Date:	Tue, 19 Apr 2011 22:04:22 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Michal Suchanek <hramrach@...trum.cz>
Cc:	Andreas Dilger <adilger@...ger.ca>, Jiri Kosina <jkosina@...e.cz>,
	Ric Wheeler <ricwheeler@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>,
	Ian Kent <ikent@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Hugh Dickins <hughd@...gle.com>, Eric Paris <eparis@...hat.com>
Subject: [PATCH] tmpfs: implement generic xattr support

Michal Suchanek <hramrach@...trum.cz> writes:
> Applying this patch is not sufficient. Apparently more xattrs are
> needed but adding them on top of this patch should be easy.
>
> The ones mentioned in the overlayfs doc are
>
> trusted.overlay.whiteout
> trusted.overlay.opaque
>
> The patch implements security.*

Here's an updated patch.  It changes a number of things:

 - it implements shmem specific xattr ops instead of using the generic_*
   functions.  Which means that there's no back and forth between the
   VFS and the filesystem.  I basically copied the btrfs way of doing
   things.

 - adds a new config option: CONFIG_TMPFS_XATTR and makes
   CONFIG_TMPFS_POSIX_ACL depend on this.  This way xattr support can be
   turned on without also adding ACL support.

 - now supports trusted.* namespace needed by overlayfs in addition to
   security.*.  Doesn't yet support user.* since that needs more thought
   wrt. kernel memory use.

 - supports xattrs on symlinks, again needed by overlayfs

Hugh, Eric, does this look acceptable?

Thanks,
Miklos

---
From: Eric Paris <eparis@...hat.com>
Subject: tmpfs: implement generic xattr support

This patch implements generic xattrs for tmpfs filesystems.  The feodra
project, while trying to replace suid apps with file capabilities,
realized that tmpfs, which is used on the build systems, does not
support file capabilities and thus cannot be used to build packages
which use file capabilities.  Xattrs are also needed for overlayfs.

The xattr interface is a bit, odd.  If a filesystem does not implement any
{get,set,list}xattr functions the VFS will call into some random LSM hooks and
the running LSM can then implement some method for handling xattrs.  SELinux
for example provides a method to support security.selinux but no other
security.* xattrs.

As it stands today when one enables CONFIG_TMPFS_POSIX_ACL tmpfs will have
xattr handler routines specifically to handle acls.  Because of this tmpfs
would loose the VFS/LSM helpers to support the running LSM.  To make up for
that tmpfs had stub functions that did nothing but call into the LSM hooks
which implement the helpers.

This new patch does not use the LSM fallback functions and instead
just implements a native get/set/list xattr feature for the full
security.* and trusted.* namespace like a normal filesystem.  This
means that tmpfs can now support both security.selinux and
security.capability, which was not previously possible.

The basic implementation is that I attach a:

struct shmem_xattr {
	struct list_head list; /* anchored by shmem_inode_info->xattr_list */
	char *name;
	size_t size;
	char value[0];
};

Into the struct shmem_inode_info for each xattr that is set.  This
implementation could easily support the user.* namespace as well,
except some care needs to be taken to prevent large amounts of
unswappable memory being allocated for unprivileged users.

Signed-off-by: Eric Paris <eparis@...hat.com>
Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
---
 fs/Kconfig               |   18 ++
 include/linux/shmem_fs.h |    1 
 mm/shmem.c               |  302 +++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 273 insertions(+), 48 deletions(-)

Index: linux-2.6/fs/Kconfig
===================================================================
--- linux-2.6.orig/fs/Kconfig	2011-04-19 21:09:33.000000000 +0200
+++ linux-2.6/fs/Kconfig	2011-04-19 21:09:35.000000000 +0200
@@ -121,9 +121,25 @@ config TMPFS
 
 	  See <file:Documentation/filesystems/tmpfs.txt> for details.
 
+config TMPFS_XATTR
+	bool "Tmpfs extended attributes"
+	depends on TMPFS
+	default y
+	help
+	  Extended attributes are name:value pairs associated with inodes by
+	  the kernel or by users (see the attr(5) manual page, or visit
+	  <http://acl.bestbits.at/> for details).
+
+	  Currently this enables support for the trusted.* and
+	  security.* namespaces.
+
+	  If unsure, say N.
+
+	  You need this for POSIX ACL support on tmpfs.
+
 config TMPFS_POSIX_ACL
 	bool "Tmpfs POSIX Access Control Lists"
-	depends on TMPFS
+	depends on TMPFS_XATTR
 	select GENERIC_ACL
 	help
 	  POSIX Access Control Lists (ACLs) support permissions for users and
Index: linux-2.6/include/linux/shmem_fs.h
===================================================================
--- linux-2.6.orig/include/linux/shmem_fs.h	2011-04-19 21:09:25.000000000 +0200
+++ linux-2.6/include/linux/shmem_fs.h	2011-04-19 21:09:35.000000000 +0200
@@ -19,6 +19,7 @@ struct shmem_inode_info {
 	struct page		*i_indirect;	/* top indirect blocks page */
 	swp_entry_t		i_direct[SHMEM_NR_DIRECT]; /* first blocks */
 	struct list_head	swaplist;	/* chain of maybes on swap */
+	struct list_head	xattr_list;	/* list of shmem_xattr */
 	struct inode		vfs_inode;
 };
 
