linux-kernel - Re: efivarfs and writev() support

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Message-ID: <20150317154630.GA25605@codeblueprint.co.uk>
Date:	Tue, 17 Mar 2015 15:46:30 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Marcel Holtmann <marcel@...tmann.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-efi@...r.kernel.org,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Jeremy Kerr <jk@...abs.org>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: efivarfs and writev() support

On Sat, 14 Mar, at 09:33:00AM, Marcel Holtmann wrote:
> 
> I say we should support writev() on efivarfs. Not supporting it seems
> odd especially since that is not documented anywhere. So yes, I am for
> adding .write_iter() support and be done with that.

Well, as Al has explained it's not that writev() isn't supported, it's
that having an iovec vector containing only the 4-byte variable
attribute isn't currently supported.

Since writev() calls are intended to be atomic, and even though efivarfs
gets fed the variable data in chunks we can process it in one go, I
think allowing the scenario you describe is fine.

> Also please note that even write(,4) and write(,n) does not work
> either. You can not write partial entries as it seems. Maybe you are
> able to append, but it seems the initial creation of the variable has
> to be done with a single write() call. Anything else ends up in a file
> with 0 length.

Yes, that's by design. I guess it's to prohibit people from creating
bogus EFI variables or accidentally deleting variables (a SetVariable()
call with length 0 is a delete).

-- 
Matt Fleming, Intel Open Source Technology Center
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