Re: [PATCH v4 1/4] x86/livepatch: align functions to ensure minimal distance between entry points

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.12.2023 09:32, Roger Pau Monné wrote:
> On Tue, Dec 19, 2023 at 07:46:11PM +0000, Andrew Cooper wrote:
>> On 15/12/2023 11:18 am, Roger Pau Monne wrote:
>>> The minimal function size requirements for livepatch are either 5 bytes (for
>>
>> "for an x86 livepatch", seeing as we're touching multiple architectures
>> worth of files.
>>
>> I know it's at the end of the sentence, but it wants to be earlier to be
>> clearer.
>>
>>> jmp) or 9 bytes (for endbr + jmp) on x86, 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 script, 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.
>>>
>>> Note that it's possible for the compiler to end up using a higher function
>>> alignment regardless of the passed value, so this change just make sure that
>>> the minimum required for livepatch to work is present.  Different compilers
>>> handle the option differently, as clang will ignore -falign-functions value
>>> if it's smaller than the one that would be set by the optimization level, 
>>> while
>>> gcc seems to always honor the function alignment passed in 
>>> -falign-functions.
>>> In order to cope with this behavior and avoid that setting -falign-functions
>>> results in an alignment inferior to what the optimization level would have
>>> selected force x86 release builds to use a function alignment of 16 bytes.
>>
>> Yuck :(
>>
>> The same will be true for all other architectures too?
> 
> I would expect that for gcc I guess.
> 
>> What happens on ARM, which also picks up an explicit choice in livepatch
>> builds?
> 
> Arm AFAICT seems to use a 4 byte function alignment with -O2 (both gcc
> and clang), so that matches what we need to enforce for livepatch.  If
> we ever need a higher alignment for livepatch reasons it would be a
> multiple of the minimum one set by the compiler, so that should be
> fine.

Thinking of it: The forcing of 16-byte alignment in release builds of x86
is based on observations with certain compiler versions, iirc. What if
future versions decide to go lower/higher for, perhaps, very good reasons?
We don't really mean to override the compiler's choice, so maybe further
probing is actually necessary?

Jan