signal: Add unsafe_put_compat_sigset()

Implement 'unsafe' version of put_compat_sigset()

For the bigendian, use unsafe_put_user() directly
to avoid intermediate copy through the stack.

For the littleendian, use a straight unsafe_copy_to_user().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/537c7082ee309a0bb9c67a50c5d9dd929aedb82d.1597770847.git.christophe.leroy@csgroup.eu

include/linux/compat.h

@@ -442,6 +442,38 @@ put_compat_sigset(compat_sigset_t __user *compat, const sigset_t *set,
 #endif
 }
 
+#ifdef CONFIG_CPU_BIG_ENDIAN
+#define unsafe_put_compat_sigset(compat, set, label) do {		\
+	compat_sigset_t __user *__c = compat;				\
+	const sigset_t *__s = set;					\
+									\
+	switch (_NSIG_WORDS) {						\
+	case 4:								\
+		unsafe_put_user(__s->sig[3] >> 32, &__c->sig[7], label);	\
+		unsafe_put_user(__s->sig[3], &__c->sig[6], label);	\
+		fallthrough;						\
+	case 3:								\
+		unsafe_put_user(__s->sig[2] >> 32, &__c->sig[5], label);	\
+		unsafe_put_user(__s->sig[2], &__c->sig[4], label);	\
+		fallthrough;						\
+	case 2:								\
+		unsafe_put_user(__s->sig[1] >> 32, &__c->sig[3], label);	\
+		unsafe_put_user(__s->sig[1], &__c->sig[2], label);	\
+		fallthrough;						\
+	case 1:								\
+		unsafe_put_user(__s->sig[0] >> 32, &__c->sig[1], label);	\
+		unsafe_put_user(__s->sig[0], &__c->sig[0], label);	\
+	}								\
+} while (0)
+#else
+#define unsafe_put_compat_sigset(compat, set, label) do {		\
+	compat_sigset_t __user *__c = compat;				\
+	const sigset_t *__s = set;					\
+									\
+	unsafe_copy_to_user(__c, __s, sizeof(*__c), label);		\
+} while (0)
+#endif
+
 extern int compat_ptrace_request(struct task_struct *child,
 				 compat_long_t request,
 				 compat_ulong_t addr, compat_ulong_t data);