diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index f229c4e8dd9322..5e735e86bdee9e 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -11507,9 +11507,6 @@ static PyObject *
 os_read_impl(PyObject *module, int fd, Py_ssize_t length)
 /*[clinic end generated code: output=dafbe9a5cddb987b input=1df2eaa27c0bf1d3]*/
 {
-    Py_ssize_t n;
-    PyObject *buffer;
-
     if (length < 0) {
         errno = EINVAL;
         return posix_error();
@@ -11517,20 +11514,18 @@ os_read_impl(PyObject *module, int fd, Py_ssize_t length)
 
     length = Py_MIN(length, _PY_READ_MAX);
 
-    buffer = PyBytes_FromStringAndSize((char *)NULL, length);
-    if (buffer == NULL)
+    PyBytesWriter *writer = PyBytesWriter_Create(length);
+    if (writer == NULL) {
         return NULL;
+    }
 
-    n = _Py_read(fd, PyBytes_AS_STRING(buffer), length);
+    Py_ssize_t n = _Py_read(fd, PyBytesWriter_GetData(writer), length);
     if (n == -1) {
-        Py_DECREF(buffer);
+        PyBytesWriter_Discard(writer);
         return NULL;
     }
 
-    if (n != length)
-        _PyBytes_Resize(&buffer, n);
-
-    return buffer;
+    return PyBytesWriter_FinishWithSize(writer, n);
 }
 
 /*[clinic input]
@@ -11708,20 +11703,20 @@ os_pread_impl(PyObject *module, int fd, Py_ssize_t length, Py_off_t offset)
 {
     Py_ssize_t n;
     int async_err = 0;
-    PyObject *buffer;
 
     if (length < 0) {
         errno = EINVAL;
         return posix_error();
     }
-    buffer = PyBytes_FromStringAndSize((char *)NULL, length);
-    if (buffer == NULL)
+    PyBytesWriter *writer = PyBytesWriter_Create(length);
+    if (writer == NULL) {
         return NULL;
+    }
 
     do {
         Py_BEGIN_ALLOW_THREADS
         _Py_BEGIN_SUPPRESS_IPH
-        n = pread(fd, PyBytes_AS_STRING(buffer), length, offset);
+        n = pread(fd, PyBytesWriter_GetData(writer), length, offset);
         _Py_END_SUPPRESS_IPH
         Py_END_ALLOW_THREADS
     } while (n < 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
@@ -11730,12 +11725,10 @@ os_pread_impl(PyObject *module, int fd, Py_ssize_t length, Py_off_t offset)
         if (!async_err) {
             posix_error();
         }
-        Py_DECREF(buffer);
+        PyBytesWriter_Discard(writer);
         return NULL;
     }
-    if (n != length)
-        _PyBytes_Resize(&buffer, n);
-    return buffer;
+    return PyBytesWriter_FinishWithSize(writer, n);
 }
 #endif /* HAVE_PREAD */
 
@@ -14945,9 +14938,6 @@ os_getxattr_impl(PyObject *module, path_t *path, path_t *attribute,
                  int follow_symlinks)
 /*[clinic end generated code: output=5f2f44200a43cff2 input=025789491708f7eb]*/
 {
-    Py_ssize_t i;
-    PyObject *buffer = NULL;
-
     if (fd_and_follow_symlinks_invalid("getxattr", path->fd, follow_symlinks))
         return NULL;
 
@@ -14955,8 +14945,7 @@ os_getxattr_impl(PyObject *module, path_t *path, path_t *attribute,
         return NULL;
     }
 
-    for (i = 0; ; i++) {
-        void *ptr;
+    for (Py_ssize_t i = 0; ; i++) {
         ssize_t result;
         static const Py_ssize_t buffer_sizes[] = {128, XATTR_SIZE_MAX, 0};
         Py_ssize_t buffer_size = buffer_sizes[i];
@@ -14964,10 +14953,11 @@ os_getxattr_impl(PyObject *module, path_t *path, path_t *attribute,
             path_error(path);
             return NULL;
         }
-        buffer = PyBytes_FromStringAndSize(NULL, buffer_size);
-        if (!buffer)
+        PyBytesWriter *writer = PyBytesWriter_Create(buffer_size);
+        if (writer == NULL) {
             return NULL;
-        ptr = PyBytes_AS_STRING(buffer);
+        }
+        void *ptr = PyBytesWriter_GetData(writer);
 
