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linux/fs/ext4/acl.c
Theodore Ts'o 210ad6aedb ext4: avoid unnecessary spinlock in critical POSIX ACL path 
If a filesystem supports POSIX ACL's, the VFS layer expects the filesystem
to do POSIX ACL checks on any files not owned by the caller, and it does
this for every single pathname component that it looks up.

That obviously can be pretty expensive if the filesystem isn't careful
about it, especially with locking. That's doubly sad, since the common
case tends to be that there are no ACL's associated with the files in
question.

ext4 already caches the ACL data so that it doesn't have to look it up
over and over again, but it does so by taking the inode->i_lock spinlock
on every lookup. Which is a noticeable overhead even if it's a private
lock, especially on CPU's where the serialization is expensive (eg Intel
Netburst aka 'P4').

For the special case of not actually having any ACL's, all that locking is
unnecessary. Even if somebody else were to be changing the ACL's on
another CPU, we simply don't care - if we've seen a NULL ACL, we might as
well use it.

So just load the ACL speculatively without any locking, and if it was
NULL, just use it. If it's non-NULL (either because we had a cached
entry, or because the cache hasn't been filled in at all), it means that
we'll need to get the lock and re-load it properly.

(This commit was ported from a patch originally authored by Linus for
ext3.)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2009-06-17 00:36:35 -04:00

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/*
* linux/fs/ext4/acl.c
*
* Copyright (C) 2001-2003 Andreas Gruenbacher, <agruen@suse.de>
*/
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/capability.h>
#include <linux/fs.h>
#include "ext4_jbd2.h"
#include "ext4.h"
#include "xattr.h"
#include "acl.h"
/*
* Convert from filesystem to in-memory representation.
*/
static struct posix_acl *
ext4_acl_from_disk(const void *value, size_t size)
{
const char *end = (char *)value + size;
int n, count;
struct posix_acl *acl;
if (!value)
return NULL;
if (size < sizeof(ext4_acl_header))
return ERR_PTR(-EINVAL);
if (((ext4_acl_header *)value)->a_version !=
cpu_to_le32(EXT4_ACL_VERSION))
return ERR_PTR(-EINVAL);
value = (char *)value + sizeof(ext4_acl_header);
count = ext4_acl_count(size);
if (count < 0)
return ERR_PTR(-EINVAL);
if (count == 0)
return NULL;
acl = posix_acl_alloc(count, GFP_NOFS);
if (!acl)
return ERR_PTR(-ENOMEM);
for (n = 0; n < count; n++) {
ext4_acl_entry *entry =
(ext4_acl_entry *)value;
if ((char *)value + sizeof(ext4_acl_entry_short) > end)
goto fail;
acl->a_entries[n].e_tag = le16_to_cpu(entry->e_tag);
acl->a_entries[n].e_perm = le16_to_cpu(entry->e_perm);
switch (acl->a_entries[n].e_tag) {
case ACL_USER_OBJ:
case ACL_GROUP_OBJ:
case ACL_MASK:
case ACL_OTHER:
value = (char *)value +
sizeof(ext4_acl_entry_short);
acl->a_entries[n].e_id = ACL_UNDEFINED_ID;
break;
case ACL_USER:
case ACL_GROUP:
value = (char *)value + sizeof(ext4_acl_entry);
if ((char *)value > end)
goto fail;
acl->a_entries[n].e_id =
le32_to_cpu(entry->e_id);
break;
default:
goto fail;
}
}
if (value != end)
goto fail;
return acl;
fail:
posix_acl_release(acl);
return ERR_PTR(-EINVAL);
}
/*
* Convert from in-memory to filesystem representation.
