win32: make the clang build less crashy

busybox-w32 binaries built using clang crashed so frequently that
they were pretty much unusable.  The main issue seems to be with
assignments to the structures containing global variables which
are used in most applets.

Upstream commit 5156b2455 (Make const ptr assign as function call
in clang) addresses this, but is insufficient for the build on
Windows.  Extend the idea to the ASSIGN_CONST_PTR() macro too.

Costs 32-80 bytes in the gcc build.

4 files changed, 22 insertions, 6 deletions

diff --git a/Makefile.flags b/Makefile.flags
index 1a2923854..bf22cec04 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -149,6 +149,11 @@ endif
 
 ifeq ($(CONFIG_PLATFORM_MINGW32),y)
 CFLAGS += -Iwin32 -DHAVE_STRING_H=1 -DHAVE_CONFIG_H=0 -fno-builtin-stpcpy -fno-builtin-stpncpy -fno-ident -fno-builtin-strndup
+# this seems to be necessary for setjmp/longjmp to work with clang
+ifeq ($(lastword $(subst -, ,$(CC))),clang)
+CFLAGS += $(call cc-option,-fexceptions,)
+endif
+
 EXEEXT = .exe
 LDLIBS += ws2_32
 endif
diff --git a/include/libbb.h b/include/libbb.h
index a7b19b1c9..a469bfe03 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -2439,10 +2439,15 @@ static ALWAYS_INLINE void* not_const_pp(const void *p)
         );
         return pp;
 }
+# if !ENABLE_PLATFORM_MINGW32
 # define ASSIGN_CONST_PTR(pptr, v) do { \
         *(void**)not_const_pp(pptr) = (void*)(v); \
         barrier(); \
 } while (0)
+#else
+/* On Windows it seems necessary for this to be a function too. */
+void ASSIGN_CONST_PTR(const void *pptr, const void *ptr) FAST_FUNC;
+#endif
 /* XZALLOC_CONST_PTR() is an out-of-line function to prevent
  * clang from reading pointer before it is assigned.
  */
diff --git a/libbb/const_hack.c b/libbb/const_hack.c
index 9575e6d67..75163fede 100644
--- a/libbb/const_hack.c
+++ b/libbb/const_hack.c
@@ -13,4 +13,14 @@ void FAST_FUNC XZALLOC_CONST_PTR(const void *pptr, size_t size)
 {
         ASSIGN_CONST_PTR(pptr, xzalloc(size));
 }
+
+# if ENABLE_PLATFORM_MINGW32
+void FAST_FUNC ASSIGN_CONST_PTR(const void *pptr, const void *v)
+{
+        do {
+                *(void**)not_const_pp(pptr) = (void*)(v);
+                barrier();
+        } while (0);
+}
+# endif
 #endif
diff --git a/shell/ash.c b/shell/ash.c
index 8b7c6c627..adab63fa7 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -17279,8 +17279,6 @@ forkshell_init(const char *idstr)
         struct forkshell *fs;
         void *map_handle;
         HANDLE h;
-        struct globals_var **gvpp;
-        struct globals_misc **gmpp;
         int i;
         char **ptr;
         char *lrelocate;
@@ -17323,10 +17321,8 @@ forkshell_init(const char *idstr)
         fs->gmp->trap_ptr = fs->gmp->trap;
 
         /* Set global variables */
-        gvpp = (struct globals_var **)&ash_ptr_to_globals_var;
-        *gvpp = fs->gvp;
-        gmpp = (struct globals_misc **)&ash_ptr_to_globals_misc;
-        *gmpp = fs->gmp;
+        ASSIGN_CONST_PTR(&ash_ptr_to_globals_var, fs->gvp);
+        ASSIGN_CONST_PTR(&ash_ptr_to_globals_misc, fs->gmp);
         cmdtable = fs->cmdtable;
 #if ENABLE_ASH_ALIAS
         atab = fs->atab;        /* will be NULL for FS_SHELLEXEC */