signal: Add unsafe_get_compat_sigset()

In the same way as commit 14026b94 ("signal: Add
unsafe_put_compat_sigset()"), this time add
unsafe_get_compat_sigset() macro which is the 'unsafe'
version of get_compat_sigset()

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

For the littleendian, use a straight unsafe_copy_from_user().

This commit adds the generic fallback for unsafe_copy_from_user().
Architectures wanting to use unsafe_get_compat_sigset() have to
make sure they have their own unsafe_copy_from_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/b05bf434ee13c76bc9df5f02653a10db5e7b54e5.1616151715.git.christophe.leroy@csgroup.eu

include/linux/compat.h

@@ -465,6 +465,34 @@ put_compat_sigset(compat_sigset_t __user *compat, const sigset_t *set,
 		unsafe_put_user(__s->sig[0], &__c->sig[0], label);	\
 	}								\
 } while (0)
+
+#define unsafe_get_compat_sigset(set, compat, label) do {		\
+	const compat_sigset_t __user *__c = compat;			\
+	compat_sigset_word hi, lo;					\
+	sigset_t *__s = set;						\
+									\
+	switch (_NSIG_WORDS) {						\
+	case 4:								\
+		unsafe_get_user(lo, &__c->sig[7], label);		\
+		unsafe_get_user(hi, &__c->sig[6], label);		\
+		__s->sig[3] = hi | (((long)lo) << 32);			\
+		fallthrough;						\
+	case 3:								\
+		unsafe_get_user(lo, &__c->sig[5], label);		\
+		unsafe_get_user(hi, &__c->sig[4], label);		\
+		__s->sig[2] = hi | (((long)lo) << 32);			\
+		fallthrough;						\
+	case 2:								\
+		unsafe_get_user(lo, &__c->sig[3], label);		\
+		unsafe_get_user(hi, &__c->sig[2], label);		\
+		__s->sig[1] = hi | (((long)lo) << 32);			\
+		fallthrough;						\
+	case 1:								\
+		unsafe_get_user(lo, &__c->sig[1], label);		\
+		unsafe_get_user(hi, &__c->sig[0], label);		\
+		__s->sig[0] = hi | (((long)lo) << 32);			\
+	}								\
+} while (0)
 #else
 #define unsafe_put_compat_sigset(compat, set, label) do {		\
 	compat_sigset_t __user *__c = compat;				\
@@ -472,6 +500,13 @@ put_compat_sigset(compat_sigset_t __user *compat, const sigset_t *set,
 									\
 	unsafe_copy_to_user(__c, __s, sizeof(*__c), label);		\
 } while (0)
+
+#define unsafe_get_compat_sigset(set, compat, label) do {		\
+	const compat_sigset_t __user *__c = compat;			\
+	sigset_t *__s = set;						\
+									\
+	unsafe_copy_from_user(__s, __c, sizeof(*__c), label);		\
+} while (0)
 #endif
 
 extern int compat_ptrace_request(struct task_struct *child,

include/linux/uaccess.h

@@ -397,6 +397,7 @@ long strnlen_user_nofault(const void __user *unsafe_addr, long count);
 #define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user(x,p),e)
 #define unsafe_put_user(x,p,e) unsafe_op_wrap(__put_user(x,p),e)
 #define unsafe_copy_to_user(d,s,l,e) unsafe_op_wrap(__copy_to_user(d,s,l),e)
+#define unsafe_copy_from_user(d,s,l,e) unsafe_op_wrap(__copy_from_user(d,s,l),e)
 static inline unsigned long user_access_save(void) { return 0UL; }
 static inline void user_access_restore(unsigned long flags) { }
 #endif