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Re: [PATCH v6 3/3] xen/livepatch: align functions to ensure minimal distance between entry points
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 13 Feb 2024 16:58:38 +0100
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- Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>, Alistair Francis <alistair.francis@xxxxxxx>, Bob Eshleman <bobbyeshleman@xxxxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 13 Feb 2024 15:58:55 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07.02.2024 15:55, Roger Pau Monne wrote:
> The minimal function size requirements for an x86 livepatch are either 5 bytes
> (for jmp) or 9 bytes (for endbr + jmp), and always 4 bytes on Arm. Ensure
> that
> distance between functions entry points is always at least of the minimal
> required size for livepatch instruction replacement to be successful.
>
> Add an additional align directive to the linker scripts, in order to ensure
> that
> the next section placed after the .text.* (per-function sections) is also
> aligned to the required boundary, so that the distance of the last function
> entry point with the next symbol is also of minimal size.
Perhaps "... minimal required size"?
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -395,8 +395,11 @@ config CRYPTO
> config LIVEPATCH
> bool "Live patching support"
> default X86
> - depends on "$(XEN_HAS_BUILD_ID)" = "y"
> + depends on "$(XEN_HAS_BUILD_ID)" = "y" && CC_HAS_FUNCTION_ALIGNMENT
> select CC_SPLIT_SECTIONS
> + select FUNCTION_ALIGNMENT_16B if XEN_IBT
> + select FUNCTION_ALIGNMENT_8B if X86
> + select FUNCTION_ALIGNMENT_4B if ARM
This isn't strictly needed, is it? Would be nice to avoid re-selection
of what the default for an arch is anyway, as otherwise this will start
looking clumsy when a couple more architectures are added. Preferably
with that dropped (or it being clarified why it's still desirable to
have):
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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