Error if creating symlink to absolute path (#272)

This commit adds an absolute path check to `path_symlink`. This behavior
is specified by [WASI][1].

[1]: https://github.com/WebAssembly/wasi-filesystem/blob/main/path-resolution.md#symlinks

fixes #271

Signed-off-by: Yage Hu <me@huyage.dev>

src/uvwasi.c

@@ -2401,6 +2401,12 @@ uvwasi_errno_t uvwasi_path_symlink(uvwasi_t* uvwasi,
   memcpy(truncated_old_path, old_path, old_path_len);
   truncated_old_path[old_path_len] = '\0';
 
+  if (old_path_len > 0 && old_path[0] == '/') {
+    uv_mutex_unlock(&wrap->mutex);
+    uvwasi__free(uvwasi, truncated_old_path);
+    return UVWASI_EPERM;
+  }
+
   err = uvwasi__resolve_path(uvwasi,
                              wrap,
                              new_path,

test/test-path-symlink-absolute-path.c

@@ -0,0 +1,47 @@
+#include <assert.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include "uvwasi.h"
+#include "uv.h"
+#include "test-common.h"
+
+#define TEST_TMP_DIR "./out/tmp"
+#define LINK_CONTENT "/absolute"
+#define TEST_FILE    "./out/tmp/test-path-symlink-absolute-path.link"
+
+int main(void) {
+  uvwasi_t uvwasi;
+  uvwasi_options_t init_options;
+  uvwasi_errno_t err;
+  uv_fs_t req;
+  int r;
+
+  setup_test_environment();
+
+  r = uv_fs_mkdir(NULL, &req, TEST_TMP_DIR, 0777, NULL);
+  uv_fs_req_cleanup(&req);
+  assert(r == 0 || r == UV_EEXIST);
+
+  uvwasi_options_init(&init_options);
+  init_options.preopenc = 1;
+  init_options.preopens = calloc(1, sizeof(uvwasi_preopen_t));
+  init_options.preopens[0].mapped_path = "/var";
+  init_options.preopens[0].real_path = TEST_TMP_DIR;
+
+  err = uvwasi_init(&uvwasi, &init_options);
+  assert(err == 0);
+
+  err = uvwasi_path_symlink(&uvwasi,
+                            LINK_CONTENT,
+                            strlen(LINK_CONTENT) + 1,
+                            3,
+                            TEST_FILE,
+                            strlen(TEST_FILE) + 1);
+  assert(err == UVWASI_EPERM);
+
+  uvwasi_destroy(&uvwasi);
+  free(init_options.preopens);
+
+  return 0;
+}