Re: fchmodat with AT_SYMLINK_NOFOLLOW

[Bruno Haible <bruno@clisp.org>]

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Hi Paul,

> I installed the attached patch to Gnulib in preparation for the upcoming 
> glibc fix. The patch causes fchmodat with AT_SYMLINK_NOFOLLOW to work on 
> non-symlinks, and similarly for lchmod on non-symlinks. The idea is to 
> avoid this sort of problem in the future, and to let Coreutils etc. work 
> on older platforms as if glibc 2.32 (or whatever) is already in place.

Some improvements:

1) Improve the cross-compilation guesses. The result of the
"fchmodat+AT_SYMLINK_NOFOLLOW works on non-symlinks" test is:
  - yes on kFreeBSD/glibc, Hurd/glibc, FreeBSD 12, AIX 7.2, Solaris 11, Haiku,
  - no on Linux/glibc, Cygwin 2.9.

2) On Cygwin, the functions fchmodat and lchown crash. The cause is an endless
recursion, because some of the #includes in fchmodat.c includes the full
<sys/stat.h>, including the '#define fchmodat rpl_fchmodat'.

3) Strengthen the unit test, and make sure that it does not write files that
other unit tests could possibly write as well.
This test is a bit tricky, because on native Windows, we cannot do arbitrary
chmods:
  - chmod of 700 is equivalent to 600 (since it does not have an execute bit
    on the file system),
  - chmod of 600 sets the mode to 666 - since there is no distinction between
    users, group, and world with this API.
  - After a chmod of 400 (= read-only), unlink() fails.


2020-02-08  Bruno Haible  <bruno@clisp.org>

	fchmodat: Strengthen tests.
	* tests/test-fchmodat.c (BASE): New macro.
	(main): Use it, to avoid conflicts with other unit tests. Verify that
	fchmodat changed the file permission bits.

2020-02-08  Bruno Haible  <bruno@clisp.org>

	fchmodat: Fix endless recursion on Cygwin (regression from 2020-02-07).
	* lib/fchmodat.c (orig_fchmodat): Move definition to immediately after
	'#undef __need_system_sys_stat_h'.

2020-02-08  Bruno Haible  <bruno@clisp.org>

	fchmodat: Improve cross-compilation guesses.
	* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): Require AC_CANONICAL_HOST. When
	cross-compiling, guess depending on the platform.
	* doc/posix-functions/fchmodat.texi: Clarify.


[-- Attachment #2: 0001-fchmodat-Improve-cross-compilation-guesses.patch --]
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From a680228fe4d39f749cb819e45202c6fec6ca9d29 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 8 Feb 2020 20:38:01 +0100
Subject: [PATCH 1/3] fchmodat: Improve cross-compilation guesses.

* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): Require AC_CANONICAL_HOST. When
cross-compiling, guess depending on the platform.
* doc/posix-functions/fchmodat.texi: Clarify.
---
 ChangeLog                         |  7 +++++++
 doc/posix-functions/fchmodat.texi | 10 +++++-----
 m4/fchmodat.m4                    | 14 +++++++++++---
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index df48e88..32d9a00 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-02-08  Bruno Haible  <bruno@clisp.org>
 
