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

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Message-Id: <20240404084450.802bae64e3ed08197fd0dad1@kernel.org>
Date: Thu, 4 Apr 2024 08:44:50 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Mark Rutland <mark.rutland@....com>, 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, 3 Apr 2024 18:52:30 +0100
Catalin Marinas <catalin.marinas@....com> wrote:

> 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>

Thanks Catalin. I'll pick this series to linux-trace tree.

Thank you!

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>