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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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."

Thanks, Roger.