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Re: [PATCH v6 3/3] xen/livepatch: align functions to ensure minimal distance between entry points
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 26 Feb 2024 13:36:32 +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: Mon, 26 Feb 2024 12:36:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.02.2024 12:32, Roger Pau Monné wrote:
> On Tue, Feb 13, 2024 at 04:58:38PM +0100, Jan Beulich wrote:
>> 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"?
>
> Yes.
>
>>> --- 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.
>
> My worry was that the default per-arch could change, ie: for example
> x86 moving from 16 to 8 and then it would hamper livepatch support if
> IBT is also enabled. I however think it's very unlikely to reduce the
> default alignment, and in any case we would hit a build time assert if
> that ever happens.
>
> So yes, I'm fine with dropping those.
Oh, no - not "those", only "that", i.e. only the last (Arm) one.
Jan
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