Index: linux-2.6/mm/shmem.c
===================================================================
--- linux-2.6.orig/mm/shmem.c	2011-04-19 21:09:25.000000000 +0200
+++ linux-2.6/mm/shmem.c	2011-04-19 21:09:35.000000000 +0200
@@ -99,6 +99,13 @@ static struct vfsmount *shm_mnt;
 /* Pretend that each entry is of this size in directory's i_size */
 #define BOGO_DIRENT_SIZE 20
 
+struct shmem_xattr {
+	struct list_head list;	/* anchored by shmem_inode_info->xattr_list */
+	char *name;		/* xattr name */
+	size_t size;
+	char value[0];
+};
+
 /* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
 enum sgp_type {
 	SGP_READ,	/* don't exceed i_size, don't allocate page */
@@ -822,6 +829,7 @@ static int shmem_notify_change(struct de
 static void shmem_evict_inode(struct inode *inode)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_xattr *xattr, *nxattr;
 
 	if (inode->i_mapping->a_ops == &shmem_aops) {
 		truncate_inode_pages(inode->i_mapping, 0);
@@ -834,6 +842,11 @@ static void shmem_evict_inode(struct ino
 			mutex_unlock(&shmem_swaplist_mutex);
 		}
 	}
+
+	list_for_each_entry_safe(xattr, nxattr, &info->xattr_list, list) {
+		kfree(xattr->name);
+		kfree(xattr);
+	}
 	BUG_ON(inode->i_blocks);
 	shmem_free_inode(inode->i_sb);
 	end_writeback(inode);
@@ -1597,6 +1610,7 @@ static struct inode *shmem_get_inode(str
 		spin_lock_init(&info->lock);
 		info->flags = flags & VM_NORESERVE;
 		INIT_LIST_HEAD(&info->swaplist);
+		INIT_LIST_HEAD(&info->xattr_list);
 		cache_no_acl(inode);
 
 		switch (mode & S_IFMT) {
@@ -2048,62 +2062,225 @@ static void shmem_put_link(struct dentry
 	}
 }
 
-static const struct inode_operations shmem_symlink_inline_operations = {
-	.readlink	= generic_readlink,
-	.follow_link	= shmem_follow_link_inline,
-};
-
-static const struct inode_operations shmem_symlink_inode_operations = {
-	.readlink	= generic_readlink,
-	.follow_link	= shmem_follow_link,
-	.put_link	= shmem_put_link,
-};
-
-#ifdef CONFIG_TMPFS_POSIX_ACL
+#ifdef CONFIG_TMPFS_XATTR
 /*
- * Superblocks without xattr inode operations will get security.* xattr
- * support from the VFS "for free". As soon as we have any other xattrs
+ * Superblocks without xattr inode operations may get some security.* xattr
+ * support from the LSM "for free". As soon as we have any other xattrs
  * like ACLs, we also need to implement the security.* handlers at
  * filesystem level, though.
  */
 
-static size_t shmem_xattr_security_list(struct dentry *dentry, char *list,
-					size_t list_len, const char *name,
-					size_t name_len, int handler_flags)
+static int shmem_xattr_get(struct dentry *dentry, const char *name,
+			   void *buffer, size_t size)
 {
-	return security_inode_listsecurity(dentry->d_inode, list, list_len);
-}
+	struct shmem_inode_info *info;
+	struct shmem_xattr *xattr;
+	int ret = -ENODATA;
 
-static int shmem_xattr_security_get(struct dentry *dentry, const char *name,
-		void *buffer, size_t size, int handler_flags)
-{
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
-	return xattr_getsecurity(dentry->d_inode, name, buffer, size);
+	info = SHMEM_I(dentry->d_inode);
+
+	spin_lock(&dentry->d_inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		if (strcmp(name, xattr->name))
+			continue;
+
+		ret = xattr->size;
+		if (buffer) {
+			if (size < xattr->size)
+				ret = -ERANGE;
+			else
+				memcpy(buffer, xattr->value, xattr->size);
+		}
+		break;
+	}
+	spin_unlock(&dentry->d_inode->i_lock);
+	return ret;
 }
 
