Linux 4.5 compat: get_link() / put_link()

The follow_link() interface was retired in favor of get_link().
In the process of phasing in get_link() the Linux kernel went
through two different versions.  The first of which depended
on put_link() and the final version on a delayed done function.

- Improved configure checks for .follow_link, .get_link, .put_link.
  - Interfaces checked from newest to oldest.
  - Strict checking for each possible known interface.
  - Configure fails when no known interface is available.

- Both versions .get_link are detected and supported as well
  two previous versions of .follow_link.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Issue #4228

config/kernel-follow-link-nameidata.m4

@@ -1,24 +0,0 @@
-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-get-link.m4

@@ -0,0 +1,100 @@
+dnl #
+dnl # Supported get_link() interfaces checked newest to oldest.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_FOLLOW_LINK], [
+	dnl #
+	dnl # 4.2 API change
+	dnl # - This kernel retired the nameidata structure.
+	dnl #
+	AC_MSG_CHECKING([whether iops->follow_link() passes cookie])
+	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(yes)
+		AC_DEFINE(HAVE_FOLLOW_LINK_COOKIE, 1,
+		    [iops->follow_link() cookie])
+	],[
+		dnl #
+		dnl # 2.6.32 API
+		dnl #
+		AC_MSG_RESULT(no)
+		AC_MSG_CHECKING(
+		   [whether iops->follow_link() passes nameidata])
+		ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+			void *follow_link(struct dentry *de, struct
+			    nameidata *nd) { return (void *)NULL; }
+			static struct inode_operations
+			    iops __attribute__ ((unused)) = {
+				.follow_link = follow_link,
+			};
+		],[
+		],[
+			AC_MSG_RESULT(yes)
+			AC_DEFINE(HAVE_FOLLOW_LINK_NAMEIDATA, 1,
+			          [iops->follow_link() nameidata])
+		],[
+                        AC_MSG_ERROR(no; please file a bug report)
+		])
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_GET_LINK], [
+	dnl #
+	dnl # 4.5 API change
+	dnl # The get_link interface has added a delayed done call and
+	dnl # used it to retire the put_link() interface.
+	dnl #
+	AC_MSG_CHECKING([whether iops->get_link() passes delayed])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+		const char *get_link(struct dentry *de, struct inode *ip,
+		    struct delayed_call *done) { return "symlink"; }
+		static struct inode_operations
+		     iops __attribute__ ((unused)) = {
+			.get_link = get_link,
+		};
+	],[
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_GET_LINK_DELAYED, 1,
+		    [iops->get_link() delayed])
+	],[
+		dnl #
+		dnl # 4.5 API change
+		dnl # The follow_link() interface has been replaced by
+		dnl # get_link() which behaves the same as before except:
+		dnl # - An inode is passed as a separate argument
+		dnl # - When called in RCU mode a NULL dentry is passed.
+		dnl #
+		AC_MSG_RESULT(no)
+		AC_MSG_CHECKING([whether iops->get_link() passes cookie])
+		ZFS_LINUX_TRY_COMPILE([
+			#include <linux/fs.h>
+			const char *get_link(struct dentry *de, struct
+			    inode *ip, void **cookie) { return "symlink"; }
+			static struct inode_operations
+			     iops __attribute__ ((unused)) = {
+				.get_link = get_link,
+			};
+		],[
+		],[
+			AC_MSG_RESULT(yes)
+			AC_DEFINE(HAVE_GET_LINK_COOKIE, 1,
+			    [iops->get_link() cookie])
+		],[
+			dnl #
+			dnl # Check for the follow_link APIs.
+			dnl #
+			AC_MSG_RESULT(no)
+			ZFS_AC_KERNEL_FOLLOW_LINK
+		])
+	])
+])

config/kernel-put-link-nameidata.m4

@@ -1,23 +0,0 @@
-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-put-link.m4

@@ -0,0 +1,60 @@
+dnl #
+dnl # Supported symlink APIs
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_PUT_LINK], [
+	dnl #
+	dnl # 4.5 API change
+	dnl # get_link() uses delayed done, there is no put_link() interface.
+	dnl #
+	ZFS_LINUX_TRY_COMPILE([
+		#if !defined(HAVE_GET_LINK_DELAYED)
+		#error "Expecting get_link() delayed done"
+		#endif
+	],[
+	],[
+		AC_DEFINE(HAVE_PUT_LINK_DELAYED, 1, [iops->put_link() delayed])
+	],[
+		dnl #
+		dnl # 4.2 API change
+		dnl # This kernel retired the nameidata structure.
+		dnl #
+		AC_MSG_CHECKING([whether iops->put_link() passes cookie])
+		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(yes)
+			AC_DEFINE(HAVE_PUT_LINK_COOKIE, 1,
+			    [iops->put_link() cookie])
+		],[
+			dnl #
+			dnl # 2.6.32 API
+			dnl #
+			AC_MSG_RESULT(no)
+			AC_MSG_CHECKING(
+			    [whether iops->put_link() passes nameidata])
+			ZFS_LINUX_TRY_COMPILE([
+				#include <linux/fs.h>
+				void put_link(struct dentry *de, struct
+				    nameidata *nd, void *ptr) { return; }
+				static struct inode_operations
+				    iops __attribute__ ((unused)) = {
+					.put_link = put_link,
+				};
+			],[
+			],[
+				AC_MSG_RESULT(yes)
+				AC_DEFINE(HAVE_PUT_LINK_NAMEIDATA, 1,
+				    [iops->put_link() nameidata])
+			],[
+				AC_MSG_ERROR(no; please file a bug report)
+			])
+		])
+	])
+])

config/kernel.m4

@@ -58,7 +58,7 @@ 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_GET_LINK
 	ZFS_AC_KERNEL_PUT_LINK
 	ZFS_AC_KERNEL_TRUNCATE_RANGE
 	ZFS_AC_KERNEL_AUTOMOUNT

module/zfs/zpl_inode.c

@@ -339,26 +339,42 @@ zpl_symlink(struct inode *dir, struct dentry *dentry, const char *name)
 	return (error);
 }
 
