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gh-81793: Always call linkat() from os.link(), if available (GH-132517)

Browse source 
This fixes os.link() on platforms (like Linux and OpenIndiana) where the
system link() function does not follow symlinks.

* On Linux, it now follows symlinks by default and if
  follow_symlinks=True is specified.
* On Windows, it now raises error if follow_symlinks=True is passed.
* On macOS, it now raises error if follow_symlinks=False is passed and
  the system linkat() function is not available at runtime.
* On other platforms, it now raises error if follow_symlinks is passed
  with a value that does not match the system link() function behavior
  if if the behavior is not known.

Co-authored-by: Joachim Henke <37883863+jo-he@users.noreply.github.com>
Co-authored-by: Thomas Kluyver <takowl@gmail.com>
This commit is contained in:
Serhiy Storchaka 2025-05-04 17:24:10 +03:00 • committed by GitHub
parent e9253ebf74
commit 5a57248b22
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GPG key ID: B5690EEEBB952194
6 changed files with 100 additions and 67 deletions
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1
Doc/library/os.rst
View file

@ -2338,6 +2338,7 @@ features:
This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
supply :ref:`paths relative to directory descriptors <dir_fd>`, and :ref:`not
following symlinks <follow_symlinks>`.
The default value of *follow_symlinks* is ``False`` on Windows.
.. audit-event:: os.link src,dst,src_dir_fd,dst_dir_fd os.link

2
Lib/test/test_inspect/test_inspect.py
View file

@ -5844,7 +5844,7 @@ class TestSignatureDefinitions(unittest.TestCase):
self._test_module_has_signatures(operator)
def test_os_module_has_signatures(self):
unsupported_signature = {'chmod', 'utime'}
unsupported_signature = {'chmod', 'link', 'utime'}
unsupported_signature |= {name for name in
['get_terminal_size', 'posix_spawn', 'posix_spawnp',
'register_at_fork', 'startfile']

44
Lib/test/test_posix.py
View file

@ -1521,6 +1521,50 @@ class PosixTester(unittest.TestCase):
self.assertEqual(cm.exception.errno, errno.EINVAL)
os.close(os.pidfd_open(os.getpid(), 0))
@unittest.skipUnless(hasattr(os, "link"), "test needs os.link()")
def test_link_follow_symlinks(self):
default_follow = sys.platform.startswith(
('darwin', 'freebsd', 'netbsd', 'openbsd', 'dragonfly', 'sunos5'))
default_no_follow = sys.platform.startswith(('win32', 'linux'))
orig = os_helper.TESTFN
symlink = orig + 'symlink'
posix.symlink(orig, symlink)
self.addCleanup(os_helper.unlink, symlink)
with self.subTest('no follow_symlinks'):
# no follow_symlinks -> platform depending
link = orig + 'link'
posix.link(symlink, link)
self.addCleanup(os_helper.unlink, link)
if os.link in os.supports_follow_symlinks or default_follow:
self.assertEqual(posix.lstat(link), posix.lstat(orig))
elif default_no_follow:
self.assertEqual(posix.lstat(link), posix.lstat(symlink))
with self.subTest('follow_symlinks=False'):
# follow_symlinks=False -> duplicate the symlink itself
link = orig + 'link_nofollow'
try:
posix.link(symlink, link, follow_symlinks=False)
except NotImplementedError:
if os.link in os.supports_follow_symlinks or default_no_follow:
raise
else:
self.addCleanup(os_helper.unlink, link)
self.assertEqual(posix.lstat(link), posix.lstat(symlink))
with self.subTest('follow_symlinks=True'):
# follow_symlinks=True -> duplicate the target file
link = orig + 'link_following'
try:
posix.link(symlink, link, follow_symlinks=True)
except NotImplementedError:
if os.link in os.supports_follow_symlinks or default_follow:
raise
else:
self.addCleanup(os_helper.unlink, link)
self.assertEqual(posix.lstat(link), posix.lstat(orig))
# tests for the posix *at functions follow
class TestPosixDirFd(unittest.TestCase):

7
Misc/NEWS.d/next/Library/2025-04-14-17-24-50.gh-issue-81793.OhRTTT.rst Normal file
View file

@ -0,0 +1,7 @@
Fix :func:`os.link` on platforms (like Linux) where the
system :c:func:`!link` function does not follow symlinks. On Linux,
it now follows symlinks by default or if
``follow_symlinks=True`` is specified. On Windows, it now raises an error if
``follow_symlinks=True`` is passed. On macOS, it now raises an error if
``follow_symlinks=False`` is passed and the system :c:func:`!linkat`
function is not available at runtime.

