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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

> 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.
> 
> Since the option (-falign-functions) is supported by both minimal required
> compiler versions of clang and gcc there's no need to add a test to check for
> its presence.
> 
> The alignment is currently only implemented for livepatch on x86, I'm unsure
> whether ARM has a mandatory function alignment high enough to cover for the
> space required by the replacement instruction(s).

Indeed I was wondering whether this shouldn't be generalized, if indeed
required.

Jan