-static int shmem_xattr_security_set(struct dentry *dentry, const char *name,
-		const void *value, size_t size, int flags, int handler_flags)
+static int shmem_xattr_set(struct dentry *dentry, const char *name,
+			   const void *value, size_t size, int flags)
 {
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
-	return security_inode_setsecurity(dentry->d_inode, name, value,
-					  size, flags);
+	struct inode *inode = dentry->d_inode;
+	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_xattr *xattr;
+	struct shmem_xattr *new_xattr = NULL;
+	size_t len;
+	int err = 0;
+
+	/* value == NULL means remove */
+	if (value) {
+		/* wrap around? */
+		len = sizeof(*new_xattr) + size;
+		if (len <= sizeof(*new_xattr))
+			return -ENOMEM;
+
+		new_xattr = kmalloc(len, GFP_NOFS);
+		if (!new_xattr)
+		return -ENOMEM;
+
+		new_xattr->name = kstrdup(name, GFP_NOFS);
+		if (!new_xattr->name) {
+			kfree(new_xattr);
+			return -ENOMEM;
+		}
+
+		new_xattr->size = size;
+		memcpy(new_xattr->value, value, size);
+	}
+
+	spin_lock(&inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		if (!strcmp(name, xattr->name)) {
+			if (flags & XATTR_CREATE) {
+				xattr = new_xattr;
+				err = -EEXIST;
+			} else if (new_xattr) {
+				list_replace(&xattr->list, &new_xattr->list);
+			} else {
+				list_del(&xattr->list);
+			}
+			goto out;
+		}
+	}
+	if (flags & XATTR_REPLACE) {
+		xattr = new_xattr;
+		err = -ENODATA;
+	} else {
+		list_add(&new_xattr->list, &info->xattr_list);
+		xattr = NULL;
+	}
+out:
+	spin_unlock(&inode->i_lock);
+	if (xattr)
+		kfree(xattr->name);
+	kfree(xattr);
+	return 0;
 }
 
-static const struct xattr_handler shmem_xattr_security_handler = {
-	.prefix = XATTR_SECURITY_PREFIX,
-	.list   = shmem_xattr_security_list,
-	.get    = shmem_xattr_security_get,
-	.set    = shmem_xattr_security_set,
-};
 
 static const struct xattr_handler *shmem_xattr_handlers[] = {
+#ifdef CONFIG_TMPFS_POSIX_ACL
 	&generic_acl_access_handler,
 	&generic_acl_default_handler,
-	&shmem_xattr_security_handler,
+#endif
 	NULL
 };
+
+static int shmem_xattr_validate(const char *name)
+{
+	struct { const char *prefix; size_t len; } arr[] = {
+		{ XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN },
+		{ XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN }};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(arr); i++) {
+		size_t preflen = arr[i].len;
+		if (strncmp(name, arr[i].prefix, preflen) == 0) {
+			if (!name[preflen])
+				return -EINVAL;
+			return 0;
+		}
+	}
+	return -EOPNOTSUPP;
+}
+
+static ssize_t shmem_getxattr(struct dentry *dentry, const char *name,
+			      void *buffer, size_t size)
+{
+	int err;
+
+	/*
+	 * If this is a request for a synthetic attribute in the system.*
+	 * namespace use the generic infrastructure to resolve a handler
+	 * for it via sb->s_xattr.
+	 */
+	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
+		return generic_getxattr(dentry, name, buffer, size);
+
+	err = shmem_xattr_validate(name);
+	if (err)
+		return err;
+
+	return shmem_xattr_get(dentry, name, buffer, size);
+}
+
+static int shmem_setxattr(struct dentry *dentry, const char *name,
+			  const void *value, size_t size, int flags)
+{
+	int err;
+
+	/*
+	 * If this is a request for a synthetic attribute in the system.*
+	 * namespace use the generic infrastructure to resolve a handler
+	 * for it via sb->s_xattr.
+	 */
+	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
+		return generic_setxattr(dentry, name, value, size, flags);
+
+	err = shmem_xattr_validate(name);
+	if (err)
+		return err;
+
+	if (size == 0)
+		value = "";  /* empty EA, do not remove */
+
+	return shmem_xattr_set(dentry, name, value, size, flags);
+
+}
+
+static int shmem_removexattr(struct dentry *dentry, const char *name)
+{
+	int err;
+
+	/*
+	 * If this is a request for a synthetic attribute in the system.*
+	 * namespace use the generic infrastructure to resolve a handler
+	 * for it via sb->s_xattr.
+	 */
+	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
+		return generic_removexattr(dentry, name);
+
+	err = shmem_xattr_validate(name);
+	if (err)
+		return err;
+
+	return shmem_xattr_set(dentry, name, NULL, 0, XATTR_REPLACE);
+}
+
+static bool xattr_is_trusted(const char *name)
+{
+	return !strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN);
+}
+
+static ssize_t shmem_listxattr(struct dentry *dentry, char *buffer, size_t size)
+{
+	bool trusted = capable(CAP_SYS_ADMIN);
+	struct shmem_xattr *xattr;
+	struct shmem_inode_info *info;
+	size_t used = 0;
+
+	info = SHMEM_I(dentry->d_inode);
+
+	spin_lock(&dentry->d_inode->i_lock);
+	list_for_each_entry(xattr, &info->xattr_list, list) {
+		/* skip "trusted." attributes for unprivileged callers */
+		if (!trusted && xattr_is_trusted(xattr->name))
+			continue;
+
+		used += strlen(xattr->name) + 1;
+		if (buffer) {
+			if (size < used) {
+				used = -ERANGE;
+				break;
+			}
+			strncpy(buffer, xattr->name, strlen(xattr->name) + 1);
+			buffer += strlen(xattr->name) + 1;
+		}
+	}
+	spin_unlock(&dentry->d_inode->i_lock);
+
+	return used;
+}
 #endif
 
