PPC get_user workaround

Linux 5.12 PPC 5.12 get_user() and __copy_from_user_inatomic()
inline helpers very indirectly include a reference to the GPL'd
array mmu_feature_keys[] and fails to build. Workaround this by
using copy_from_user() and throwing EFAULT for any calls to
__copy_from_user_inatomic(). This is a workaround until a fix
for Linux commit 7613f5a66becfd0e43a0f34de8518695888f5458
"powerpc/64s/kuap: Use mmu_has_feature()" is fully addressed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Authored-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes openzfs#11958
Closes openzfs#12590
Closes openzfs#13367

config/kernel-copy-from-user-inatomic.m4

@@ -0,0 +1,26 @@
+dnl #
+dnl # On certain architectures `__copy_from_user_inatomic`
+dnl # is a GPL exported variable and cannot be used by OpenZFS.
+dnl #
+
+dnl #
+dnl # Checking if `__copy_from_user_inatomic` is available.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC___COPY_FROM_USER_INATOMIC], [
+	ZFS_LINUX_TEST_SRC([__copy_from_user_inatomic], [
+		#include <linux/uaccess.h>
+	], [
+		int result __attribute__ ((unused)) = __copy_from_user_inatomic(NULL, NULL, 0);
+	], [], [ZFS_META_LICENSE])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL___COPY_FROM_USER_INATOMIC], [
+	AC_MSG_CHECKING([whether __copy_from_user_inatomic is available])
+	ZFS_LINUX_TEST_RESULT([__copy_from_user_inatomic_license], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE___COPY_FROM_USER_INATOMIC, 1,
+		    [__copy_from_user_inatomic is available])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -142,6 +142,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_ADD_DISK
 	ZFS_AC_KERNEL_SRC_KTHREAD
 	ZFS_AC_KERNEL_SRC_ZERO_PAGE
+	ZFS_AC_KERNEL_SRC___COPY_FROM_USER_INATOMIC
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -257,6 +258,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_ADD_DISK
 	ZFS_AC_KERNEL_KTHREAD
 	ZFS_AC_KERNEL_ZERO_PAGE
+	ZFS_AC_KERNEL___COPY_FROM_USER_INATOMIC
 ])
 
 dnl #

module/os/linux/zfs/zfs_uio.c

@@ -75,6 +75,7 @@ zfs_uiomove_iov(void *p, size_t n, zfs_uio_rw_t rw, zfs_uio_t *uio)
 			} else {
 				unsigned long b_left = 0;
 				if (uio->uio_fault_disable) {
+#if defined(HAVE___COPY_FROM_USER_INATOMIC)
 					if (!zfs_access_ok(VERIFY_READ,
 					    (iov->iov_base + skip), cnt)) {
 						return (EFAULT);
@@ -84,6 +85,9 @@ zfs_uiomove_iov(void *p, size_t n, zfs_uio_rw_t rw, zfs_uio_t *uio)
 					    __copy_from_user_inatomic(p,
 					    (iov->iov_base + skip), cnt);
 					pagefault_enable();
+#else
+					return (EFAULT);
+#endif
 				} else {
 					b_left =
 					    copy_from_user(p,
@@ -248,15 +252,15 @@ zfs_uio_prefaultpages(ssize_t n, zfs_uio_t *uio)
 			/* touch each page in this segment. */
 			p = iov->iov_base + skip;
 			while (cnt) {
-				if (get_user(tmp, (uint8_t *)p))
+				if (copy_from_user(&tmp, p, 1))
 					return (EFAULT);
 				ulong_t incr = MIN(cnt, PAGESIZE);
 				p += incr;
 				cnt -= incr;
 			}
 			/* touch the last byte in case it straddles a page. */
 			p--;
-			if (get_user(tmp, (uint8_t *)p))
+			if (copy_from_user(&tmp, p, 1))
 				return (EFAULT);
 		}
 	}