linux-kernel - Re: [PATCH] efi/arm: fix allocation failure when reserving the kernel base

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Message-ID: <CAKv+Gu8DBA8SoJ49dMeYjXHchGZpAiLgxLEQF7jQGtdQ6camdw@mail.gmail.com>
Date:   Tue, 20 Aug 2019 15:27:23 +0300
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Chester Lin <clin@...e.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "ren_guo@...ky.com" <ren_guo@...ky.com>,
        Juergen Gross <JGross@...e.com>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "guillaume.gardet@....com" <guillaume.gardet@....com>,
        Joey Lee <JLee@...e.com>, Gary Lin <GLin@...e.com>
Subject: Re: [PATCH] efi/arm: fix allocation failure when reserving the kernel base

On Tue, 20 Aug 2019 at 14:54, Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Sun, Aug 04, 2019 at 10:57:00AM +0300, Ard Biesheuvel wrote:
> > (The first TEXT_OFFSET bytes are no longer used in practice, which is
> > why putting a reserved region of 4 KB bytes works at the moment, but
> > this is fragile).
>
> That is not correct for 32-bit ARM.  The swapper page table is still
> located 16kiB below the kernel.
>

Right. So that means we can only permit reserved regions in the first
(TEXT_OFFSET - 16 kiB) bytes starting at the first 128 MiB aligned
address covered by system RAM, if we want to ensure that the
decompressor or the early kernel don't trample on it. (or TEXT_OFFSET
- 20 kiB for LPAE kernels)