Linux 4.2 compat: follow_link() / put_link()

As of Linux 4.2 the kernel has completely retired the nameidata
structure.  One of the few remaining consumers of this interface
were the follow_link() and put_link() callbacks.

This patch adds the required checks to configure to detect the
interface change and updates the functions accordingly.  Migrating
to the simple_follow_link() interface was considered but was decided
against ironically due to the increased complexity.

It also should be noted that the kernel follow_link() and put_link()
interfaces changes several times after 4.1 and but before 4.2.  This
means there is a narrow range of kernel commits which never appear
in an official tag of the Linux kernel which ZoL will not build.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Issue #3596

config/kernel-create-nameidata.m4

@@ -2,7 +2,7 @@ dnl #
 dnl # 3.6 API change
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_CREATE_NAMEIDATA], [
-	AC_MSG_CHECKING([whether iops->create() takes struct nameidata])
+	AC_MSG_CHECKING([whether iops->create() passes nameidata])
 	ZFS_LINUX_TRY_COMPILE([
 		#include <linux/fs.h>
 
@@ -22,7 +22,7 @@ AC_DEFUN([ZFS_AC_KERNEL_CREATE_NAMEIDATA], [
 	],[
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_CREATE_NAMEIDATA, 1,
-		          [iops->create() operation takes nameidata])
+		          [iops->create() passes nameidata])
 	],[
 		AC_MSG_RESULT(no)
 	])

config/kernel-follow-link-nameidata.m4

@@ -0,0 +1,24 @@
+dnl #
+dnl # 4.2 API change
+dnl # This kernel retired the nameidata structure which forced the
+dnl # restructuring of the follow_link() prototype and how it is called.
+dnl # We check for the new interface rather than detecting the old one.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_FOLLOW_LINK], [
+	AC_MSG_CHECKING([whether iops->follow_link() passes nameidata])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+		const char *follow_link(struct dentry *de, void **cookie)
+		    { return "symlink"; }
+		static struct inode_operations iops __attribute__ ((unused)) = {
+			.follow_link = follow_link,
+		};
+	],[
+	],[
+		AC_MSG_RESULT(no)
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_FOLLOW_LINK_NAMEIDATA, 1,
+		          [iops->follow_link() nameidata])
+	])
+])

config/kernel-lookup-nameidata.m4

@@ -2,7 +2,7 @@ dnl #
 dnl # 3.6 API change
 dnl #
 AC_DEFUN([ZFS_AC_KERNEL_LOOKUP_NAMEIDATA], [
-	AC_MSG_CHECKING([whether iops->lookup() takes struct nameidata])
+	AC_MSG_CHECKING([whether iops->lookup() passes nameidata])
 	ZFS_LINUX_TRY_COMPILE([
 		#include <linux/fs.h>
 
@@ -18,7 +18,7 @@ AC_DEFUN([ZFS_AC_KERNEL_LOOKUP_NAMEIDATA], [
 	],[
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_LOOKUP_NAMEIDATA, 1,
-		          [iops->lookup() operation takes nameidata])
+		          [iops->lookup() passes nameidata])
 	],[
 		AC_MSG_RESULT(no)
 	])

config/kernel-put-link-nameidata.m4

@@ -0,0 +1,23 @@
+dnl #
+dnl # 4.2 API change
+dnl # This kernel retired the nameidata structure which forced the
+dnl # restructuring of the put_link() prototype and how it is called.
+dnl # We check for the new interface rather than detecting the old one.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_PUT_LINK], [
+	AC_MSG_CHECKING([whether iops->put_link() passes nameidata])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+		void put_link(struct inode *ip, void *cookie) { return; }
+		static struct inode_operations iops __attribute__ ((unused)) = {
+			.put_link = put_link,
+		};
+	],[
+	],[
+		AC_MSG_RESULT(no)
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_PUT_LINK_NAMEIDATA, 1,
+		          [iops->put_link() nameidata])
+	])
+])

config/kernel.m4

@@ -70,6 +70,8 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_MKDIR_UMODE_T
 	ZFS_AC_KERNEL_LOOKUP_NAMEIDATA
 	ZFS_AC_KERNEL_CREATE_NAMEIDATA
+	ZFS_AC_KERNEL_FOLLOW_LINK
+	ZFS_AC_KERNEL_PUT_LINK
 	ZFS_AC_KERNEL_TRUNCATE_RANGE
 	ZFS_AC_KERNEL_AUTOMOUNT
 	ZFS_AC_KERNEL_ENCODE_FH_WITH_INODE

module/zfs/zpl_inode.c

@@ -348,8 +348,13 @@ zpl_symlink(struct inode *dir, struct dentry *dentry, const char *name)
 	return (error);
 }
 
+#ifdef HAVE_FOLLOW_LINK_NAMEIDATA
 static void *
 zpl_follow_link(struct dentry *dentry, struct nameidata *nd)
+#else
+const char *
+zpl_follow_link(struct dentry *dentry, void **symlink_cookie)
+#endif
 {
 	cred_t *cr = CRED();
 	struct inode *ip = dentry->d_inode;
@@ -372,17 +377,28 @@ zpl_follow_link(struct dentry *dentry, struct nameidata *nd)
 	cookie = spl_fstrans_mark();
 	error = -zfs_readlink(ip, &uio, cr);
 	spl_fstrans_unmark(cookie);
-	if (error) {
+
+	if (error)
 		kmem_free(link, MAXPATHLEN);
-		nd_set_link(nd, ERR_PTR(error));
-	} else {
-		nd_set_link(nd, link);
-	}
 
 	crfree(cr);
+
+#ifdef HAVE_FOLLOW_LINK_NAMEIDATA
+	if (error)
+		nd_set_link(nd, ERR_PTR(error));
+	else
+		nd_set_link(nd, link);
+
 	return (NULL);
+#else
+	if (error)
+		return (ERR_PTR(error));
+	else
+		return (*symlink_cookie = link);
+#endif
 }
 
+#ifdef HAVE_PUT_LINK_NAMEIDATA
 static void
 zpl_put_link(struct dentry *dentry, struct nameidata *nd, void *ptr)
 {
@@ -391,6 +407,13 @@ zpl_put_link(struct dentry *dentry, struct nameidata *nd, void *ptr)
 	if (!IS_ERR(link))
 		kmem_free(link, MAXPATHLEN);
 }
+#else
+static void
+zpl_put_link(struct inode *unused, void *symlink_cookie)
+{
+	kmem_free(symlink_cookie, MAXPATHLEN);
+}
+#endif
 
 static int
 zpl_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)