6
Modules/clinic/posixmodule.c.h generated
View file

@ -1471,7 +1471,7 @@ os_getcwdb(PyObject *module, PyObject *Py_UNUSED(ignored))
PyDoc_STRVAR(os_link__doc__,
"link($module, /, src, dst, *, src_dir_fd=None, dst_dir_fd=None,\n"
" follow_symlinks=True)\n"
" follow_symlinks=(os.name != \'nt\'))\n"
"--\n"
"\n"
"Create a hard link to a file.\n"
@ -1530,7 +1530,7 @@ os_link(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwn
path_t dst = PATH_T_INITIALIZE_P("link", "dst", 0, 0, 0, 0);
int src_dir_fd = DEFAULT_DIR_FD;
int dst_dir_fd = DEFAULT_DIR_FD;
int follow_symlinks = 1;
int follow_symlinks = -1;
args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser,
/*minpos*/ 2, /*maxpos*/ 2, /*minkw*/ 0, /*varpos*/ 0, argsbuf);
@ -13398,4 +13398,4 @@ os__emscripten_debugger(PyObject *module, PyObject *Py_UNUSED(ignored))
#ifndef OS__EMSCRIPTEN_DEBUGGER_METHODDEF
#define OS__EMSCRIPTEN_DEBUGGER_METHODDEF
#endif /* !defined(OS__EMSCRIPTEN_DEBUGGER_METHODDEF) */
/*[clinic end generated code: output=a5ca2541f2af5462 input=a9049054013a1b77]*/
/*[clinic end generated code: output=f7b5635e0b948be4 input=a9049054013a1b77]*/

