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

Catalin Marinas
2024-04-03T17:52:35Z

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@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.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@arm.com>