Fix read() on a directory fd returning EBADF instead of EISDIR

libc-bottom-half/cloudlibc/src/libc/unistd/read.c

@@ -21,6 +21,13 @@ ssize_t read(int fildes, void *buf, size_t nbyte) {
   size_t bytes_read;
   __wasi_errno_t error = __wasi_fd_read(fildes, &iov, 1, &bytes_read);
   if (error != 0) {
+    __wasi_fdstat_t fds;
+    if ((error == __WASI_ERRNO_BADF || error == ENOTCAPABLE) &&
+        __wasi_fd_fdstat_get(fildes, &fds) == 0 &&
+        fds.fs_filetype == __WASI_FILETYPE_DIRECTORY) {
+      errno = EISDIR;
+      return -1;
+    }
     errno = error == ENOTCAPABLE ? EBADF : error;
     return -1;
   }

libc-bottom-half/sources/file.c

@@ -81,6 +81,29 @@ static void file_free(void *data) {
   free(file);
 }
 
+// True when `file` refers to a directory. Used to return POSIX EISDIR
+// instead of the host's `bad-descriptor`. On wasip2 this runs only after
+// `read-via-stream` fails. On wasip3 that call traps, so the check runs
+// before opening the stream.
+static bool file_is_directory(file_t *file) {
+  filesystem_error_code_t error;
+#ifdef __wasip2__
+  filesystem_descriptor_type_t ty;
+  if (!filesystem_method_descriptor_get_type(
+          filesystem_borrow_descriptor(file->file_handle), &ty, &error))
+    return false;
+  return ty == FILESYSTEM_DESCRIPTOR_TYPE_DIRECTORY;
+#else
+  filesystem_descriptor_stat_t st;
+  if (!filesystem_method_descriptor_stat(
+          filesystem_borrow_descriptor(file->file_handle), &st, &error))
+    return false;
+  bool is_dir = st.type.tag == FILESYSTEM_DESCRIPTOR_TYPE_DIRECTORY;
+  filesystem_descriptor_stat_free(&st);
+  return is_dir;
+#endif
+}
+
 #ifndef __wasip2__
 static int file_read_eof(void *data) {
   file_t *file = (file_t *)data;
@@ -94,6 +117,11 @@ static int file_read_eof(void *data) {
     filesystem_future_result_void_error_code_drop_readable(file->read_result);
     file->read_result = 0;
     if (result.is_err) {
+      if (result.val.err.tag == FILESYSTEM_ERROR_CODE_BAD_DESCRIPTOR &&
+          file_is_directory(file)) {
+        errno = EISDIR;
+        return -1;
+      }
       translate_error(&result.val.err);
       return -1;
     }
@@ -115,6 +143,11 @@ static int file_get_read_stream(void *data, wasi_read_t *read) {
         filesystem_borrow_descriptor(file->file_handle), file->offset,
         &file->read_stream, &error_code);
     if (!ok) {
+      if (error_code == FILESYSTEM_ERROR_CODE_BAD_DESCRIPTOR &&
+          file_is_directory(file)) {
+        errno = EISDIR;
+        return -1;
+      }
       translate_error(&error_code);
       return -1;
     }
@@ -123,6 +156,12 @@ static int file_get_read_stream(void *data, wasi_read_t *read) {
   read->pollable = &file->read_pollable;
 #else
   if (!wasip3_io_state_present(&file->read)) {
+    // wasip3 `read-via-stream` returns a stream, not a result. Wasmtime
+    // traps with ErrorCode::BadDescriptor on a directory. Check first.
+    if (file_is_directory(file)) {
+      errno = EISDIR;
+      return -1;
+    }
     assert(!file->read_result);
     filesystem_tuple2_stream_u8_future_result_void_error_code_t result;
     filesystem_method_descriptor_read_via_stream(

test/CMakeLists.txt

@@ -488,6 +488,7 @@ add_wasilibc_test(ppoll.c FS)
 add_wasilibc_test(pselect.c FS)
 add_wasilibc_test(preadvwritev.c FS)
 add_wasilibc_test(preadwrite.c FS)
+add_wasilibc_test(read-dir-eisdir.c FS)
 add_wasilibc_test(readlink.c FS)
 add_wasilibc_test(readv.c FS)
 add_wasilibc_test(rename.c FS)

test/src/read-dir-eisdir.c

@@ -0,0 +1,42 @@
+#include "test.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#define TEST(c)                                                                \
+  do {                                                                         \
+    errno = 0;                                                                 \
+    if (!(c))                                                                  \
+      t_error("%s failed (errno = %d)\n", #c, errno);                          \
+  } while (0)
+
+// POSIX read() on a directory fd must fail with EISDIR, not EBADF.
+// wasip2 read-via-stream fails with bad-descriptor (mapped to EBADF).
+// wasip3 read-via-stream traps with BadDescriptor unless we check type first.
+int main(void) {
+  char buf[16];
+
+  TEST(mkdir("read-dir-eisdir", 0755) == 0);
+  int dirfd = open("read-dir-eisdir", O_RDONLY | O_DIRECTORY);
+  TEST(dirfd > 2);
+
+  TEST(read(dirfd, buf, sizeof buf) == -1 && errno == EISDIR);
+
+  TEST(close(dirfd) == 0);
+
+  // Genuine bad fds stay EBADF; regular files still read.
+  TEST(read(-1, buf, 1) == -1 && errno == EBADF);
+
+  int fd;
+  TEST((fd = open("read-dir-eisdir/f", O_RDWR | O_CREAT | O_EXCL, 0600)) > 2);
+  TEST(write(fd, "x", 1) == 1);
+  TEST(lseek(fd, 0, SEEK_SET) == 0);
+  TEST(read(fd, buf, 1) == 1);
+  TEST(buf[0] == 'x');
+  TEST(close(fd) == 0);
+  TEST(unlink("read-dir-eisdir/f") == 0);
+  TEST(rmdir("read-dir-eisdir") == 0);
+
+  return t_status;
+}