win32: implement symlink(2)

Provide an implementation of symlink(2).

Calls to symlink(2) will fail in default Windows installations unless
running with elevated privileges.  Failure to create a symlink when
extracting files from an archive is therefore treated as a non-fatal
error.

There are two ways to permit the creation of symlinks:

- Edit security policy to give users the 'Create symbolic links'
  privilege.  Unfortunately this doesn't work for users who are an
  Administrator.

- Enable developer mode, which is available in later versions of
  Windows 10.

The ability to create symlinks is not available in Windows XP
or ReactOS.

3 files changed, 55 insertions, 12 deletions

diff --git a/archival/libarchive/unsafe_symlink_target.c b/archival/libarchive/unsafe_symlink_target.c
index b5f5a2670..8e4cd4380 100644
--- a/archival/libarchive/unsafe_symlink_target.c
+++ b/archival/libarchive/unsafe_symlink_target.c
@@ -5,12 +5,17 @@
 #include "libbb.h"
 #include "bb_archive.h"
 
+/* symlink may not be available for WIN32, just issue a warning */
+#if ENABLE_PLATFORM_MINGW32
+# undef bb_perror_msg_and_die
+# define bb_perror_msg_and_die(...) bb_perror_msg(__VA_ARGS__)
+#endif
+
 void FAST_FUNC create_or_remember_link(llist_t **link_placeholders,
                 const char *target,
                 const char *linkname,
                 int hard_link)
 {
-#if !ENABLE_PLATFORM_MINGW32
         if (hard_link || target[0] == '/' || strstr(target, "..")) {
                 llist_add_to_end(link_placeholders,
                         xasprintf("%c%s%c%s", hard_link, linkname, '\0', target)
@@ -23,16 +28,6 @@ void FAST_FUNC create_or_remember_link(llist_t **link_placeholders,
                         "sym", linkname, target
                 );
         }
-#else
-        if (hard_link) {
-                llist_add_to_end(link_placeholders,
-                        xasprintf("%c%s%c%s", hard_link, linkname, '\0', target)
-                );
-                return;
-        }
-        /* symlink isn't implemented for WIN32, just issue a warning */
-        bb_perror_msg("can't create symlink '%s' to '%s'", linkname, target);
-#endif
 }
 
 void FAST_FUNC create_links_from_list(llist_t *list)
@@ -42,11 +37,24 @@ void FAST_FUNC create_links_from_list(llist_t *list)
 
                 target = list->data + 1 + strlen(list->data + 1) + 1;
                 if ((*list->data ? link : symlink) (target, list->data + 1)) {
+#if !ENABLE_PLATFORM_MINGW32
                         /* shared message */
                         bb_error_msg_and_die("can't create %slink '%s' to '%s'",
                                 *list->data ? "hard" : "sym",
                                 list->data + 1, target
                         );
+#else
+                        if (!*list->data)
+                                bb_error_msg("can't create %slink '%s' to '%s'",
+                                        "sym",
+                                        list->data + 1, target
+                                );
+                        else
+                                bb_error_msg_and_die("can't create %slink '%s' to '%s'",
+                                        "hard",
+                                        list->data + 1, target
+                                );
+#endif
                 }
                 list = list->link;
         }
diff --git a/include/mingw.h b/include/mingw.h
index 713205ef9..e2c02babf 100644
--- a/include/mingw.h
+++ b/include/mingw.h
@@ -440,7 +440,7 @@ NOIMPL(setsid,void);
 NOIMPL(setuid,uid_t gid UNUSED_PARAM);
 NOIMPL(seteuid,uid_t gid UNUSED_PARAM);
 unsigned int sleep(unsigned int seconds);
-NOIMPL(symlink,const char *oldpath UNUSED_PARAM, const char *newpath UNUSED_PARAM);
+int symlink(const char *target, const char *linkpath);
 static inline void sync(void) {}
 long sysconf(int name);
 IMPL(getpagesize,int,4096,void);
diff --git a/win32/mingw.c b/win32/mingw.c
index 41dba9857..a6362333d 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -1063,6 +1063,41 @@ int link(const char *oldpath, const char *newpath)
         return 0;
 }
 
+#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
+
+int symlink(const char *target, const char *linkpath)
+{
+        DWORD flag = SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
+        struct stat st;
+        DECLARE_PROC_ADDR(BOOL, CreateSymbolicLinkA, LPCSTR, LPCSTR, DWORD);
+
+        if (!INIT_PROC_ADDR(kernel32.dll, CreateSymbolicLinkA)) {
+                return -1;
+        }
+
+        if (stat(target, &st) != -1 && S_ISDIR(st.st_mode))
+                flag |= SYMBOLIC_LINK_FLAG_DIRECTORY;
+
+ retry:
+        if (!CreateSymbolicLinkA(linkpath, target, flag)) {
+                /* Old Windows versions see 'UNPRIVILEGED_CREATE' as an invalid
+                 * parameter.  Retry without it. */
+                if ((flag & SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE) &&
+                                GetLastError() == ERROR_INVALID_PARAMETER) {
+                        flag &= ~SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
+                        goto retry;
+                }
+                errno = err_win_to_posix();
+                return -1;
+        }
+        return 0;
+}
+
 static char *normalize_ntpathA(char *buf)
 {
         /* fix absolute path prefixes */