Re: [RFC PATCH] CIFS posix acl permission checking

[Jon Severinsson]

On Monday 01 March 2010 18:03:49, Matthew Wilcox wrote:
> On Mon, Mar 01, 2010 at 04:15:10PM +0100, Jon Severinsson wrote:
>> Firstly, please note that I'm new to the kernel community, so if I'm
>> doing something wrong, just please politely point it out to me and I'll
>> try to fix it, or at the very least not do it again. Also please bear in
>> mind that English isn't my native language.
> 
> Okey dokey, here's some minor mistakes you made.

Thank you.

>  - Generally a good idea to cc the CIFS development list for CIFS patches.
>    I'm sure they'll notice it here, but just to be on the safe side, I've
>    added them to the cc.

Sure thing. I used linux-fsdevel as I didn't find anything more specific on 
vger.kernel.org, didn't think to look at lists.samba.org.

>  - You've included ifdefs around the check_acl entry in inode_operations,
>    *and* inside the definition of cifs_check_acl.  You only need to do one
>    or the other, and opinion is divided on which is better.

While I did recognize the redundancy, I decided to follow the same convention 
the other functions in xattr.c did, and include ifdefs at both locations.

I also considered the possible reasons for the existing functions to do both, 
and and came up with two reasons. The first simply being the paradigm of 
defensive programming, always check before doing a call, but never assume that 
the check has been done before being called. The second one is that of 
performance. The ifdefs has to be in cifs_check_acl to protect against other 
callers (while this patch doesn't introduce any, it doesn't prevent further 
patches from adding them), and not including the ifdefs in inode_operations 
would mean a completely useless function call on every permission check when a 
feature was turned off at compile time. The second one is a micro-optimization 
I 
don't really care fore, but defensive programming I do respect.

With this in mind, what do you recommend, double protection, breaking 
convention or changing the existing code?

>> --- a/fs/cifs/xattr.c
>> +++ b/fs/cifs/xattr.c
>> @@ -374,3 +374,74 @@ ssize_t cifs_listxattr(struct dentry *direntry, char
>> *data, size_t buf_size) #endif
>>  	return rc;
>>  }
>> +
>> +int cifs_check_acl(struct inode *inode, int mask)
>> +{
>> +	int rc = -EOPNOTSUPP;
>> +#ifdef CONFIG_CIFS_XATTR
>> +#ifdef CONFIG_CIFS_POSIX
>> +	struct dentry *dentry = NULL;
>> +	size_t buf_size;
>> +	void *ea_value = NULL;
>> +	ssize_t ea_size;
>> +	struct posix_acl *acl = NULL;
> 
> I don't think you need to initialise ea_value, do you?

While currently correct, I find it a good idea to immediately null any pointer 
that is freed in the exit section. Otherwise it is very easy to forget to do 
that the day someone adds a goto prior to the first assignment, and not nulling 
then can have unintended consequences in unrelated code. That being said, if 
you say that the kernel community frowns upon that kind of defensive 
programming I will definitely remove it.

With your suggested improvement below, there is no need to NULL dentry though, 
so I'll remove that.

>> +	/* CIFS gets acl from server by path, and thus needs a dentry rather 
>> than 
>> +	   an inode. Note that the path of each dentry will point to the
>> same inode 
>> +	   on the backing fs at the server, so their acls will be
>> the same, and it 
>> +	   doesn't matter which one we pick, so just pick the
>> fist. */ 
>> +	if (list_empty(&inode->i_dentry))
>> +		dentry = list_first_entry(&inode->i_dentry, struct dentry, 
>> d_alias);
>> +
>> +	/* If we didn't get an dentry for the inode, something went terribly
>> wrong. 
>> +	   All we can do at this point is to return an error. */
>> +	if (!dentry || IS_ERR(dentry))
>> +		return PTR_ERR(dentry);
> 
> I don't think dentry can possibly be IS_ERR here, and returning
> PTR_ERR(NULL) doesn't do what you think it does.  Also, you've got the
>  sense of list_empty backwards.  I think what you meant to do here was:
> 
> 	if (!list_empty(&inode->i_dentry))
> 		dentry = list_first_entry(&inode->i_dentry, struct dentry,
> 								d_alias);
> 	else
> 		return -EINVAL;

Sure, this makes more sense. Originally I used d_find_alias(inode), but later 
thought it was better to use the information in the inode struct directly. 
I never really understood how list.h lists was supposed to work, but stopped 
when cut-and-paste got it working for me. Probably a bad idea...

Attaching an updated patch, mostly for the convenience of anyone only 
following linux-cifs-client.

And thanks again for your input.

Best Regards
Jon Severinsson
commit fa0b9415cda17b31966542101bc4ceb0c97c87cb
Author: Jon Severinsson <jon@xxxxxxxxxxxxxxx>
Date:   Mon Mar 1 19:24:30 2010 +0100

    [CIFS] Adds support for permision checking vs. posix acl.
    
