From 58a03a80744bdb8e1a21c9ea321df1159f8eb283 Mon Sep 17 00:00:00 2001
From: "Samuel D. Leslie" <sdl@nexiom.net>
Date: Sat, 27 May 2017 21:26:18 +1000
Subject: [PATCH] Add support for creating symlinks as an unprivileged user

This requires Windows 10 Creators Update (v1703) or newer as the
underlying API support isn't present in earlier releases of Windows.
Further, it's currently restricted by the OS to systems which have
Developer Mode enabled. If these requirements aren't met, symlink
handling remains unchanged from previous releases of Windows.

The support is provided by a new flag to CreateSymbolicLinkW:
SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE

More details are on the official developer blog announcement:
https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/
---
 compat/mingw.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/compat/mingw.c b/compat/mingw.c
index 1898a012e7b950..48855da67ac405 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -272,6 +272,16 @@ int mingw_core_config(const char *var, const char *value)
 
 DECLARE_PROC_ADDR(kernel32.dll, BOOL, CreateSymbolicLinkW, LPCWSTR, LPCWSTR, DWORD);
 
+#ifndef SYMBOLIC_LINK_FLAG_DIRECTORY
+#define SYMBOLIC_LINK_FLAG_DIRECTORY 0x1
+#endif
+
+#ifndef SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+#define SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE 0x2
+#endif
+
+static int has_unprivileged_symlinks = 0;
+
 enum phantom_symlink_result {
 	PHANTOM_SYMLINK_RETRY,
 	PHANTOM_SYMLINK_DONE,
@@ -286,6 +296,7 @@ static enum phantom_symlink_result process_phantom_symlink(
 		const wchar_t *wtarget, const wchar_t *wlink) {
 	HANDLE hnd;
 	BY_HANDLE_FILE_INFORMATION fdata;
+	DWORD symlink_flags = SYMBOLIC_LINK_FLAG_DIRECTORY;
 
 	/* check that wlink is still a file symlink */
 	if ((GetFileAttributesW(wlink)
@@ -313,8 +324,13 @@ static enum phantom_symlink_result process_phantom_symlink(
 	if (!(fdata.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY))
 		return PHANTOM_SYMLINK_DONE;
 
+	/* Permit creating symlink as an unprivileged user if supported */
+	if (has_unprivileged_symlinks == 1) {
+		symlink_flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
+	}
+
 	/* otherwise recreate the symlink with directory flag */
-	if (DeleteFileW(wlink) && CreateSymbolicLinkW(wlink, wtarget, 1))
+	if (DeleteFileW(wlink) && CreateSymbolicLinkW(wlink, wtarget, symlink_flags))
 		return PHANTOM_SYMLINK_DIRECTORY;
 
 	errno = err_win_to_posix(GetLastError());
@@ -2639,6 +2655,7 @@ int symlink(const char *target, const char *link)
 {
 	wchar_t wtarget[MAX_LONG_PATH], wlink[MAX_LONG_PATH];
 	int len;
+	DWORD symlink_flags = 0;
 
 	/* fail if symlinks are disabled or API is not supported (WinXP) */
 	if (!has_symlinks || !INIT_PROC_ADDR(CreateSymbolicLinkW)) {
@@ -2655,8 +2672,13 @@ int symlink(const char *target, const char *link)
 		if (wtarget[len] == '/')
 			wtarget[len] = '\\';
 
+	/* Permit creating symlink as an unprivileged user if supported */
+	if (has_unprivileged_symlinks == 1) {
+		symlink_flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
+	}
+
 	/* create file symlink */
-	if (!CreateSymbolicLinkW(wlink, wtarget, 0)) {
+	if (!CreateSymbolicLinkW(wlink, wtarget, symlink_flags)) {
 		errno = err_win_to_posix(GetLastError());
 		return -1;
 	}
@@ -3021,6 +3043,18 @@ static void setup_windows_environment(void)
 	if (!(tmp = getenv("MSYS")) || !strstr(tmp, "winsymlinks:nativestrict"))
 		has_symlinks = 0;
 
+	/* Check if this release of Windows supports creating symbolic links as an
+	 * unprivileged user. The underlying API support was introduced in the
+	 * Windows 10 Creators Update (v1703). Older releases of Windows don't
+	 * understand the new flag to the CreateSymbolicLink family of functions.
+	 */
+	struct utsname winver;
+	if (uname(&winver) == 0) {
+		if (atoi(winver.version) >= 15063) {
+			has_unprivileged_symlinks = 1;
+		}
+	}
+
 	if (!getenv("LC_ALL") && !getenv("LC_CTYPE") && !getenv("LANG"))
 		setenv("LC_CTYPE", "C", 1);
 }