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Date:   Thu, 28 Jan 2021 21:50:41 -0500
From:   "bfields@fieldses.org" <bfields@fieldses.org>
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "guy@vastdata.com" <guy@vastdata.com>,
        "schumakeranna@gmail.com" <schumakeranna@gmail.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] nfs: we don't support removing system.nfs4_acl
Message-ID: <20210129025041.GA12151@fieldses.org>
References: <20210128223638.GE29887@fieldses.org>
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 <cbc7115cc5d5aeb7ffb9e9b3880e453bf54ecbdb.camel@hammerspace.com>
 <20210129023527.GA11864@fieldses.org>
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On Thu, Jan 28, 2021 at 09:35:27PM -0500, bfields@fieldses.org wrote:
> Note that this patch doesn't prevent an application from setting a
> zero-length ACL.  The xattr format is XDR with the first four bytes
> representing the number of ACEs, so you'd set a zero-length ACL by
> passing down a 4-byte all-zero buffer as the new value of the
> system.nfs4_acl xattr.
> 
> A zero-length NULL buffer is what's used to implement removexattr:
> 
> int
> __vfs_removexattr(struct dentry *dentry, const char *name)
> {
> 	...
> 	return handler->set(handler, dentry, inode, name, NULL, 0, XATTR_REPLACE);
> }
> 
> That's the case this patch covers.

So, I should have said in the changelog, apologies--the behavior without
this patch is that when it gets a removexattr, the client sends a
SETATTR with a bitmap claiming there's an ACL attribute, but a
zero-length attribute value list, and the server (correctly) returns
BADXDR.

--b.