-#ifdef HAVE_FOLLOW_LINK_NAMEIDATA
+#if defined(HAVE_PUT_LINK_COOKIE)
-static void *
+static void
-zpl_follow_link(struct dentry *dentry, struct nameidata *nd)
+zpl_put_link(struct inode *unused, void *cookie)
-#else
-const char *
-zpl_follow_link(struct dentry *dentry, void **symlink_cookie)
-#endif
 {
+	kmem_free(cookie, MAXPATHLEN);
+}
+#elif defined(HAVE_PUT_LINK_NAMEIDATA)
+static void
+zpl_put_link(struct dentry *dentry, struct nameidata *nd, void *ptr)
+{
+	const char *link = nd_get_link(nd);
+
+	if (!IS_ERR(link))
+		kmem_free(link, MAXPATHLEN);
+}
+#elif defined(HAVE_PUT_LINK_DELAYED)
+static void
+zpl_put_link(void *ptr)
+{
+	kmem_free(ptr, MAXPATHLEN);
+}
+#endif
+
+static int
+zpl_get_link_common(struct dentry *dentry, struct inode *ip, char **link)
+{
+	fstrans_cookie_t cookie;
 	cred_t *cr = CRED();
-	struct inode *ip = dentry->d_inode;
 	struct iovec iov;
 	uio_t uio;
-	char *link;
 	int error;
-	fstrans_cookie_t cookie;
 
 	crhold(cr);
-
+	*link = NULL;
 	iov.iov_len = MAXPATHLEN;
-	iov.iov_base = link = kmem_zalloc(MAXPATHLEN, KM_SLEEP);
+	iov.iov_base = kmem_zalloc(MAXPATHLEN, KM_SLEEP);
 
 	uio.uio_iov = &iov;
 	uio.uio_iovcnt = 1;
@@ -369,41 +385,78 @@ zpl_follow_link(struct dentry *dentry, void **symlink_cookie)
 	cookie = spl_fstrans_mark();
 	error = -zfs_readlink(ip, &uio, cr);
 	spl_fstrans_unmark(cookie);
-
-	if (error)
-		kmem_free(link, MAXPATHLEN);
-
 	crfree(cr);
 
-#ifdef HAVE_FOLLOW_LINK_NAMEIDATA
+	if (error)
+		kmem_free(iov.iov_base, MAXPATHLEN);
+	else
+		*link = iov.iov_base;
+
+	return (error);
+}
+
+#if defined(HAVE_GET_LINK_DELAYED)
+const char *
+zpl_get_link(struct dentry *dentry, struct inode *inode,
+    struct delayed_call *done)
+{
+	char *link = NULL;
+	int error;
+
+	if (!dentry)
+		return (ERR_PTR(-ECHILD));
+
+	error = zpl_get_link_common(dentry, inode, &link);
+	if (error)
+		return (ERR_PTR(error));
+
+	set_delayed_call(done, zpl_put_link, link);
+
+	return (link);
+}
+#elif defined(HAVE_GET_LINK_COOKIE)
+const char *
+zpl_get_link(struct dentry *dentry, struct inode *inode, void **cookie)
+{
+	char *link = NULL;
+	int error;
+
+	if (!dentry)
+		return (ERR_PTR(-ECHILD));
+
+	error = zpl_get_link_common(dentry, inode, &link);
+	if (error)
+		return (ERR_PTR(error));
+
+	return (*cookie = link);
+}
+#elif defined(HAVE_FOLLOW_LINK_COOKIE)
+const char *
+zpl_follow_link(struct dentry *dentry, void **cookie)
+{
+	char *link = NULL;
+	int error;
+
+	error = zpl_get_link_common(dentry, dentry->d_inode, &link);
+	if (error)
+		return (ERR_PTR(error));
+
+	return (*cookie = link);
+}
+#elif defined(HAVE_FOLLOW_LINK_NAMEIDATA)
+static void *
+zpl_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+	char *link = NULL;
+	int error;
+
+	error = zpl_get_link_common(dentry, dentry->d_inode, &link);
 	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)
-{
-	const char *link = nd_get_link(nd);
-
-	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
 
@@ -591,8 +644,14 @@ const struct inode_operations zpl_dir_inode_operations = {
 
 const struct inode_operations zpl_symlink_inode_operations = {
 	.readlink	= generic_readlink,
+#if defined(HAVE_GET_LINK_DELAYED) || defined(HAVE_GET_LINK_COOKIE)
+	.get_link	= zpl_get_link,
+#elif defined(HAVE_FOLLOW_LINK_COOKIE) || defined(HAVE_FOLLOW_LINK_NAMEIDATA)
 	.follow_link	= zpl_follow_link,
+#endif
+#if defined(HAVE_PUT_LINK_COOKIE) || defined(HAVE_PUT_LINK_NAMEIDATA)
 	.put_link	= zpl_put_link,
+#endif
 	.setattr	= zpl_setattr,
 	.getattr	= zpl_getattr,
 	.setxattr	= generic_setxattr,