Prevent offset wrapping in fd_pread (#258)

This commit inserts a bounds check in `fd_pread` for the `offset`
parameter.  This is necessary because an implicit unsigned-to-signed
integer conversion is performed when `uv_fs_read` is called.  Such a
conversion results in implementation-defined behavior.  One such
behavior is the offset wrapping.

fixes #257

src/uvwasi.c

@@ -1158,7 +1158,7 @@ uvwasi_errno_t uvwasi_fd_pread(uvwasi_t* uvwasi,
                offset,
                nread);
 
-  if (uvwasi == NULL || iovs == NULL || nread == NULL)
+  if (uvwasi == NULL || iovs == NULL || nread == NULL || offset > INT64_MAX)
     return UVWASI_EINVAL;
 
   err = uvwasi_fd_table_get(uvwasi->fds,

test/test-fd-pread-large-offset.c

@@ -0,0 +1,84 @@
+#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"
+
+int main(void) {
+  const char* path = "file.txt";
+  uvwasi_t uvwasi;
+  uvwasi_iovec_t* iovs;
+  uvwasi_fd_t fd;
+  uvwasi_filesize_t large_offset = (uint64_t) INT64_MAX + 1;
+  uvwasi_options_t init_options;
+  uvwasi_rights_t fs_rights_base;
+  uvwasi_size_t nread;
+  uvwasi_size_t iovs_len = 1;
+  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);
+
+  // Create a file.
+  fs_rights_base = UVWASI_RIGHT_FD_DATASYNC |
+                   UVWASI_RIGHT_FD_FILESTAT_GET |
+                   UVWASI_RIGHT_FD_FILESTAT_SET_SIZE |
+                   UVWASI_RIGHT_FD_READ |
+                   UVWASI_RIGHT_FD_SEEK |
+                   UVWASI_RIGHT_FD_SYNC |
+                   UVWASI_RIGHT_FD_TELL |
+                   UVWASI_RIGHT_FD_WRITE |
+                   UVWASI_RIGHT_PATH_UNLINK_FILE;
+  err = uvwasi_path_open(&uvwasi,
+                         3,
+                         1,
+                         path,
+                         strlen(path) + 1,
+                         UVWASI_O_CREAT,
+                         fs_rights_base,
+                         0,
+                         0,
+                         &fd);
+  assert(err == 0);
+
+  iovs = calloc(iovs_len, sizeof(uvwasi_iovec_t));
+  assert(iovs != NULL);
+
+  for (int i = 0; i < iovs_len; i++) {
+    iovs[i].buf_len = 1;
+    iovs[i].buf = malloc(1);
+  }
+
+  err = uvwasi_fd_pread(&uvwasi, fd, iovs, iovs_len, large_offset, &nread);
+  assert(err == UVWASI_EINVAL);
+
+  uvwasi_destroy(&uvwasi);
+
+  for (int i = 0; i < iovs_len; i++) {
+    free((void *) iovs[i].buf);
+  }
+
+  free(iovs);
+  free(init_options.preopens);
+
+  return 0;
+}
+