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
