selftests/bpf: Update sockopt_sk test to the use bpf_set_retval

The tests would break without this patch, because at one point it calls
  getsockopt(fd, SOL_TCP, TCP_ZEROCOPY_RECEIVE, &buf, &optlen)
This getsockopt receives the kernel-set -EINVAL. Prior to this patch
series, the eBPF getsockopt hook's -EPERM would override kernel's
-EINVAL, however, after this patch series, return 0's automatic
-EPERM will not; the eBPF prog has to explicitly bpf_set_retval(-EPERM)
if that is wanted.

I also removed the explicit mentions of EPERM in the comments in the
prog.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/4f20b77cb46812dbc2bdcd7e3fa87c7573bde55e.1639619851.git.zhuyifei@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/sockopt_sk.c

@@ -173,11 +173,11 @@ static int getsetsockopt(void)
 	}
 
 	memset(&buf, 0, sizeof(buf));
-	buf.zc.address = 12345; /* rejected by BPF */
+	buf.zc.address = 12345; /* Not page aligned. Rejected by tcp_zerocopy_receive() */
 	optlen = sizeof(buf.zc);
 	errno = 0;
 	err = getsockopt(fd, SOL_TCP, TCP_ZEROCOPY_RECEIVE, &buf, &optlen);
-	if (errno != EPERM) {
+	if (errno != EINVAL) {
 		log_err("Unexpected getsockopt(TCP_ZEROCOPY_RECEIVE) err=%d errno=%d",
 			err, errno);
 		goto err;

tools/testing/selftests/bpf/progs/sockopt_sk.c

@@ -73,17 +73,17 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		 */
 
 		if (optval + sizeof(struct tcp_zerocopy_receive) > optval_end)
-			return 0; /* EPERM, bounds check */
+			return 0; /* bounds check */
 
 		if (((struct tcp_zerocopy_receive *)optval)->address != 0)
-			return 0; /* EPERM, unexpected data */
+			return 0; /* unexpected data */
 
 		return 1;
 	}
 
 	if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
 		if (optval + 1 > optval_end)
-			return 0; /* EPERM, bounds check */
+			return 0; /* bounds check */
 
 		ctx->retval = 0; /* Reset system call return value to zero */
 
@@ -96,24 +96,24 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		 * bytes of data.
 		 */
 		if (optval_end - optval != page_size)
-			return 0; /* EPERM, unexpected data size */
+			return 0; /* unexpected data size */
 
 		return 1;
 	}
 
 	if (ctx->level != SOL_CUSTOM)
-		return 0; /* EPERM, deny everything except custom level */
+		return 0; /* deny everything except custom level */
 
 	if (optval + 1 > optval_end)
-		return 0; /* EPERM, bounds check */
+		return 0; /* bounds check */
 
 	storage = bpf_sk_storage_get(&socket_storage_map, ctx->sk, 0,
 				     BPF_SK_STORAGE_GET_F_CREATE);
 	if (!storage)
-		return 0; /* EPERM, couldn't get sk storage */
+		return 0; /* couldn't get sk storage */
 
 	if (!ctx->retval)
-		return 0; /* EPERM, kernel should not have handled
+		return 0; /* kernel should not have handled
 			   * SOL_CUSTOM, something is wrong!
 			   */
 	ctx->retval = 0; /* Reset system call return value to zero */
@@ -152,7 +152,7 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		/* Overwrite SO_SNDBUF value */
 
 		if (optval + sizeof(__u32) > optval_end)
-			return 0; /* EPERM, bounds check */
+			return 0; /* bounds check */
 
 		*(__u32 *)optval = 0x55AA;
 		ctx->optlen = 4;
@@ -164,7 +164,7 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		/* Always use cubic */
 
 		if (optval + 5 > optval_end)
-			return 0; /* EPERM, bounds check */
+			return 0; /* bounds check */
 
 		memcpy(optval, "cubic", 5);
 		ctx->optlen = 5;
@@ -175,10 +175,10 @@ int _setsockopt(struct bpf_sockopt *ctx)
 	if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
 		/* Original optlen is larger than PAGE_SIZE. */
 		if (ctx->optlen != page_size * 2)
-			return 0; /* EPERM, unexpected data size */
+			return 0; /* unexpected data size */
 
 		if (optval + 1 > optval_end)
-			return 0; /* EPERM, bounds check */
+			return 0; /* bounds check */
 
 		/* Make sure we can trim the buffer. */
 		optval[0] = 0;
@@ -189,21 +189,21 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		 * bytes of data.
 		 */
 		if (optval_end - optval != page_size)
-			return 0; /* EPERM, unexpected data size */
+			return 0; /* unexpected data size */
 
 		return 1;
 	}
 
 	if (ctx->level != SOL_CUSTOM)
-		return 0; /* EPERM, deny everything except custom level */
+		return 0; /* deny everything except custom level */
 
 	if (optval + 1 > optval_end)
-		return 0; /* EPERM, bounds check */
+		return 0; /* bounds check */
 
 	storage = bpf_sk_storage_get(&socket_storage_map, ctx->sk, 0,
 				     BPF_SK_STORAGE_GET_F_CREATE);
 	if (!storage)
-		return 0; /* EPERM, couldn't get sk storage */
+		return 0; /* couldn't get sk storage */
 
 	storage->val = optval[0];
 	ctx->optlen = -1; /* BPF has consumed this option, don't call kernel