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    SubjectRe: [PATCH] tmpfs: implement generic xattr support
    Quoting Miklos Szeredi (miklos@szeredi.hu):
    > Michal Suchanek <hramrach@centrum.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@redhat.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@redhat.com>
    > Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

    Looks good to me.

    Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>

    thanks,
    -serge

    > ---
    > 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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