linux-kernel - Re: [PATCH v2 1/4] arm64: patching: always use fixmap

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Message-ID: <Zg2XXmMb_wWS4fJH@arm.com>
Date: Wed, 3 Apr 2024 18:52:30 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
	anil.s.keshavamurthy@...el.com, aou@...s.berkeley.edu,
	davem@...emloft.net, jarkko@...nel.org,
	linux-arm-kernel@...ts.infradead.org, mhiramat@...nel.org,
	naveen.n.rao@...ux.ibm.com, palmer@...belt.com,
	paul.walmsley@...ive.com
Subject: Re: [PATCH v2 1/4] arm64: patching: always use fixmap

On Wed, Apr 03, 2024 at 04:01:51PM +0100, Mark Rutland wrote:
> For historical reasons, patch_map() won't bother to fixmap non-image
> addresses when CONFIG_STRICT_MODULE_RWX=n, matching the behaviour prior
> to the introduction of CONFIG_STRICT_MODULE_RWX. However, as arm64
> doesn't select CONFIG_ARCH_OPTIONAL_KERNEL_RWX, CONFIG_MODULES implies
> CONFIG_STRICT_MODULE_RWX, so any kernel built with module support will
> use the fixmap for any non-image address.
> 
> Historically we only used patch_map() for the kernel image and modules,
> but these days its also used by BPF and KPROBES to write to read-only
> pages of executable text. Currently these both depend on CONFIG_MODULES,
> but we'd like to change that in subsequent patches, which will require
> using the fixmap regardless of CONFIG_STRICT_MODULE_RWX.
> 
> This patch changes patch_map() to always use the fixmap, and simplifies
> the logic:
> 
> * Use is_image_text() directly in the if-else, rather than using a
>   temporary boolean variable.
> 
> * Use offset_in_page() to get the offset within the mapping.
> 
> * Remove uintaddr and cast the address directly when using
>   is_image_text().
> 
> For kernels built with CONFIG_MODULES=y, there should be no functional
> change as a result of this patch.
> 
> For kernels built with CONFIG_MODULES=n, patch_map() will use the fixmap
> for non-image addresses, but there are no extant users with non-image
> addresses when CONFIG_MODULES=n, and hence there should be no functional
> change as a result of this patch alone.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> ---
>  arch/arm64/kernel/patching.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> Catalin, Will, this is a prerequisite for the final two patches in the
> series. Are you happy for this go via the tracing tree?

Fine by me.

Acked-by: Catalin Marinas <catalin.marinas@....com>