Re: [PATCH v8 05/14] x86: conditionally place regular ASM functions into separate sections

Ard Biesheuvel
2021-12-04T08:31:52Z

On Fri, 3 Dec 2021 at 20:46, Nicolas Pitre <nico@fluxnic.net> wrote:
>
> On Fri, 3 Dec 2021, Peter Zijlstra wrote:
>
> > On Fri, Dec 03, 2021 at 03:10:51PM +0100, Alexander Lobakin wrote:
> > > From: Peter Zijlstra <peterz@infradead.org>
> > > Date: Fri, 3 Dec 2021 10:44:10 +0100
> > >
> > > > On Thu, Dec 02, 2021 at 11:32:05PM +0100, Alexander Lobakin wrote:
> > > > > Use the newly introduces macros to create unique separate sections
> > > > > for (almost) every "regular" ASM function (i.e. for those which
> > > > > aren't explicitly put into a specific one).
> > > > > There should be no leftovers as input .text will be size-asserted
> > > > > in the LD script generated for FG-KASLR.
> > > >
> > > > *groan*...
> > > >
> > > > Please, can't we do something like:
> > > >
> > > > #define SYM_PUSH_SECTION(name) \
> > > > .if section == .text \
> > > > .push_section .text.##name \
> > > > .else \
> > > > .push_section .text \
> > > > .endif
> > > >
> > > > #define SYM_POP_SECTION() \
> > > > .pop_section
> > > >
> > > > and wrap that inside the existing SYM_FUNC_START*() SYM_FUNC_END()
> > > > macros.
> > >
> > > Ah I see. I asked about this in my previous mail and you replied
> > > already (: Cool stuff, I'll use it, it simplifies things a lot.
> >
> > Note, I've no idea if it works. GAS and me aren't really on speaking
> > terms. It would be my luck for that to be totally impossible, hjl?
>
> Surely this would do it:
>
> http://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=451133cefa839104
>

That seems rather useful, actually. It will also fix a problem with
subsections, which are sometimes difficult to construct from a macro,
as they cannot be created using pushsection/popsection unless you know
the current section name, and the alternative syntax (.subsection /
.previous) does not permit nesting. This makes their use from a macro
risky, given that it may not be obvious to the macro's caller that it
uses a subsection under the hood.