10=fp0
; builder->layer_offsets[layer_idx] = builder->cur_off;
86: (6b) *(u16 *)(r4 +10) = r2        ; R2_w=invP14 R4_w=fp-44 fp-40=mmmmmmmm
; layer = ctx_data(ctx) + builder->cur_off;
87: (0f) r1 += r7                     ; R1_w=pkt(id=0,off=14,r=54,imm=0) R7=invP14
; builder->cur_off += hdrsize;
88: (69) r2 = *(u16 *)(r10 -40)       ; R2_w=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)) R10=fp0
```

At instruction 85 we know the offset is `14` exactly. It is then written
back to the stack. And at 88 we have lost bounds data. Since this is
an offset we use to construct the pointers, this uncertainty becomes
part of all pointers causing the need for additional checks.

To mitigate the result of the above apparent verifier bug/limitation,
I have also changes our offset value from 16 bit to 64 bit which tracks
better.

Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>

bpf/tests/ipv6_test.c

@@ -144,6 +144,9 @@ int ipv6_with_hop_auth_tcp_pktgen(struct __ctx_buff *ctx)
 		return TEST_ERROR;
 
 	authhdr = (struct ipv6_authhdr *)l3_next;
+	if ((void *) authhdr + sizeof(struct ipv6_authhdr) > ctx_data_end(ctx))
+		return TEST_ERROR;
+
 	authhdr->spi = 0x222;
 	authhdr->seq = 1;
 

bpf/tests/pktgen.h

@@ -147,8 +147,8 @@ enum pkt_layer {
 /* Packet builder */
 struct pktgen {
 	struct __ctx_buff *ctx;
-	__u16 cur_off;
-	__u16 layer_offsets[PKT_BUILDER_LAYERS];
+	__u64 cur_off;
+	__u64 layer_offsets[PKT_BUILDER_LAYERS];
 	enum pkt_layer layers[PKT_BUILDER_LAYERS];
 };
 
@@ -176,29 +176,6 @@ int pktgen__free_layer(const struct pktgen *builder)
 	return -1;
 }
 
-static __always_inline
-__attribute__((warn_unused_result))
-int ctx_data_valid(const struct __ctx_buff *ctx, __maybe_unused __u16 off)
-{
-	int ret = 0;
-
-	/* try to open up a range on the pkt reg */
-	asm volatile("r1 = *(u32 *)(%[ctx] +0)\n\t"
-		     "r2 = *(u32 *)(%[ctx] +4)\n\t"
-		     "%[off] &= %[offmax]\n\t"
-		     "r1 += %[off]\n\t"
-		     "if r1 > r2 goto +2\n\t"
-		     "%[ret] = 0\n\t"
-		     "goto +1\n\t"
-		     "%[ret] = %[errno]\n\t"
-		     : [ret]"=r"(ret)
-		     : [ctx]"r"(ctx), [off]"r"(off),
-		       [offmax]"i"(0xff), [errno]"i"(-EINVAL)
-		     : "r1", "r2");
-
-	return ret;
-}
-
 /* Push an empty ethernet header onto the packet */
 static __always_inline
 __attribute__((warn_unused_result))
@@ -398,11 +375,7 @@ struct ipv6_opt_hdr *pktgen__append_ipv6_extension_header(struct pktgen *builder
 	if (!hdr)
 		return NULL;
 
-	/* after multiple extension headers are added, LLVM tends to generate
-	 * code that verifier doesn't understand. try to open up a range on
-	 * the pkt reg
-	 */
-	if (ctx_data_valid(builder->ctx, builder->cur_off))
+	if ((void *) hdr + sizeof(struct ipv6_opt_hdr) > ctx_data_end(builder->ctx))
 		return NULL;
 
 	hdr->hdrlen = hdrlen;
@@ -537,7 +510,7 @@ void *pktgen__push_data_room(struct pktgen *builder, int len)
 	/* Check that any value within the struct will not exceed a u16 which
 	 * is the max allowed offset within a packet from ctx->data.
 	 */
-	if (builder->cur_off >= MAX_PACKET_OFF - len)
+	if ((__s64)builder->cur_off >= MAX_PACKET_OFF - len)
 		return 0;
 
 	layer = ctx_data(ctx) + builder->cur_off;
@@ -581,10 +554,10 @@ void pktgen__finish(const struct pktgen *builder)
 	struct ipv6hdr *ipv6_layer;
 	struct ipv6_opt_hdr *ipv6_opt_layer;
 	struct tcphdr *tcp_layer;
-	__u16 layer_off;
+	__u64 layer_off;
 	__u16 v4len;
 	__be16 v6len;
-	__u16 hdr_size;
+	__u64 hdr_size;
 
 	#pragma unroll
 	for (int i = 0; i < PKT_BUILDER_LAYERS; i++) {
@@ -780,7 +753,7 @@ void pktgen__finish(const struct pktgen *builder)
 						builder->layer_offsets[i];
 			}
 
-			tcp_layer->doff = hdr_size / 4;
+			tcp_layer->doff = (__u16)hdr_size / 4;
 
 			break;