win32: retry when command needs elevated privileges

Some installer programs have an entry in their manifest to indicate
that they need elevated privileges.  The shell in busybox-w32 was
unable to run such programs.

When a program fails to run with ERROR_ELEVATION_REQUIRED, try
again using ShellExecuteEx() with the 'runas' verb to give it
elevated privileges.

Adds 272-288 bytes.

(GitHub issue #481)

2 files changed, 64 insertions, 0 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index 7a5198ccf..cb1f84f30 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -2538,3 +2538,25 @@ char * FAST_FUNC exe_relative_path(const char *tail)
         free(exepath);
         return relpath;
 }
+
+int mingw_shell_execute(SHELLEXECUTEINFO *info)
+{
+        DECLARE_PROC_ADDR(BOOL, ShellExecuteExA, SHELLEXECUTEINFOA *);
+        char *lpath;
+        int ret;
+
+        if (!INIT_PROC_ADDR(shell32.dll, ShellExecuteExA)) {
+                errno = ENOSYS;
+                return FALSE;
+        }
+
+        // ShellExecuteEx() needs backslash as separator in UNC paths.
+        lpath = xstrdup(info->lpFile);
+        slash_to_bs(lpath);
+        info->lpFile = lpath;
+
+        ret = ShellExecuteExA(info);
+
+        free(lpath);
+        return ret;
+}
diff --git a/win32/process.c b/win32/process.c
index fd89707ca..33f45ee42 100644
--- a/win32/process.c
+++ b/win32/process.c
@@ -501,6 +501,38 @@ static NORETURN void wait_for_child(HANDLE child, const char *cmd)
         exit((int)code);
 }
 
+static intptr_t
+shell_execute(const char *path, char *const *argv)
+{
+        SHELLEXECUTEINFO info;
+        char *args;
+
+        memset(&info, 0, sizeof(SHELLEXECUTEINFO));
+        info.cbSize = sizeof(SHELLEXECUTEINFO);
+        info.fMask = SEE_MASK_NOCLOSEPROCESS;
+        /* info.hwnd = NULL; */
+        info.lpVerb = "runas";
+        info.lpFile = path;
+
+        args = NULL;
+        if (*argv++) {
+                while (*argv) {
+                        char *q = quote_arg(*argv++);
+                        args = xappendword(args, q);
+                        free(q);
+                }
+        }
+
+        info.lpParameters = args;
+        /* info.lpDirectory = NULL; */
+        info.nShow = SW_SHOWNORMAL;
+
+        mingw_shell_execute(&info);
+
+        free(args);
+        return info.hProcess ? (intptr_t)info.hProcess : -1;
+}
+
 int
 mingw_execvp(const char *cmd, char *const *argv)
 {
@@ -514,6 +546,16 @@ int
 mingw_execve(const char *cmd, char *const *argv, char *const *envp)
 {
         intptr_t ret = mingw_spawn_interpreter(P_NOWAIT, cmd, argv, envp, 0);
+
+        if (ret == -1 && GetLastError() == ERROR_ELEVATION_REQUIRED) {
+                // Command exists but failed because it wants elevated privileges.
+                // Try again using ShellExecuteEx().
+                SetLastError(0);
+                ret = shell_execute(cmd, argv);
+                if (GetLastError())
+                        exit(1);
+        }
+
         if (ret != -1)
                 wait_for_child((HANDLE)ret, cmd);
         return ret;