gh-153400: Add syscall fallbacks for copy_file_range/memfd_create

glibc only grew copy_file_range() and memfd_create() in 2.27, and we compile
the os functions out when the libc we build against doesn't have them. That
loses them for good in a redistributable built against an older glibc, such
as the python-build-standalone builds targeting glibc 2.17, even when the
kernel it runs on implements the syscalls.

Keep calling the libc wrappers when they are available, so we don't lose
their symbol versioning and _FORTIFY_SOURCE checks, and issue the syscall
directly when they aren't. If the syscall number is missing as well, the
functions are still left out. pidfd_open() and pidfd_getfd() already use raw
syscalls, so nothing changes for them.

Include <sys/syscall.h> whenever it exists rather than only when the
getrandom() syscall was detected, since __NR_* is now needed for more than
getrandom().

Signed-off-by: Daan De Meyer <daan@amutable.com>

Author: daandemeyer

Doc/library/os.rst

@@ -1011,10 +1011,14 @@ as internal buffering of data.
       It will always copy no bytes and return 0 as if the file was empty
       because of a known Linux kernel issue.
 
-   .. availability:: Linux >= 4.5 with glibc >= 2.27.
+   .. availability:: Linux >= 4.5.
 
    .. versionadded:: 3.8
 
+   .. versionchanged:: 3.16
+      The function is now also available when Python is built against a libc
+      that lacks ``copy_file_range()``, such as glibc older than 2.27.
+
 
 .. function:: device_encoding(fd)
 
@@ -4400,10 +4404,14 @@ The following flags are used in :attr:`statvfs_result.f_flag`.
    the file descriptor, and as such multiple files can have the same name
    without any side effects.
 
-   .. availability:: Linux >= 3.17 with glibc >= 2.27.
+   .. availability:: Linux >= 3.17.
 
    .. versionadded:: 3.8
 
+   .. versionchanged:: 3.16
+      The function is now also available when Python is built against a libc
+      that lacks ``memfd_create()``, such as glibc older than 2.27.
+
 
 .. data:: MFD_CLOEXEC
           MFD_ALLOW_SEALING

Misc/NEWS.d/next/Library/2026-08-10-22-23-29.gh-issue-153400.nucElC.rst

@@ -0,0 +1,4 @@
+On Linux, :func:`os.copy_file_range` and :func:`os.memfd_create` now fall back
+to the raw syscall when the libc Python is built against does not provide the
+wrapper function, so they stay available on interpreters built against a libc
+older than glibc 2.27.

Modules/clinic/posixmodule.c.h

@@ -153,8 +153,8 @@
 #ifdef HAVE_LINUX_RANDOM_H
 #  include <linux/random.h>       // GRND_RANDOM
 #endif
-#ifdef HAVE_GETRANDOM_SYSCALL
-#  include <sys/syscall.h>        // syscall()
+#ifdef HAVE_SYS_SYSCALL_H
+#  include <sys/syscall.h>        // syscall(), __NR_xxx syscall numbers
 #endif
 
 #ifdef HAVE_POSIX_SPAWN
@@ -13044,7 +13044,12 @@ os_pwritev_impl(PyObject *module, int fd, PyObject *buffers, Py_off_t offset,
 }
 #endif /* HAVE_PWRITEV */
 
-#ifdef HAVE_COPY_FILE_RANGE
+#if defined(HAVE_COPY_FILE_RANGE) || \
+    (defined(__linux__) && defined(__NR_copy_file_range))
+#  define _Py_HAVE_COPY_FILE_RANGE
+#endif
+
+#ifdef _Py_HAVE_COPY_FILE_RANGE
 /*[clinic input]
 
 os.copy_file_range
@@ -13096,7 +13101,13 @@ os_copy_file_range_impl(PyObject *module, int src, int dst, Py_ssize_t count,
 
     do {
         Py_BEGIN_ALLOW_THREADS
+#ifdef HAVE_COPY_FILE_RANGE
         ret = copy_file_range(src, p_offset_src, dst, p_offset_dst, count, flags);
+#else
+        /* Largefile support makes off_t 64-bit, as the kernel expects. */
+        ret = syscall(__NR_copy_file_range, src, p_offset_src, dst, p_offset_dst,
+                      count, flags);
+#endif
         Py_END_ALLOW_THREADS
     } while (ret < 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
 
@@ -13106,7 +13117,7 @@ os_copy_file_range_impl(PyObject *module, int src, int dst, Py_ssize_t count,
 
     return PyLong_FromSsize_t(ret);
 }
-#endif /* HAVE_COPY_FILE_RANGE*/
+#endif /* _Py_HAVE_COPY_FILE_RANGE */
 
 #if (defined(HAVE_SPLICE) && !defined(_AIX))
 /*[clinic input]
@@ -15808,7 +15819,12 @@ os_urandom_impl(PyObject *module, Py_ssize_t size)
     return PyBytesWriter_Finish(writer);
 }
 
-#ifdef HAVE_MEMFD_CREATE
+#if defined(HAVE_MEMFD_CREATE) || \
+    (defined(__linux__) && defined(__NR_memfd_create) && defined(MFD_CLOEXEC))
+#  define _Py_HAVE_MEMFD_CREATE
+#endif
+
+#ifdef _Py_HAVE_MEMFD_CREATE
 /*[clinic input]
 os.memfd_create
 
@@ -15824,7 +15840,11 @@ os_memfd_create_impl(PyObject *module, PyObject *name, unsigned int flags)
     int fd;
     const char *bytes = PyBytes_AS_STRING(name);
     Py_BEGIN_ALLOW_THREADS
+#ifdef HAVE_MEMFD_CREATE
     fd = memfd_create(bytes, flags);
+#else
+    fd = syscall(__NR_memfd_create, bytes, flags);
+#endif
     Py_END_ALLOW_THREADS
     if (fd == -1) {
         return PyErr_SetFromErrno(PyExc_OSError);
@@ -18433,7 +18453,7 @@ all_ins(PyObject *m)
     if (PyModule_AddIntMacro(m, GRND_RANDOM)) return -1;
     if (PyModule_AddIntMacro(m, GRND_NONBLOCK)) return -1;
 #endif
-#ifdef HAVE_MEMFD_CREATE
+#ifdef _Py_HAVE_MEMFD_CREATE
     if (PyModule_AddIntMacro(m, MFD_CLOEXEC)) return -1;
     if (PyModule_AddIntMacro(m, MFD_ALLOW_SEALING)) return -1;
 #ifdef MFD_HUGETLB
@@ -18481,7 +18501,7 @@ all_ins(PyObject *m)
 #ifdef MFD_HUGE_16GB
     if (PyModule_AddIntMacro(m, MFD_HUGE_16GB)) return -1;
 #endif
-#endif /* HAVE_MEMFD_CREATE */
+#endif /* _Py_HAVE_MEMFD_CREATE */
 
 #if defined(HAVE_EVENTFD) && defined(EFD_CLOEXEC)
     if (PyModule_AddIntMacro(m, EFD_CLOEXEC)) return -1;
@@ -18737,7 +18757,7 @@ static const struct have_function {
     { "HAVE_LUTIMES", NULL },
 #endif
 
-#ifdef HAVE_MEMFD_CREATE
+#ifdef _Py_HAVE_MEMFD_CREATE
     { "HAVE_MEMFD_CREATE", NULL },
 #endif