*/
static void *
ext4_acl_to_disk(const struct posix_acl *acl, size_t *size)
{
ext4_acl_header *ext_acl;
char *e;
size_t n;
*size = ext4_acl_size(acl->a_count);
ext_acl = kmalloc(sizeof(ext4_acl_header) + acl->a_count *
sizeof(ext4_acl_entry), GFP_NOFS);
if (!ext_acl)
return ERR_PTR(-ENOMEM);
ext_acl->a_version = cpu_to_le32(EXT4_ACL_VERSION);
e = (char *)ext_acl + sizeof(ext4_acl_header);
for (n = 0; n < acl->a_count; n++) {
ext4_acl_entry *entry = (ext4_acl_entry *)e;
entry->e_tag = cpu_to_le16(acl->a_entries[n].e_tag);
entry->e_perm = cpu_to_le16(acl->a_entries[n].e_perm);
switch (acl->a_entries[n].e_tag) {
case ACL_USER:
case ACL_GROUP:
entry->e_id = cpu_to_le32(acl->a_entries[n].e_id);
e += sizeof(ext4_acl_entry);
break;
case ACL_USER_OBJ:
case ACL_GROUP_OBJ:
case ACL_MASK:
case ACL_OTHER:
e += sizeof(ext4_acl_entry_short);
break;
default:
goto fail;
}
}
return (char *)ext_acl;
fail:
kfree(ext_acl);
return ERR_PTR(-EINVAL);
}
static inline struct posix_acl *
ext4_iget_acl(struct inode *inode, struct posix_acl **i_acl)
{
struct posix_acl *acl = ACCESS_ONCE(*i_acl);
if (acl) {
spin_lock(&inode->i_lock);
acl = *i_acl;
if (acl != EXT4_ACL_NOT_CACHED)
acl = posix_acl_dup(acl);
spin_unlock(&inode->i_lock);
}
return acl;
}
static inline void
ext4_iset_acl(struct inode *inode, struct posix_acl **i_acl,
struct posix_acl *acl)
{
spin_lock(&inode->i_lock);
if (*i_acl != EXT4_ACL_NOT_CACHED)
posix_acl_release(*i_acl);
*i_acl = posix_acl_dup(acl);
spin_unlock(&inode->i_lock);
}
/*
* Inode operation get_posix_acl().
*
* inode->i_mutex: don't care
*/
static struct posix_acl *
ext4_get_acl(struct inode *inode, int type)
{
struct ext4_inode_info *ei = EXT4_I(inode);
int name_index;
char *value = NULL;
struct posix_acl *acl;
int retval;
if (!test_opt(inode->i_sb, POSIX_ACL))
return NULL;
switch (type) {
case ACL_TYPE_ACCESS:
acl = ext4_iget_acl(inode, &ei->i_acl);
if (acl != EXT4_ACL_NOT_CACHED)
return acl;
name_index = EXT4_XATTR_INDEX_POSIX_ACL_ACCESS;
break;
case ACL_TYPE_DEFAULT:
acl = ext4_iget_acl(inode, &ei->i_default_acl);
if (acl != EXT4_ACL_NOT_CACHED)
return acl;
name_index = EXT4_XATTR_INDEX_POSIX_ACL_DEFAULT;
break;
default:
return ERR_PTR(-EINVAL);
}
retval = ext4_xattr_get(inode, name_index, "", NULL, 0);
if (retval > 0) {
value = kmalloc(retval, GFP_NOFS);
if (!value)
return ERR_PTR(-ENOMEM);
retval = ext4_xattr_get(inode, name_index, "", value, retval);
}
if (retval > 0)
acl = ext4_acl_from_disk(value, retval);
else if (retval == -ENODATA || retval == -ENOSYS)
acl = NULL;
else
acl = ERR_PTR(retval);
kfree(value);
if (!IS_ERR(acl)) {
switch (type) {
case ACL_TYPE_ACCESS:
ext4_iset_acl(inode, &ei->i_acl, acl);
break;
case ACL_TYPE_DEFAULT:
ext4_iset_acl(inode, &ei->i_default_acl, acl);
break;
}
}
return acl;
}
/*
* Set the access or default ACL of an inode.