         Py_BEGIN_ALLOW_THREADS;
         if (path->fd >= 0)
@@ -14979,23 +14969,16 @@ os_getxattr_impl(PyObject *module, path_t *path, path_t *attribute,
         Py_END_ALLOW_THREADS;
 
         if (result < 0) {
+            PyBytesWriter_Discard(writer);
             if (errno == ERANGE) {
-                Py_DECREF(buffer);
                 continue;
             }
             path_error(path);
-            Py_DECREF(buffer);
             return NULL;
         }
 
-        if (result != buffer_size) {
-            /* Can only shrink. */
-            _PyBytes_Resize(&buffer, result);
-        }
-        break;
+        return PyBytesWriter_FinishWithSize(writer, result);
     }
-
-    return buffer;
 }
 
 
@@ -15222,19 +15205,22 @@ static PyObject *
 os_urandom_impl(PyObject *module, Py_ssize_t size)
 /*[clinic end generated code: output=42c5cca9d18068e9 input=58a0def87dbc2c22]*/
 {
-    PyObject *bytes;
-    int result;
+    if (size < 0) {
+        return PyErr_Format(PyExc_ValueError,
+                            "negative argument not allowed");
+    }
 
-    bytes = PyBytes_FromStringAndSize(NULL, size);
-    if (bytes == NULL)
+    PyBytesWriter *writer = PyBytesWriter_Create(size);
+    if (writer == NULL) {
         return NULL;
+    }
 
-    result = _PyOS_URandom(PyBytes_AS_STRING(bytes), PyBytes_GET_SIZE(bytes));
+    int result = _PyOS_URandom(PyBytesWriter_GetData(writer), size);
     if (result == -1) {
-        Py_DECREF(bytes);
+        PyBytesWriter_Discard(writer);
         return NULL;
     }
-    return bytes;
+    return PyBytesWriter_Finish(writer);
 }
 
 #ifdef HAVE_MEMFD_CREATE
@@ -16704,25 +16690,20 @@ static PyObject *
 os_getrandom_impl(PyObject *module, Py_ssize_t size, int flags)
 /*[clinic end generated code: output=b3a618196a61409c input=59bafac39c594947]*/
 {
-    PyObject *bytes;
-    Py_ssize_t n;
-
     if (size < 0) {
         errno = EINVAL;
         return posix_error();
     }
 
-    bytes = PyBytes_FromStringAndSize(NULL, size);
-    if (bytes == NULL) {
-        PyErr_NoMemory();
+    PyBytesWriter *writer = PyBytesWriter_Create(size);
+    if (writer == NULL) {
         return NULL;
     }
+    void *data = PyBytesWriter_GetData(writer);
 
+    Py_ssize_t n;
     while (1) {
-        n = syscall(SYS_getrandom,
-                    PyBytes_AS_STRING(bytes),
-                    PyBytes_GET_SIZE(bytes),
-                    flags);
+        n = syscall(SYS_getrandom, data, size, flags);
         if (n < 0 && errno == EINTR) {
             if (PyErr_CheckSignals() < 0) {
                 goto error;
@@ -16739,14 +16720,10 @@ os_getrandom_impl(PyObject *module, Py_ssize_t size, int flags)
         goto error;
     }
 
-    if (n != size) {
-        _PyBytes_Resize(&bytes, n);
-    }
-
-    return bytes;
+    return PyBytesWriter_FinishWithSize(writer, n);
 
 error:
-    Py_DECREF(bytes);
+    PyBytesWriter_Discard(writer);
     return NULL;
 }
 #endif   /* HAVE_GETRANDOM_SYSCALL */