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Re: [PATCH v2 1/5] x86/livepatch: set function alignment to ensure minimal function size
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 1 Dec 2023 07:53:29 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 01 Dec 2023 06:53:45 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.11.2023 18:37, Roger Pau Monné wrote:
> On Thu, Nov 30, 2023 at 05:55:07PM +0100, Jan Beulich wrote:
>> On 28.11.2023 11:03, Roger Pau Monne wrote:
>>> The minimal function size requirements for livepatch are either 5 bytes (for
>>> jmp) or 9 bytes (for endbr + jmp). Ensure that functions are always at
>>> least
>>> that size by requesting the compiled to align the functions to 8 or 16
>>> bytes,
>>> depending on whether Xen is build with IBT support.
>>
>> How is alignment going to enforce minimum function size? If a function is
>> last in a section, there may not be any padding added (ahead of linking at
>> least). The trailing padding also isn't part of the function.
>
> If each function lives in it's own section (by using
> -ffunction-sections), and each section is aligned, then I think we can
> guarantee that there will always be enough padding space?
>
> Even the last function/section on the .text block would still be
> aligned, and as long as the function alignment <= SECTION_ALIGN
> there will be enough padding left. I should add some build time
> assert that CONFIG_CC_FUNCTION_ALIGNMENT <= SECTION_ALIGN.
I'm not sure of there being a requirement for a section to be padded to
its alignment. If the following section has smaller alignment, it could
be made start earlier. Of course our linker scripts might guarantee
this ...
> While the trailing padding is not part of the function itself, I don't
> see issues in overwriting it with the replacement function code.
If it's really padding, yes.
Jan
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