*
* inode->i_mutex: down unless called from ext4_new_inode
*/
static int
ext4_set_acl(handle_t *handle, struct inode *inode, int type,
struct posix_acl *acl)
{
struct ext4_inode_info *ei = EXT4_I(inode);
int name_index;
void *value = NULL;
size_t size = 0;
int error;
if (S_ISLNK(inode->i_mode))
return -EOPNOTSUPP;
switch (type) {
case ACL_TYPE_ACCESS:
name_index = EXT4_XATTR_INDEX_POSIX_ACL_ACCESS;
if (acl) {
mode_t mode = inode->i_mode;
error = posix_acl_equiv_mode(acl, &mode);
if (error < 0)
return error;
else {
inode->i_mode = mode;
ext4_mark_inode_dirty(handle, inode);
if (error == 0)
acl = NULL;
}
}
break;
case ACL_TYPE_DEFAULT:
name_index = EXT4_XATTR_INDEX_POSIX_ACL_DEFAULT;
if (!S_ISDIR(inode->i_mode))
return acl ? -EACCES : 0;
break;
default:
return -EINVAL;
}
if (acl) {
value = ext4_acl_to_disk(acl, &size);
if (IS_ERR(value))
return (int)PTR_ERR(value);
}
error = ext4_xattr_set_handle(handle, inode, name_index, "",
value, size, 0);
kfree(value);
if (!error) {
switch (type) {
case ACL_TYPE_ACCESS:
ext4_iset_acl(inode, &ei->i_acl, acl);
break;
case ACL_TYPE_DEFAULT:
ext4_iset_acl(inode, &ei->i_default_acl, acl);
break;
}
}
return error;
}
static int
ext4_check_acl(struct inode *inode, int mask)
{
struct posix_acl *acl = ext4_get_acl(inode, ACL_TYPE_ACCESS);
if (IS_ERR(acl))
return PTR_ERR(acl);
if (acl) {
int error = posix_acl_permission(inode, acl, mask);
posix_acl_release(acl);
return error;
}
return -EAGAIN;
}
int
ext4_permission(struct inode *inode, int mask)
{
return generic_permission(inode, mask, ext4_check_acl);
}
/*
* Initialize the ACLs of a new inode. Called from ext4_new_inode.
*
* dir->i_mutex: down
* inode->i_mutex: up (access to inode is still exclusive)
*/
int
ext4_init_acl(handle_t *handle, struct inode *inode, struct inode *dir)
{
struct posix_acl *acl = NULL;
int error = 0;
if (!S_ISLNK(inode->i_mode)) {
if (test_opt(dir->i_sb, POSIX_ACL)) {
acl = ext4_get_acl(dir, ACL_TYPE_DEFAULT);
if (IS_ERR(acl))
return PTR_ERR(acl);
}
if (!acl)
inode->i_mode &= ~current_umask();
}
if (test_opt(inode->i_sb, POSIX_ACL) && acl) {
struct posix_acl *clone;
mode_t mode;
if (S_ISDIR(inode->i_mode)) {
error = ext4_set_acl(handle, inode,
ACL_TYPE_DEFAULT, acl);
if (error)
goto cleanup;
}
clone = posix_acl_clone(acl, GFP_NOFS);
error = -ENOMEM;
if (!clone)
goto cleanup;
mode = inode->i_mode;
error = posix_acl_create_masq(clone, &mode);
if (error >= 0) {
inode->i_mode = mode;
if (error > 0) {
/* This is an extended ACL */
error = ext4_set_acl(handle, inode,
ACL_TYPE_ACCESS, clone);
}
}
posix_acl_release(clone);
}
cleanup:
posix_acl_release(acl);
return error;
}
/*
* Does chmod for an inode that may have an Access Control List. The
* inode->i_mode field must be updated to the desired value by the caller
* before calling this function.
* Returns 0 on success, or a negative error number.
*
* We change the ACL rather than storing some ACL entries in the file
* mode permission bits (which would be more efficient), because that
* would break once additional permissions (like ACL_APPEND, ACL_DELETE
* for directories) are added. There are no more bits available in the
* file mode.