+	fchmodat: Improve cross-compilation guesses.
+	* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): Require AC_CANONICAL_HOST. When
+	cross-compiling, guess depending on the platform.
+	* doc/posix-functions/fchmodat.texi: Clarify.
+
+2020-02-08  Bruno Haible  <bruno@clisp.org>
+
 	Fix compilation errors in a testdir created with --with-c++-tests.
 	* lib/c++defs.h (_GL_CXXALIASWARN1_2): Do not use __typeof__ (func),
 	since it does not work any more with g++ >= 4.4.
diff --git a/doc/posix-functions/fchmodat.texi b/doc/posix-functions/fchmodat.texi
index 4d19031..a295f83 100644
--- a/doc/posix-functions/fchmodat.texi
+++ b/doc/posix-functions/fchmodat.texi
@@ -9,15 +9,15 @@ Gnulib module: fchmodat
 Portability problems fixed by Gnulib:
 @itemize
 @item
-When given the @code{AT_SYMLINK_NOFOLLOW} flag,
-this function fails with @code{errno} set to @code{ENOTSUP},
-even when the file is not a symbolic link:
-GNU/Linux and Cygwin with glibc 2.31.
-@item
 This function is missing on some platforms:
 glibc 2.3.6, Mac OS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8,
 AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 10, Cygwin 1.5.x, mingw, MSVC 14.
 But the replacement function is not safe to be used in libraries and is not multithread-safe.
+@item
+When given the @code{AT_SYMLINK_NOFOLLOW} flag,
+this function fails with @code{errno} set to @code{ENOTSUP},
+even when the file is not a symbolic link:
+GNU/Linux with glibc 2.31, Cygwin 2.9.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/fchmodat.m4 b/m4/fchmodat.m4
index 8195ef6..f284485 100644
--- a/m4/fchmodat.m4
+++ b/m4/fchmodat.m4
@@ -1,4 +1,4 @@
-# fchmodat.m4 serial 2
+# fchmodat.m4 serial 3
 dnl Copyright (C) 2004-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -10,6 +10,7 @@ AC_DEFUN([gl_FUNC_FCHMODAT],
 [
   AC_REQUIRE([gl_SYS_STAT_H_DEFAULTS])
   AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
   AC_CHECK_FUNCS_ONCE([fchmodat lchmod])
   if test $ac_cv_func_fchmodat != yes; then
     HAVE_FCHMODAT=0
@@ -17,7 +18,9 @@ AC_DEFUN([gl_FUNC_FCHMODAT],
     AC_CACHE_CHECK(
       [whether fchmodat+AT_SYMLINK_NOFOLLOW works on non-symlinks],
       [gl_cv_func_fchmodat_works],
-      [AC_RUN_IFELSE(
+      [dnl This test fails on GNU/Linux with glibc 2.31 (but not on
+       dnl GNU/kFreeBSD nor GNU/Hurd) and Cygwin 2.9.
+       AC_RUN_IFELSE(
          [AC_LANG_PROGRAM(
             [
               AC_INCLUDES_DEFAULT[
@@ -53,7 +56,12 @@ AC_DEFUN([gl_FUNC_FCHMODAT],
             ]])],
          [gl_cv_func_fchmodat_works=yes],
          [gl_cv_func_fchmodat_works=no],
-         [gl_cv_func_fchmodat_works=$gl_cross_guess_normal])
+         [case "$host_os" in
+            dnl Guess no on Linux with glibc and Cygwin, yes otherwise.
+            linux-gnu* | cygwin*) gl_cv_func_fchmodat_works="guessing no" ;;
+            *)                    gl_cv_func_fchmodat_works="$gl_cross_guess_normal" ;;
+          esac
+         ])
        rm -f conftest.fchmodat])
     case $gl_cv_func_fchmodat_works in
       *yes) ;;
-- 
2.7.4


[-- Attachment #3: 0002-fchmodat-Fix-endless-recursion-on-Cygwin-regression-.patch --]
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From 1bbbce29dc592833e79ab6d21472528a01ab67b2 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 8 Feb 2020 20:41:20 +0100
Subject: [PATCH 2/3] fchmodat: Fix endless recursion on Cygwin (regression
 from 2020-02-07).

