linux-kernel - Re: Re: [PATCH v2 3/3] efi: Capsule update with user helper interface

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Date:	Tue, 10 Mar 2015 13:26:03 -0400
From:	Peter Jones <pjones@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Matt Fleming <matt@...sole-pimps.org>,
	"Kweh, Hock Leong" <hock.leong.kweh@...el.com>,
	Sam Protsenko <semen.protsenko@...aro.org>,
	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Ong, Boon Leong" <boon.leong.ong@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>
Subject: Re: Re: [PATCH v2 3/3] efi: Capsule update with user helper interface

On Tue, Mar 10, 2015 at 08:51:59AM -0700, Andy Lutomirski wrote:
> On Tue, Mar 10, 2015 at 8:40 AM, Peter Jones <pjones@...hat.com> wrote:
> >
> >> >> So, for the sysfs interface, let's not allow loading from /lib. Let's
> >> >> not require a userland tool. Let's just do,
> >> >>
> >> >>   # echo /path/to/my/awesome/capsule.bin > /sys/../capsule
> >> >
> >> >>
> >> >> and be done with it.
> >> >>
> >> >> Hmmm?
> >> >
> >> > I assume you're implying a) the capsule header with the guid is embedded
> >> > in the .bin there already, and b) one contiguous write(2) with error
> >> > reporting coming through something like vars.c's efi_status_to_err()?
> >> >
> >> > If so, yes, I prefer this API.
> >> >
> >>
> >> Is using a char device really so bad?  I have a "simple_char" that
> >> makes this really easy that's pending review.
> >
> > As long as there's straightforward propagation of the EFI_STATUS return
> > from UpdateCapsule() back, sysfs file vs char device makes very little
> > difference to me.  Either way it's open(), write(), close().  Using the
> > runtime firmware upload interface designed for wifi and scsi devices is
> > the part I don't really like.
> >
> 
> I'm not 100% happy with write(2) (which is all we have in sysfs) for
> two reasons:
> 
> 1. If we write a file name, eww.  That's more complicated, requires
> temporary files, has annoying mount namespace issues, etc.
> 
> 2. If we write the full contents, we need to do it in a single call to
> write.  That means that we can't use cat, which mostly defeats the
> purpose.  In fact, using cat could be actively harmful.

So if what we wind up with is:

fd = open("/sys/.../capsule", O_RDWR);
write(fd, buf, size/N);
...
write(fd, buf + M*size/N, size/N);
close(fd);

You're suggesting the error code would post on close()?  My worry about
that is that I imagine a lot less code in the wild checks the error code
on close() than on write() - though gnu cat does do so on both.  But
there are other questions still - will it post on fdatasync()?  On
fsync()?

-- 
        Peter
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