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From:   Matthew Garrett <mjg59@google.com>
Date:   Tue, 20 Feb 2018 23:39:27 +0000
Message-ID: <CACdnJussCnLqdGGyOc=iUwEdot_U2DZjGcVWRcQa-DxVcUhabA@mail.gmail.com>
Subject: Re: [PATCH 1/2] fs/efivarfs: restrict inode permissions
To:     tony.luck@intel.com
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        joe.konno@linux.intel.com, linux-efi <linux-efi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        matthew.garrett@nebula.com, Jeremy Kerr <jk@ozlabs.org>,
        ak@linux.intel.com, pjones@redhat.com, luto@kernel.org,
        James Bottomley <james.bottomley@hansenpartnership.com>
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On Tue, Feb 20, 2018 at 3:30 PM Luck, Tony <tony.luck@intel.com> wrote:
> [1] I didn't dig through the Linux code to check whether we manage to
> get those four SMIs from a single EFI call, or whether we make multiple
> EFI calls to open/read/close one file. It is possible that we stink a
> bit too if we are doing more EFI calls than required.

read() will make two calls - one to obtain the size of the variable, the
other to read it. It looks like cat will also trigger an fstat(), so we're
probably also making a call for that. There's presumably some optimisation
that could be made there if we trust the firmware not to change the size
behind our back.