* lib/fchmodat.c (orig_fchmodat): Move definition to immediately after
'#undef __need_system_sys_stat_h'.
---
 ChangeLog      |  6 ++++++
 lib/fchmodat.c | 12 ++++++------
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 32d9a00..a5ca210 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2020-02-08  Bruno Haible  <bruno@clisp.org>
 
+	fchmodat: Fix endless recursion on Cygwin (regression from 2020-02-07).
+	* lib/fchmodat.c (orig_fchmodat): Move definition to immediately after
+	'#undef __need_system_sys_stat_h'.
+
+2020-02-08  Bruno Haible  <bruno@clisp.org>
+
 	fchmodat: Improve cross-compilation guesses.
 	* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): Require AC_CANONICAL_HOST. When
 	cross-compiling, guess depending on the platform.
diff --git a/lib/fchmodat.c b/lib/fchmodat.c
index c6b8ef7..87aa0d1 100644
--- a/lib/fchmodat.c
+++ b/lib/fchmodat.c
@@ -26,12 +26,6 @@
 #include <sys/stat.h>
 #undef __need_system_sys_stat_h
 
-#include <errno.h>
-#include <fcntl.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <unistd.h>
-
 #if HAVE_FCHMODAT
 static int
 orig_fchmodat (int dir, char const *file, mode_t mode, int flags)
@@ -40,6 +34,12 @@ orig_fchmodat (int dir, char const *file, mode_t mode, int flags)
 }
 #endif
 
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
 #ifdef __osf__
 /* Write "sys/stat.h" here, not <sys/stat.h>, otherwise OSF/1 5.1 DTK cc
    eliminates this include because of the preliminary #include <sys/stat.h>
-- 
2.7.4


[-- Attachment #4: 0003-fchmodat-Strengthen-tests.patch --]
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From c2d184186846851f173bec9e60781a05001837c6 Mon Sep 17 00:00:00 2001
From: Bruno Haible <bruno@clisp.org>
Date: Sat, 8 Feb 2020 20:47:58 +0100
Subject: [PATCH 3/3] fchmodat: Strengthen tests.

* tests/test-fchmodat.c (BASE): New macro.
(main): Use it, to avoid conflicts with other unit tests. Verify that
fchmodat changed the file permission bits.
---
 ChangeLog             |  7 +++++++
 tests/test-fchmodat.c | 14 +++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index a5ca210..fc07914 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-02-08  Bruno Haible  <bruno@clisp.org>
 
+	fchmodat: Strengthen tests.
+	* tests/test-fchmodat.c (BASE): New macro.
+	(main): Use it, to avoid conflicts with other unit tests. Verify that
+	fchmodat changed the file permission bits.
+
+2020-02-08  Bruno Haible  <bruno@clisp.org>
+
 	fchmodat: Fix endless recursion on Cygwin (regression from 2020-02-07).
 	* lib/fchmodat.c (orig_fchmodat): Move definition to immediately after
 	'#undef __need_system_sys_stat_h'.
diff --git a/tests/test-fchmodat.c b/tests/test-fchmodat.c
index df0f604..f6b695b 100644
--- a/tests/test-fchmodat.c
+++ b/tests/test-fchmodat.c
@@ -28,6 +28,8 @@ SIGNATURE_CHECK (fchmodat, int, (int, const char *, mode_t, int));
 
 #include "macros.h"
 
+#define BASE "test-fchmodat."
+
 int
 main (void)
 {
@@ -47,9 +49,15 @@ main (void)
   /* Test that fchmodat works on non-symlinks, when given
      the AT_SYMLINK_NOFOLLOW flag.  */
   {
-    ASSERT (close (creat ("file", 0600)) == 0);
-    ASSERT (fchmodat (AT_FDCWD, "file", 0700, AT_SYMLINK_NOFOLLOW) == 0);
-    ASSERT (unlink ("file") == 0);
+    struct stat statbuf;
+    unlink (BASE "file");
+    ASSERT (close (creat (BASE "file", 0600)) == 0);
+    ASSERT (fchmodat (AT_FDCWD, BASE "file", 0400, AT_SYMLINK_NOFOLLOW) == 0);
+    ASSERT (stat (BASE "file", &statbuf) >= 0);
+    ASSERT ((statbuf.st_mode & 0700) == 0400);
+    /* Clean up.  */
+    ASSERT (chmod (BASE "file", 0600) == 0);
+    ASSERT (unlink (BASE "file") == 0);
   }
 
   return 0;
-- 
2.7.4