107
Modules/posixmodule.c
View file

@ -573,7 +573,11 @@ extern char *ctermid_r(char *);
# define HAVE_FACCESSAT_RUNTIME 1
# define HAVE_FCHMODAT_RUNTIME 1
# define HAVE_FCHOWNAT_RUNTIME 1
#ifdef __wasi__
# define HAVE_LINKAT_RUNTIME 0
# else
# define HAVE_LINKAT_RUNTIME 1
# endif
# define HAVE_FDOPENDIR_RUNTIME 1
# define HAVE_MKDIRAT_RUNTIME 1
# define HAVE_RENAMEAT_RUNTIME 1
@ -4346,7 +4350,7 @@ os.link
*
src_dir_fd : dir_fd = None
dst_dir_fd : dir_fd = None
follow_symlinks: bool = True
follow_symlinks: bool(c_default="-1", py_default="(os.name != 'nt')") = PLACEHOLDER
Create a hard link to a file.
@ -4364,31 +4368,46 @@ src_dir_fd, dst_dir_fd, and follow_symlinks may not be implemented on your
static PyObject *
os_link_impl(PyObject *module, path_t *src, path_t *dst, int src_dir_fd,
int dst_dir_fd, int follow_symlinks)
/*[clinic end generated code: output=7f00f6007fd5269a input=b0095ebbcbaa7e04]*/
/*[clinic end generated code: output=7f00f6007fd5269a input=1d5e602d115fed7b]*/
{
#ifdef MS_WINDOWS
BOOL result = FALSE;
#else
int result;
#endif
#if defined(HAVE_LINKAT)
int linkat_unavailable = 0;
#endif
#ifndef HAVE_LINKAT
if ((src_dir_fd != DEFAULT_DIR_FD) || (dst_dir_fd != DEFAULT_DIR_FD)) {
argument_unavailable_error("link", "src_dir_fd and dst_dir_fd");
return NULL;
#ifdef HAVE_LINKAT
if (HAVE_LINKAT_RUNTIME) {
if (follow_symlinks < 0) {
follow_symlinks = 1;
}
}
else
#endif
{
if ((src_dir_fd != DEFAULT_DIR_FD) || (dst_dir_fd != DEFAULT_DIR_FD)) {
argument_unavailable_error("link", "src_dir_fd and dst_dir_fd");
return NULL;
}
/* See issue 85527: link() on Linux works like linkat without AT_SYMLINK_FOLLOW,
but on Mac it works like linkat *with* AT_SYMLINK_FOLLOW. */
#if defined(MS_WINDOWS) || defined(__linux__)
if (follow_symlinks == 1) {
argument_unavailable_error("link", "follow_symlinks=True");
return NULL;
}
#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__) || (defined(__sun) && defined(__SVR4))
if (follow_symlinks == 0) {
argument_unavailable_error("link", "follow_symlinks=False");
return NULL;
}
#else
if (follow_symlinks >= 0) {
argument_unavailable_error("link", "follow_symlinks");
return NULL;
}
#endif
#ifndef MS_WINDOWS
if ((src->narrow && dst->wide) || (src->wide && dst->narrow)) {
PyErr_SetString(PyExc_NotImplementedError,
"link: src and dst must be the same type");
return NULL;
}
#endif
if (PySys_Audit("os.link", "OOii", src->object, dst->object,
src_dir_fd == DEFAULT_DIR_FD ? -1 : src_dir_fd,
@ -4406,44 +4425,18 @@ os_link_impl(PyObject *module, path_t *src, path_t *dst, int src_dir_fd,
#else
Py_BEGIN_ALLOW_THREADS
#ifdef HAVE_LINKAT
if ((src_dir_fd != DEFAULT_DIR_FD) ||
(dst_dir_fd != DEFAULT_DIR_FD) ||
(!follow_symlinks)) {
if (HAVE_LINKAT_RUNTIME) {
result = linkat(src_dir_fd, src->narrow,
dst_dir_fd, dst->narrow,
follow_symlinks ? AT_SYMLINK_FOLLOW : 0);
}
#ifdef __APPLE__
else {
if (src_dir_fd == DEFAULT_DIR_FD && dst_dir_fd == DEFAULT_DIR_FD) {
/* See issue 41355: This matches the behaviour of !HAVE_LINKAT */
result = link(src->narrow, dst->narrow);
} else {
linkat_unavailable = 1;
}
}
#endif
if (HAVE_LINKAT_RUNTIME) {
result = linkat(src_dir_fd, src->narrow,
dst_dir_fd, dst->narrow,
follow_symlinks ? AT_SYMLINK_FOLLOW : 0);
}
else
#endif /* HAVE_LINKAT */
result = link(src->narrow, dst->narrow);
Py_END_ALLOW_THREADS
#ifdef HAVE_LINKAT
if (linkat_unavailable) {
/* Either or both dir_fd arguments were specified */
if (src_dir_fd != DEFAULT_DIR_FD) {
argument_unavailable_error("link", "src_dir_fd");
} else {
argument_unavailable_error("link", "dst_dir_fd");
}
return NULL;
}
#endif
{
/* linkat not available */
result = link(src->narrow, dst->narrow);
}
Py_END_ALLOW_THREADS
if (result)
return path_error2(src, dst);
@ -5935,12 +5928,6 @@ internal_rename(path_t *src, path_t *dst, int src_dir_fd, int dst_dir_fd, int is
return path_error2(src, dst);
#else
if ((src->narrow && dst->wide) || (src->wide && dst->narrow)) {
PyErr_Format(PyExc_ValueError,
"%s: src and dst must be the same type", function_name);
return NULL;
}
Py_BEGIN_ALLOW_THREADS
#ifdef HAVE_RENAMEAT
if (dir_fd_specified) {
@ -10613,12 +10600,6 @@ os_symlink_impl(PyObject *module, path_t *src, path_t *dst,
#else
if ((src->narrow && dst->wide) || (src->wide && dst->narrow)) {
PyErr_SetString(PyExc_ValueError,
"symlink: src and dst must be the same type");
return NULL;
}
Py_BEGIN_ALLOW_THREADS
#ifdef HAVE_SYMLINKAT
if (dir_fd != DEFAULT_DIR_FD) {