lkl: add LKL_HIJACK_SYSCTL to configure sysctl values

Documentation/lkl.txt

@@ -238,6 +238,14 @@ are the list of those variable for your environment.
 ```
      $ LKL_HIJACK_OFFLOAD=0x8002 lkl-hijack.sh ./netserver -D -f
 ```
+* LKL_HIJACK_SYSCTL
+
+  Configure sysctl values of the booted kernel via the hijack library. Multiple
+  entries can be specified.
+```
+     $ LKL_HIJACK_SYSCTL="net.ipv4.tcp_wmem=4096 87380 2147483647"
+     ./bin/lkl-hijack.sh ip address show
+```
 
 FAQ
 ===

tools/lkl/include/lkl.h

@@ -15,6 +15,10 @@ extern "C" {
 #undef class
 #endif
 
+#if defined(__MINGW32__)
+#define strtok_r strtok_s
+#endif
+
 #if __LKL__BITS_PER_LONG == 64
 #define lkl_sys_stat lkl_sys_newstat
 #define lkl_sys_lstat lkl_sys_newlstat
@@ -479,6 +483,21 @@ int lkl_qdisc_add(int ifindex, char *root, char *type);
  */
 void lkl_qdisc_parse_add(int ifindex, char *entries);
 
+/**
+ * lkl_sysctl - write a sysctl value
+ *
+ * @path - the path to an sysctl entry (e.g., "net.ipv4.tcp_wmem");
+ * @value - the value of the sysctl (e.g., "4096 87380 2147483647")
+ */
+int lkl_sysctl(const char *path, const char *value);
+
+/**
+ * lkl_sysctl_parse_write - Configure sysctl parameters with strings
+ *
+ * @sysctls - Configure sysctl parameters as the form of "key=value;..."
+ */
+void lkl_sysctl_parse_write(const char *sysctls);
+
 #ifdef __cplusplus
 }
 #endif

tools/lkl/lib/hijack/init.c

@@ -220,6 +220,7 @@ hijack_init(void)
 	char *offload1 = getenv("LKL_HIJACK_OFFLOAD");
 	int offload = 0;
 	char boot_cmdline[256] = "\0";
+	char *sysctls = getenv("LKL_HIJACK_SYSCTL");
 
 	memset(&nd_args, 0, sizeof(struct lkl_netdev_args));
 	if (!debug) {
@@ -436,6 +437,8 @@ hijack_init(void)
 	if (nd_ifindex >= 0 && qdisc_entries)
 		lkl_qdisc_parse_add(nd_ifindex, qdisc_entries);
 
+	if (sysctls)
+		lkl_sysctl_parse_write(sysctls);
 }
 
 void __attribute__((destructor))

tools/lkl/lib/utils.c

@@ -1,5 +1,6 @@
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 #include <lkl_host.h>
 
 static const char * const lkl_err_strings[] = {
@@ -205,3 +206,58 @@ void lkl_bug(const char *fmt, ...)
 
 	lkl_host_ops.panic();
 }
+
+int lkl_sysctl(const char *path, const char *value)
+{
+	int ret;
+	int fd;
+	char *delim, *p;
+	char full_path[256];
+
+	lkl_mount_fs("proc");
+
+	snprintf(full_path, sizeof(full_path), "/proc/sys/%s", path);
+	p = full_path;
+	while ((delim = strstr(p, "."))) {
+		*delim = '/';
+		p = delim + 1;
+	}
+
+	fd = lkl_sys_open(full_path, LKL_O_WRONLY | LKL_O_CREAT, 0);
+	if (fd < 0) {
+		lkl_printf("lkl_sys_open %s: %s\n",
+			   full_path, lkl_strerror(fd));
+		return -1;
+	}
+	ret = lkl_sys_write(fd, value, strlen(value));
+	if (ret < 0) {
+		lkl_printf("lkl_sys_write %s: %s\n",
+			full_path, lkl_strerror(fd));
+	}
+
+	lkl_sys_close(fd);
+
+	return 0;
+}
+
+/* Configure sysctl parameters as the form of "key=value;key=value;..." */
+void lkl_sysctl_parse_write(const char *sysctls)
+{
+	char *saveptr = NULL, *token = NULL;
+	char *key = NULL, *value = NULL;
+	char strings[256];
+	int ret = 0;
+
+	strcpy(strings, sysctls);
+	for (token = strtok_r(strings, ";", &saveptr); token;
+	     token = strtok_r(NULL, ";", &saveptr)) {
+		key = strtok(token, "=");
+		value = strtok(NULL, "=");
+		ret = lkl_sysctl(key, value);
+		if (ret) {
+			lkl_printf("Failed to configure sysctl entries: %s\n",
+				   lkl_strerror(ret));
+			return;
+		}
+	}
+}