 static struct dentry *shmem_get_parent(struct dentry *child)
@@ -2384,8 +2561,10 @@ int shmem_fill_super(struct super_block
 	sb->s_magic = TMPFS_MAGIC;
 	sb->s_op = &shmem_ops;
 	sb->s_time_gran = 1;
-#ifdef CONFIG_TMPFS_POSIX_ACL
+#ifdef CONFIG_TMPFS_XATTR
 	sb->s_xattr = shmem_xattr_handlers;
+#endif
+#ifdef CONFIG_TMPFS_POSIX_ACL
 	sb->s_flags |= MS_POSIXACL;
 #endif
 
@@ -2483,16 +2662,41 @@ static const struct file_operations shme
 static const struct inode_operations shmem_inode_operations = {
 	.setattr	= shmem_notify_change,
 	.truncate_range	= shmem_truncate_range,
+#ifdef CONFIG_TMPFS_XATTR
+	.setxattr	= shmem_setxattr,
+	.getxattr	= shmem_getxattr,
+	.listxattr	= shmem_listxattr,
+	.removexattr	= shmem_removexattr,
+#endif
 #ifdef CONFIG_TMPFS_POSIX_ACL
-	.setxattr	= generic_setxattr,
-	.getxattr	= generic_getxattr,
-	.listxattr	= generic_listxattr,
-	.removexattr	= generic_removexattr,
 	.check_acl	= generic_check_acl,
 #endif
 
 };
 
+static const struct inode_operations shmem_symlink_inline_operations = {
+	.readlink	= generic_readlink,
+	.follow_link	= shmem_follow_link_inline,
+#ifdef CONFIG_TMPFS_XATTR
+	.setxattr	= shmem_setxattr,
+	.getxattr	= shmem_getxattr,
+	.listxattr	= shmem_listxattr,
+	.removexattr	= shmem_removexattr,
+#endif
+};
+
+static const struct inode_operations shmem_symlink_inode_operations = {
+	.readlink	= generic_readlink,
+	.follow_link	= shmem_follow_link,
+	.put_link	= shmem_put_link,
+#ifdef CONFIG_TMPFS_XATTR
+	.setxattr	= shmem_setxattr,
+	.getxattr	= shmem_getxattr,
+	.listxattr	= shmem_listxattr,
+	.removexattr	= shmem_removexattr,
+#endif
+};
+
 static const struct inode_operations shmem_dir_inode_operations = {
 #ifdef CONFIG_TMPFS
 	.create		= shmem_create,
@@ -2505,23 +2709,27 @@ static const struct inode_operations shm
 	.mknod		= shmem_mknod,
 	.rename		= shmem_rename,
 #endif
-#ifdef CONFIG_TMPFS_POSIX_ACL
-	.setattr	= shmem_notify_change,
+#ifdef CONFIG_TMPFS_XATTR
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= generic_listxattr,
 	.removexattr	= generic_removexattr,
+#endif
+#ifdef CONFIG_TMPFS_POSIX_ACL
+	.setattr	= shmem_notify_change,
 	.check_acl	= generic_check_acl,
 #endif
 };
 
 static const struct inode_operations shmem_special_inode_operations = {
-#ifdef CONFIG_TMPFS_POSIX_ACL
-	.setattr	= shmem_notify_change,
+#ifdef CONFIG_TMPFS_XATTR
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= generic_listxattr,
 	.removexattr	= generic_removexattr,
+#endif
+#ifdef CONFIG_TMPFS_POSIX_ACL
+	.setattr	= shmem_notify_change,
 	.check_acl	= generic_check_acl,
 #endif
 };
--
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