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

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Message-Id: <48E5761B-8142-4CDE-9AAD-BCD70C34BC13@holtmann.org>
Date:	Tue, 17 Mar 2015 09:03:38 -0700
From:	Marcel Holtmann <marcel@...tmann.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
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

Hi Matt,

>> 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.

meaning only writev() with a single vector is supported.

>> 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).

I have no problems with these semantics, but it means you need to add support for .write_iter since otherwise multiple vectors will not work. And really think that is better than forcing userspace to work around this limitation.

As I mentioned earlier, for me it was really obvious to use a vectored write for EFI variables. That just made sense to me. So seeing it not working caused confusion.

Regards

Marcel

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