ash: workaround for UCRT bug

There seems to be a bug in UCRT such that if a process has been
started by passing a non-NULL environment block to CreateProcess()
any subsequent call to spawnve() with a non-NULL environment pointer
results in a crash.

Commit 5b48ca53b (win32: pass NULL to spawnve, not environ) fixed
the problem in busybox-w32 for those cases where a NULL environment
pointer was sufficient.  It didn't handle the case where the shell
passes a modified environment to its child.

All calls to spawnve() in the shell occur in a process which will
terminate whether or not the call succeeds.  It therefore doesn't
matter if we mess with the environment of this process to allow
spawnve() to be passed a NULL environment pointer.  Do this for
UCRT builds only.

(GitHub issue #234)

1 files changed, 12 insertions, 0 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 46c4f1675..c9122b291 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -8899,9 +8899,13 @@ tryexec(IF_FEATURE_SH_STANDALONE(int applet_no,) const char *cmd, char **argv, c
 # else
                 if (APPLET_IS_NOEXEC(applet_no)) {
 # endif
+#if ENABLE_PLATFORM_MINGW32 && !defined(_UCRT)
+                        /* If building for UCRT move this up into shellexec() to
+                         * work around a bug. */
                         clearenv();
                         while (*envp)
                                 putenv(*envp++);
+#endif
                         popredir(/*drop:*/ 1);
                         run_noexec_applet_and_exit(applet_no, cmd, argv);
                 }
@@ -8971,6 +8975,14 @@ static void shellexec(char *prog, char **argv, const char *path, int idx)
         int applet_no = -1; /* used only by FEATURE_SH_STANDALONE */
 
         envp = listvars(VEXPORT, VUNSET, /*strlist:*/ NULL, /*end:*/ NULL);
+#if ENABLE_PLATFORM_MINGW32 && defined(_UCRT)
+        /* Avoid UCRT bug by updating parent's environment and passing a
+         * NULL environment pointer to execve(). */
+        clearenv();
+        while (*envp)
+                putenv(*envp++);
+        envp = NULL;
+#endif
 #if !ENABLE_PLATFORM_MINGW32
         if (strchr(prog, '/') != NULL
 #else