From 06082c7d377152594bddfcaf2017cf4a6f9b6990 Mon Sep 17 00:00:00 2001
From: Justine Tunney <jtunney@gmail.com>
Date: Fri, 28 Jul 2023 07:37:25 -0700
Subject: [PATCH] Polyfill fchmodat()

---
 libc/calls/fchmodat-linux.c         | 81 +++++++++++++++++++++++++++++
 libc/calls/fchmodat.c               |  8 ++-
 test/libc/calls/mkntenvblock_test.c |  5 ++
 3 files changed, 93 insertions(+), 1 deletion(-)
 create mode 100644 libc/calls/fchmodat-linux.c

diff --git a/libc/calls/fchmodat-linux.c b/libc/calls/fchmodat-linux.c
new file mode 100644
index 000000000..5e6b1f392
--- /dev/null
+++ b/libc/calls/fchmodat-linux.c
@@ -0,0 +1,81 @@
+/*-*- mode:c;indent-tabs-mode:t;c-basic-offset:8;tab-width:8;coding:utf-8   -*-│
+│vi: set et ft=c ts=8 tw=8 fenc=utf-8                                       :vi│
+╚──────────────────────────────────────────────────────────────────────────────╝
+│                                                                              │
+│  Musl Libc                                                                   │
+│  Copyright © 2005-2014 Rich Felker, et al.                                   │
+│                                                                              │
+│  Permission is hereby granted, free of charge, to any person obtaining       │
+│  a copy of this software and associated documentation files (the             │
+│  "Software"), to deal in the Software without restriction, including         │
+│  without limitation the rights to use, copy, modify, merge, publish,         │
+│  distribute, sublicense, and/or sell copies of the Software, and to          │
+│  permit persons to whom the Software is furnished to do so, subject to       │
+│  the following conditions:                                                   │
+│                                                                              │
+│  The above copyright notice and this permission notice shall be              │
+│  included in all copies or substantial portions of the Software.             │
+│                                                                              │
+│  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,             │
+│  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF          │
+│  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.      │
+│  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY        │
+│  CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,        │
+│  TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE           │
+│  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                      │
+│                                                                              │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/struct/stat.internal.h"
+#include "libc/calls/syscall-sysv.internal.h"
+#include "libc/errno.h"
+#include "libc/sysv/consts/at.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/sysv/consts/s.h"
+#include "libc/sysv/errfuns.h"
+
+asm(".ident\t\"\\n\\n\
+Musl libc (MIT License)\\n\
+Copyright 2005-2014 Rich Felker, et. al.\"");
+asm(".include \"libc/disclaimer.inc\"");
+// clang-format off
+
+static void __procfdname(char *buf, unsigned fd) {
+	unsigned i, j;
+	for (i = 0; (buf[i] = "/proc/self/fd/"[i]); i++);
+	if (!fd) {
+		buf[i] = '0';
+		buf[i + 1] = 0;
+		return;
+	}
+	for (j = fd; j; j /= 10, i++);
+	buf[i] = 0;
+	for (; fd; fd /= 10) buf[--i] = '0' + fd % 10;
+}
+
+int sys_fchmodat_linux(int fd, const char *path, unsigned mode, int flag) {
+	struct stat st;
+	int ret, fd2;
+	char proc[15 + 3 * sizeof(int)];
+	if (flag != AT_SYMLINK_NOFOLLOW)
+		return einval();
+	if (sys_fstatat(fd, path, &st, flag))
+		return -1;
+	if (S_ISLNK(st.st_mode))
+		return eopnotsupp();
+	if ((fd2 = sys_openat(fd, path, (O_RDONLY | O_PATH | O_NOFOLLOW |
+					 O_NOCTTY | O_CLOEXEC), 0)) < 0) {
+		if (fd2 == -ELOOP)
+			return eopnotsupp();
+		return fd2;
+	}
+	__procfdname(proc, fd2);
+	ret = sys_fstatat(AT_FDCWD, proc, &st, 0);
+	if (!ret) {
+		if (S_ISLNK(st.st_mode))
+			ret = eopnotsupp();
+		else
+			ret = sys_fchmodat(AT_FDCWD, proc, mode, 0);
+	}
+	sys_close(fd2);
+	return ret;
+}
diff --git a/libc/calls/fchmodat.c b/libc/calls/fchmodat.c
index 6dbf00637..1b4de8647 100644
--- a/libc/calls/fchmodat.c
+++ b/libc/calls/fchmodat.c
@@ -27,6 +27,8 @@
 #include "libc/sysv/errfuns.h"
 #include "libc/zipos/zipos.internal.h"
 
+int sys_fchmodat_linux(int, const char *, unsigned, int);
+
 /**
  * Changes permissions on file, e.g.:
  *
@@ -50,7 +52,11 @@ int fchmodat(int dirfd, const char *path, uint32_t mode, int flags) {
              (rc = __zipos_notat(dirfd, path)) == -1) {
     rc = enotsup();
   } else if (!IsWindows()) {
-    rc = sys_fchmodat(dirfd, path, mode, flags);
+    if (IsLinux() && flags) {
+      rc = sys_fchmodat_linux(dirfd, path, mode, flags);
+    } else {
+      rc = sys_fchmodat(dirfd, path, mode, flags);
+    }
   } else {
     rc = sys_fchmodat_nt(dirfd, path, mode, flags);
   }
diff --git a/test/libc/calls/mkntenvblock_test.c b/test/libc/calls/mkntenvblock_test.c
index 6cd545462..7ddd6eccd 100644
--- a/test/libc/calls/mkntenvblock_test.c
+++ b/test/libc/calls/mkntenvblock_test.c
@@ -18,11 +18,16 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/ntspawn.h"
 #include "libc/mem/gc.internal.h"
+#include "libc/runtime/runtime.h"
 #include "libc/testlib/testlib.h"
 
 char tmp[ARG_MAX];
 char16_t envvars[ARG_MAX / 2];
 
+void SetUpOnce(void) {
+  environ = 0;  // pacify systemroot
+}
+
 TEST(mkntenvblock, emptyList_onlyOutputsDoubleNulStringTerminator) {
   char *envp[] = {NULL};
   ASSERT_NE(-1, mkntenvblock(envvars, envp, NULL, tmp));