linux-kernel - Re: [PATCH v2] EFI loader: remove redundant code

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Date:   Wed, 2 Nov 2016 13:57:58 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Eugene Korenevsky <ekorenevsky@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Linn Crosetto <linn@....com>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] EFI loader: remove redundant code

On 2 November 2016 at 13:56, Eugene Korenevsky <ekorenevsky@...il.com> wrote:
>> > *e820ext is always NULL in 'alloc_e820ext()' (see the code of 'exit_boot()').
>> > Without loss of generality we can replace freeing with returning
>> > EFI_INVALID_PARAMETER. So if the caller would ever incorrectly pass non-NULL
>> > *e820ext, he will obtain a returned error code.
>> >
>>
>> What exactly are you trying to fix here? Adding new artificial failure
>> modes is hardly worth it when all you are doing is cleaning up code
>> that by itself is correct to begin with, but is simply never called.
>
> This code (free_pool, assignments) is dead whether it is correct or
> not. So it is to be removed.
> The check gives some assurance that memory is not leaked if the calling
> code is changed.
>

No, it doesn't. This code executes in the context of the UEFI
firmware, which is riddled with bugs on older x86 platforms (as well
as non newer non-x86 platforms). By returning an error at runtime, you
may be breaking the boot for someone who wil have *no* idea whatsoever
what is going on.

If you want to clean this up in a way that prevents future issues,
please use something like BUILD_BUG()