linux-kernel - Re: [PATCH v4 2/2] efi: an sysfs interface for user to update efi firmware

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Date:	Wed, 22 Apr 2015 10:45:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Kweh Hock Leong <hock.leong.kweh@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	"Ong, Boon Leong" <boon.leong.ong@...el.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: [PATCH v4 2/2] efi: an sysfs interface for user to update efi firmware

On Wed, Apr 22, 2015 at 10:34 AM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Wed, 2015-04-22 at 09:50 -0700, Andy Lutomirski wrote:
>> On Apr 21, 2015 9:51 PM, "James Bottomley"
>> <James.Bottomley@...senpartnership.com> wrote:
>> >
>> > On Tue, 2015-04-21 at 20:24 -0700, Andy Lutomirski wrote:
>> > > On Tue, Apr 21, 2015 at 7:20 PM, James Bottomley
>> > > <James.Bottomley@...senpartnership.com> wrote:
>> > > > On Tue, 2015-04-21 at 18:58 -0700, Andy Lutomirski wrote:
>> > > >> On Tue, Apr 21, 2015 at 6:21 PM, James Bottomley
>> > > >> <James.Bottomley@...senpartnership.com> wrote:
>> > > >> > Andy, just on the misc device idea, what about triggering the capsule
>> > > >> > update from close()?  In theory close returns an error code (not sure if
>> > > >> > most tools actually check this, though).  That means we can do the write
>> > > >> > in chunks but pass it in atomically on the close and cat will work
>> > > >> > (provided it checks the return code of close).
>> > > >>
>> > > >> I thought about this but IIRC cat doesn't check the return value from close.
>> > > >
>> > > > It does in my copy (coreutils-8.23) :
>> > > >
>> > > >       if (!STREQ (infile, "-") && close (input_desc) < 0)
>> > > >         {
>> > > >           error (0, errno, "%s", infile);
>> > > >           ok = false;
>> > > >         }
>> > > > [...]
>> > > >   if (have_read_stdin && close (STDIN_FILENO) < 0)
>> > > >     error (EXIT_FAILURE, errno, _("closing standard input"));
>> > > >
>> > >
>> > > True, but it's stdout that we care about, not stdin :(
>> >
>> > Gosh you're determined to force me to wade through this source code,
>> > aren't you?  That's handled in lib/closeout.c:
>> >
>> > /* Close standard output.  On error, issue a diagnostic and _exit
>> >    with status 'exit_failure'.
>> >
>> > ...
>> >
>> >
>> > The point is that, admittedly much to my surprise, it all looks to be
>> > handled by cat ... so we could proceed to have the transaction completed
>> > in close in a misc device (or a sysfs file).
>> >
>> > Unless there are any other rabbits you'd like to pull out of the hat?
>>
>> No, maybe it's okay, unless there's an issue where the error would
>> only be returned on the close of the last reference of the struct
>> file.  After all, 'cat foo >/sys/bar' doesn't fully close /sys/bar
>> until after cat exits.
>
> No, cat handles that too.  It has an atexit() handler for closing
> stdout.

Indeed.  Strace tells me that:

$ cat foo >bar

opens bar and then execs cat, so cat has the only reference to bar.

So no more rabbits from me :)

--Andy
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