*
* inode->i_mutex: down
*/
int
ext4_acl_chmod(struct inode *inode)
{
struct posix_acl *acl, *clone;
int error;
if (S_ISLNK(inode->i_mode))
return -EOPNOTSUPP;
if (!test_opt(inode->i_sb, POSIX_ACL))
return 0;
acl = ext4_get_acl(inode, ACL_TYPE_ACCESS);
if (IS_ERR(acl) || !acl)
return PTR_ERR(acl);
clone = posix_acl_clone(acl, GFP_KERNEL);
posix_acl_release(acl);
if (!clone)
return -ENOMEM;
error = posix_acl_chmod_masq(clone, inode->i_mode);
if (!error) {
handle_t *handle;
int retries = 0;
retry:
handle = ext4_journal_start(inode,
EXT4_DATA_TRANS_BLOCKS(inode->i_sb));
if (IS_ERR(handle)) {
error = PTR_ERR(handle);
ext4_std_error(inode->i_sb, error);
goto out;
}
error = ext4_set_acl(handle, inode, ACL_TYPE_ACCESS, clone);
ext4_journal_stop(handle);
if (error == -ENOSPC &&
ext4_should_retry_alloc(inode->i_sb, &retries))
goto retry;
}
out:
posix_acl_release(clone);
return error;
}
/*
* Extended attribute handlers
*/
static size_t
ext4_xattr_list_acl_access(struct inode *inode, char *list, size_t list_len,
const char *name, size_t name_len)
{
const size_t size = sizeof(POSIX_ACL_XATTR_ACCESS);
if (!test_opt(inode->i_sb, POSIX_ACL))
return 0;
if (list && size <= list_len)
memcpy(list, POSIX_ACL_XATTR_ACCESS, size);
return size;
}
static size_t
ext4_xattr_list_acl_default(struct inode *inode, char *list, size_t list_len,
const char *name, size_t name_len)
{
const size_t size = sizeof(POSIX_ACL_XATTR_DEFAULT);
if (!test_opt(inode->i_sb, POSIX_ACL))
return 0;
if (list && size <= list_len)
memcpy(list, POSIX_ACL_XATTR_DEFAULT, size);
return size;
}
static int
ext4_xattr_get_acl(struct inode *inode, int type, void *buffer, size_t size)
{
struct posix_acl *acl;
int error;
if (!test_opt(inode->i_sb, POSIX_ACL))
return -EOPNOTSUPP;
acl = ext4_get_acl(inode, type);
if (IS_ERR(acl))
return PTR_ERR(acl);
if (acl == NULL)
return -ENODATA;
error = posix_acl_to_xattr(acl, buffer, size);
posix_acl_release(acl);
return error;
}
static int
ext4_xattr_get_acl_access(struct inode *inode, const char *name,
void *buffer, size_t size)
{
if (strcmp(name, "") != 0)
return -EINVAL;
return ext4_xattr_get_acl(inode, ACL_TYPE_ACCESS, buffer, size);
}
static int
ext4_xattr_get_acl_default(struct inode *inode, const char *name,
void *buffer, size_t size)
{
if (strcmp(name, "") != 0)
return -EINVAL;
return ext4_xattr_get_acl(inode, ACL_TYPE_DEFAULT, buffer, size);
}
static int
ext4_xattr_set_acl(struct inode *inode, int type, const void *value,
size_t size)
{
handle_t *handle;
struct posix_acl *acl;
int error, retries = 0;
if (!test_opt(inode->i_sb, POSIX_ACL))
return -EOPNOTSUPP;
if (!is_owner_or_cap(inode))
return -EPERM;
if (value) {
acl = posix_acl_from_xattr(value, size);
if (IS_ERR(acl))
return PTR_ERR(acl);
else if (acl) {
error = posix_acl_valid(acl);
if (error)
goto release_and_out;
}
} else
acl = NULL;
retry:
handle = ext4_journal_start(inode, EXT4_DATA_TRANS_BLOCKS(inode->i_sb));
if (IS_ERR(handle))
return PTR_ERR(handle);
error = ext4_set_acl(handle, inode, type, acl);
ext4_journal_stop(handle);
if (error == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
goto retry;
release_and_out:
posix_acl_release(acl);
return error;
}
static int
ext4_xattr_set_acl_access(struct inode *inode, const char *name,
const void *value, size_t size, int flags)
{
if (strcmp(name, "") != 0)
return -EINVAL;
return ext4_xattr_set_acl(inode, ACL_TYPE_ACCESS, value, size);
}
static int
ext4_xattr_set_acl_default(struct inode *inode, const char *name,
const void *value, size_t size, int flags)
{
if (strcmp(name, "") != 0)
return -EINVAL;
return ext4_xattr_set_acl(inode, ACL_TYPE_DEFAULT, value, size);
}
struct xattr_handler ext4_xattr_acl_access_handler = {
.prefix = POSIX_ACL_XATTR_ACCESS,
.list = ext4_xattr_list_acl_access,
.get = ext4_xattr_get_acl_access,
.set = ext4_xattr_set_acl_access,
};
struct xattr_handler ext4_xattr_acl_default_handler = {
.prefix = POSIX_ACL_XATTR_DEFAULT,
.list = ext4_xattr_list_acl_default,
.get = ext4_xattr_get_acl_default,
.set = ext4_xattr_set_acl_default,
};
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