Re: [PATCH v2 1/5] x86/livepatch: set function alignment to ensure minimal function size

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.12.2023 09:50, Roger Pau Monné wrote:
> On Fri, Dec 01, 2023 at 07:53:29AM +0100, Jan Beulich wrote:
>> 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 ...
> 
> I do think so, given our linker script arrangements for the .text
> section:
> 
> DECL_SECTION(.text) {
>     [...]
> } PHDR(text) = 0x9090
> 
> . = ALIGN(SECTION_ALIGN);
> 
> The end of the text section is aligned to SECTION_ALIGN, so as long as
> SECTION_ALIGN >= CONFIG_CC_FUNCTION_ALIGNMENT the alignment should
> guarantee a minimal function size.
> 
> Do you think it would be clearer if I add the following paragraph:
> 
> "Given the Xen linker script arrangement of the .text section, we can
> ensure that when all functions are aligned to the given boundary the
> function size will always be a multiple of such alignment, even for
> the last function in .text, as the linker script aligns the end of the
> section to SECTION_ALIGN."

I think this would be useful to have there. Beyond that, assembly code
also needs considering btw.

Jan