um: fix stub location calculation

In commit 9f0b4807 ("um: rework userspace stubs to not hard-code
stub location") I changed stub_segv_handler() to do a calculation with
a pointer to a stack variable to find the data page that we're using
for the stack and the rest of the data. This same commit was meant to
do it as well for stub_clone_handler(), but the change inadvertently
went into commit 84b2789d ("um: separate child and parent errors
in clone stub") instead.

This was reported to not be compiled correctly by gcc 5, causing the
code to crash here. I'm not sure why, perhaps it's UB because the var
isn't initialized? In any case, this trick always seemed bad, so just
create a new inline function that does the calculation in assembly.

Reported-by: subashab@codeaurora.org
Fixes: 9f0b4807 ("um: rework userspace stubs to not hard-code stub location")
Fixes: 84b2789d ("um: separate child and parent errors in clone stub")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/skas/clone.c

@@ -24,8 +24,7 @@
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_clone_handler(void)
 {
-	int stack;
-	struct stub_data *data = (void *) ((unsigned long)&stack & ~(UM_KERN_PAGE_SIZE - 1));
+	struct stub_data *data = get_stub_page();
 	long err;
 
 	err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD,

arch/x86/um/shared/sysdep/stub_32.h

@@ -101,4 +101,16 @@ static inline void remap_stack_and_trap(void)
 		"memory");
 }
 
+static __always_inline void *get_stub_page(void)
+{
+	unsigned long ret;
+
+	asm volatile (
+		"movl %%esp,%0 ;"
+		"andl %1,%0"
+		: "=a" (ret)
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)));
+
+	return (void *)ret;
+}
 #endif

arch/x86/um/shared/sysdep/stub_64.h

@@ -108,4 +108,16 @@ static inline void remap_stack_and_trap(void)
 		__syscall_clobber, "r10", "r8", "r9");
 }
 
+static __always_inline void *get_stub_page(void)
+{
+	unsigned long ret;
+
+	asm volatile (
+		"movq %%rsp,%0 ;"
+		"andq %1,%0"
+		: "=a" (ret)
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)));
+
+	return (void *)ret;
+}
 #endif

arch/x86/um/stub_segv.c

@@ -11,9 +11,8 @@
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_segv_handler(int sig, siginfo_t *info, void *p)
 {
-	int stack;
+	struct faultinfo *f = get_stub_page();
 	ucontext_t *uc = p;
-	struct faultinfo *f = (void *)(((unsigned long)&stack) & ~(UM_KERN_PAGE_SIZE - 1));
 
 	GET_FAULTINFO_FROM_MC(*f, &uc->uc_mcontext);
 	trap_myself();