    CIFS already supports getting and setting acls through getfacl and setfacl, but
    prior to this patch, any acls was ignored when doing permission checking.

diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 29f1da7..0605e11 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -284,7 +284,7 @@ static int cifs_permission(struct inode *inode, int mask)
 		on the client (above and beyond ACL on servers) for
 		servers which do not support setting and viewing mode bits,
 		so allowing client to check permissions is useful */
-		return generic_permission(inode, mask, NULL);
+		return generic_permission(inode, mask, inode->i_op->check_acl);
 }
 
 static struct kmem_cache *cifs_inode_cachep;
@@ -702,6 +702,9 @@ const struct inode_operations cifs_dir_inode_ops = {
 	.getxattr = cifs_getxattr,
 	.listxattr = cifs_listxattr,
 	.removexattr = cifs_removexattr,
+#ifdef CONFIG_CIFS_POSIX
+	.check_acl = cifs_check_acl,
+#endif
 #endif
 };
 
@@ -716,6 +719,9 @@ const struct inode_operations cifs_file_inode_ops = {
 	.getxattr = cifs_getxattr,
 	.listxattr = cifs_listxattr,
 	.removexattr = cifs_removexattr,
+#ifdef CONFIG_CIFS_POSIX
+	.check_acl = cifs_check_acl,
+#endif
 #endif
 };
 
@@ -732,6 +738,9 @@ const struct inode_operations cifs_symlink_inode_ops = {
 	.getxattr = cifs_getxattr,
 	.listxattr = cifs_listxattr,
 	.removexattr = cifs_removexattr,
+#ifdef CONFIG_CIFS_POSIX
+	.check_acl = cifs_check_acl,
+#endif
 #endif
 };
 
diff --git a/fs/cifs/cifsfs.h b/fs/cifs/cifsfs.h
index ac2b24c..6409a83 100644
--- a/fs/cifs/cifsfs.h
+++ b/fs/cifs/cifsfs.h
@@ -102,11 +102,15 @@ extern int cifs_readlink(struct dentry *direntry, char __user *buffer,
 			 int buflen);
 extern int cifs_symlink(struct inode *inode, struct dentry *direntry,
 			const char *symname);
+
+/* Functions related to extended attributes */
 extern int	cifs_removexattr(struct dentry *, const char *);
 extern int 	cifs_setxattr(struct dentry *, const char *, const void *,
 			size_t, int);
 extern ssize_t	cifs_getxattr(struct dentry *, const char *, void *, size_t);
 extern ssize_t	cifs_listxattr(struct dentry *, char *, size_t);
+extern int cifs_check_acl(struct inode *inode, int mask);
+
 extern long cifs_ioctl(struct file *filep, unsigned int cmd, unsigned long arg);
 
 #ifdef CONFIG_CIFS_EXPERIMENTAL
diff --git a/fs/cifs/xattr.c b/fs/cifs/xattr.c
index a75afa3..a07633b 100644
--- a/fs/cifs/xattr.c
+++ b/fs/cifs/xattr.c
@@ -374,3 +374,71 @@ ssize_t cifs_listxattr(struct dentry *direntry, char *data, size_t buf_size)
 #endif
 	return rc;
 }
+
+int cifs_check_acl(struct inode *inode, int mask)
+{
+	int rc = -EOPNOTSUPP;
+#ifdef CONFIG_CIFS_XATTR
+#ifdef CONFIG_CIFS_POSIX
+	struct dentry *dentry;
+	size_t buf_size;
+	void *ea_value = NULL;
+	ssize_t ea_size;
+	struct posix_acl *acl = NULL;
+
+	/* CIFS gets acl from server by path, and thus needs a dentry rather than
+	   an inode. Note that the path of each dentry will point to the same inode
+	   on the backing fs at the server, so their acls will be the same, and it
+	   doesn't matter which one we pick, so just pick the fist. */
+	if (!list_empty(&inode->i_dentry))
+		dentry = list_first_entry(&inode->i_dentry, struct dentry, d_alias);
+	else
+		return -EINVAL;
+
+	/* Try to fit the extended attribute corresponding to the posix acl in 4k
+	   memory. 4k was chosen because it always fits in a single page, and is
+	   the maximum on a default ext2/3/4 backing fs. */
+	buf_size = 4096;
+	ea_value = kmalloc(buf_size, GFP_KERNEL);
+	if (!ea_value) {
+		rc = -EAGAIN;
+		goto check_acl_exit;
+	}
+	ea_size = cifs_getxattr(dentry, POSIX_ACL_XATTR_ACCESS, ea_value, buf_size);
+
+	/* If 4k wasn't enough, try 64k, the maximum on any current backing fs. */
+	if (ea_size == -ERANGE) {
+		kfree(ea_value);
+		buf_size = 65536;
+		ea_value = kmalloc(buf_size, GFP_KERNEL);
+		if (!ea_value) {
+			rc = -EAGAIN;
+			goto check_acl_exit;
+		}
+		ea_size = cifs_getxattr(dentry, POSIX_ACL_XATTR_ACCESS, ea_value, buf_size);
+	}
+
+	/* If we didn't get any extended attribute, set the error code and exit */
+	if (ea_size <= 0) {
+		rc = -EAGAIN;
+		if (ea_size == -EOPNOTSUPP || ea_size == -EIO || ea_size == -ENOTDIR || ea_size == -ENOENT || ea_size == -EFAULT || ea_size == -EACCES)
+			rc = ea_size;
+		goto check_acl_exit;
+	}
+
+	/* Set the appropriate return value. Adapted from ext4. */
+	acl = posix_acl_from_xattr(ea_value, ea_size);
+	if (IS_ERR(acl))
+		rc = PTR_ERR(acl);
+	else if (acl)
+		rc = posix_acl_permission(inode, acl, mask);
+	else
+		rc = -EAGAIN;
+
+check_acl_exit:
+	posix_acl_release(acl);
+	kfree(ea_value);
+#endif /* CONFIG_CIFS_POSIX */
+#endif /* CONFIG_CIFS_XATTR */
+	return rc;
+}