wasi: support resolving symlinks

This commit adds support for resolving symlinks during
path resolution. This commit also ports all of the remaining
tests from nodejs/node#27850, with
the 'poll' test currently disabled.

Author: cjihrig

deps/uvwasi/src/uvwasi.c

@@ -34,11 +34,12 @@ static uvwasi_errno_t uvwasi__resolve_path(const struct uvwasi_fd_wrap_t* fd,
   /* TODO(cjihrig): path_len is treated as a size. Need to verify if path_len is
      really a string length or a size. Also need to verify if it is null
      terminated. */
-  /* TODO(cjihrig): flags is currently unused. */
+  uv_fs_t realpath_req;
   uvwasi_errno_t err;
   char* abs_path;
   char* tok;
   char* ptr;
+  int realpath_size;
   int abs_size;
   int input_is_absolute;
   int r;
@@ -96,6 +97,27 @@ static uvwasi_errno_t uvwasi__resolve_path(const struct uvwasi_fd_wrap_t* fd,
     ptr += r;
   }
 
+  if ((flags & UVWASI_LOOKUP_SYMLINK_FOLLOW) == UVWASI_LOOKUP_SYMLINK_FOLLOW) {
+    r = uv_fs_realpath(NULL, &realpath_req, resolved_path, NULL);
+    if (r == 0) {
+      realpath_size = strlen(realpath_req.ptr) + 1;
+      if (realpath_size > PATH_MAX_BYTES) {
+        err = UVWASI_ENOBUFS;
+        uv_fs_req_cleanup(&realpath_req);
+        goto exit;
+      }
+
+      memcpy(resolved_path, realpath_req.ptr, realpath_size);
+    } else if (r != UV_ENOENT) {
+      /* Report errors except ENOENT. */
+      err = uvwasi__translate_uv_error(r);
+      uv_fs_req_cleanup(&realpath_req);
+      goto exit;
+    }
+
+    uv_fs_req_cleanup(&realpath_req);
+  }
+
   /* Verify that the resolved path is still in the sandbox. */
   if (resolved_path != strstr(resolved_path, fd->real_path)) {
     err = UVWASI_ENOTCAPABLE;

test/fixtures/outside.txt

@@ -0,0 +1,2 @@
+this file is part of the WASI tests. it exists outside of the sandbox, and
+should be inaccessible from the WASI tests.

test/fixtures/wasi/subdir/input_link.txt

@@ -0,0 +1 @@
+../input.txt

test/fixtures/wasi/subdir/loop1

@@ -0,0 +1 @@
+./loop2

test/fixtures/wasi/subdir/loop2

@@ -0,0 +1 @@
+./loop1

test/fixtures/wasi/subdir/outside.txt

@@ -0,0 +1 @@
+../../outside.txt

test/wasi/c/follow_symlink.c

@@ -0,0 +1,14 @@
+#include <assert.h>
+#include <stdio.h>
+
+int main() {
+  FILE* file = fopen("/sandbox/subdir/input_link.txt", "r");
+  assert(file != NULL);
+
+  char c = fgetc(file);
+  while (c != EOF) {
+    int wrote = fputc(c, stdout);
+    assert(wrote != EOF);
+    c = fgetc(file);
+  }
+}

test/wasi/c/poll.c

@@ -0,0 +1,31 @@
+#include <assert.h>
+#include <poll.h>
+#include <time.h>
+#include <unistd.h>
+
+int main(void) {
+  struct pollfd fds[2];
+  time_t before, now;
+  int ret;
+
+  fds[0] = (struct pollfd){.fd = 1, .events = POLLOUT, .revents = 0};
+  fds[1] = (struct pollfd){.fd = 2, .events = POLLOUT, .revents = 0};
+
+  ret = poll(fds, 2, -1);
+  assert(ret == 2);
+  assert(fds[0].revents == POLLOUT);
+  assert(fds[1].revents == POLLOUT);
+
+  fds[0] = (struct pollfd){.fd = 0, .events = POLLIN, .revents = 0};
+  time(&before);
+  ret = poll(fds, 1, 2000);
+  time(&now);
+  assert(ret == 0);
+  assert(now - before >= 2);
+
+  sleep(1);
+  time(&now);
+  assert(now - before >= 3);
+
+  return 0;
+}

test/wasi/c/symlink_escape.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+
+int main() {
+  FILE* file = fopen("/sandbox/subdir/outside.txt", "r");
+  assert(file == NULL);
+  assert(errno == ENOTCAPABLE);
+}

test/wasi/c/symlink_loop.c

@@ -0,0 +1,9 @@
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+
+int main() {
+  FILE* file = fopen("/sandbox/subdir/loop1", "r");
+  assert(file == NULL);
+  assert(errno == ELOOP);
+}