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Message-ID: <CAMj1kXGYYGRMXAa+k+ysDmfbW2XsTF56yEr8=3Q__mw_Jt4j8Q@mail.gmail.com>
Date: Mon, 30 Mar 2020 14:36:25 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
kernel-hardening@...ts.openwall.com,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature
On Mon, 30 Mar 2020 at 13:29, Mark Rutland <mark.rutland@....com> wrote:
>
> On Sun, Mar 29, 2020 at 04:12:58PM +0200, Ard Biesheuvel wrote:
> > When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with
> > different permissions (r-x for .text, r-- for .rodata, rw- for .data,
> > etc) are rounded up to 2 MiB so they can be mapped more efficiently.
> > In particular, it permits the segments to be mapped using level 2
> > block entries when using 4k pages, which is expected to result in less
> > TLB pressure.
> >
> > However, the mappings for the bulk of the kernel will use level 2
> > entries anyway, and the misaligned fringes are organized such that they
> > can take advantage of the contiguous bit, and use far fewer level 3
> > entries than would be needed otherwise.
> >
> > This makes the value of this feature dubious at best, and since it is not
> > enabled in defconfig or in the distro configs, it does not appear to be
> > in wide use either. So let's just remove it.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>
> No strong feelings either way, but getting rid of code is usually good,
> so:
>
> Acked-by: Mark Rutland <mark.rutland@....com>
>
Thanks Mark.
This is related to [0], which increases the PE/COFF section alignment
to 64k so that a KASLR enabled kernel always lands at an address at
which it can execute without being moved around first. This is an
improvement in itself, but also provides 5 bits (log2(2M / 64k)) of
wiggle room for the virtual as well as the physical placement of the
kernel. CONFIG_DEBUG_ALIGN_RODATA kind of interferes with that, so I'd
like to get rid of it.
Catalin, Will: if you have no objections, I can include this in my
series for v5.8 and take it via the EFI tree.
Thanks,
Ard.
[0] https://lore.kernel.org/linux-efi/20200326165905.2240-1-ardb@kernel.org/
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