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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.12.2023 12:18, Roger Pau Monne wrote:
> The minimal function size requirements for livepatch are either 5 bytes (for
> 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.
> 
> The compiler option -falign-functions is not available on at least clang 3.8,
> so introduce a Kconfig check for it and make the livepatch option depend on 
> the
> compiler supporting the option.
> 
> The naming of the option(s) CONFIG_FUNCTION_ALIGNMENT is explicitly not
> mentioning CC in preparation for the option also being used by assembly code.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>