linux-kernel - Re: [PATCH v2 0/7] linkage: better symbol aliasing

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Date:   Tue, 25 Jan 2022 15:58:51 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        acme@...hat.com, Borislav Petkov <bp@...en8.de>,
        Mark Brown <broonie@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 0/7] linkage: better symbol aliasing

On Tue, Jan 25, 2022 at 04:49:03PM +0100, Ard Biesheuvel wrote:
> On Tue, 25 Jan 2022 at 16:46, Mark Rutland <mark.rutland@....com> wrote:
> >
> > On Tue, Jan 25, 2022 at 04:28:11PM +0100, Ard Biesheuvel wrote:
> > > On Tue, 25 Jan 2022 at 12:32, Mark Rutland <mark.rutland@....com> wrote:
> > > >
> > > > This series aims to make symbol aliasing simpler and more consistent.
> > > > The basic idea is to replace SYM_FUNC_START_ALIAS(alias) and
> > > > SYM_FUNC_END_ALIAS(alias) with a new SYM_FUNC_ALIAS(alias, name), so
> > > > that e.g.
> > > >
> > > >     SYM_FUNC_START(func)
> > > >     SYM_FUNC_START_ALIAS(alias1)
> > > >     SYM_FUNC_START_ALIAS(alias2)
> > > >         ... asm insns ...
> > > >     SYM_FUNC_END(func)
> > > >     SYM_FUNC_END_ALIAS(alias1)
> > > >     SYM_FUNC_END_ALIAS(alias2)
> > > >     EXPORT_SYMBOL(alias1)
> > > >     EXPORT_SYMBOL(alias2)
> > > >
> > > > ... can become:
> > > >
> > > >     SYM_FUNC_START(name)
> > > >         ... asm insns ...
> > > >     SYM_FUNC_END(name)
> > > >
> > > >     SYM_FUNC_ALIAS(alias1, func)
> > > >     EXPORT_SYMBOL(alias1)
> > > >
> > > >     SYM_FUNC_ALIAS(alias2, func)
> > > >     EXPORT_SYMBOL(alias2)
> > > >
> > > > This avoids repetition and hopefully make it easier to ensure
> > > > consistency (e.g. so each function has a single canonical name and
> > > > associated metadata).
> > > >
> > >
> > > I take it this affects the sizes of the alias ELF symbols? Does that matter?
> >
> > The alias should be given the same size as the original symbol, unless I've
> > made an error. If you look at patch 3:
> >
> > * In SYM_FUNC_START(name), via SYM_ENTRY_AT(name, ...), we create a local label
> >   for the start of the function: .L____sym_entry__##name
> >
> > * In SYM_FUNC_END(name), via SYM_END_AT(name, ...), we create a local label for
> >   the end of the function: .L____sym_end__##name
> >
> > * In SYM_FUNC_ALIAS*(alias,name), we define the start and end of the alias as
> >   the start and end of the original symbol using those local labels, e.g.
> >
> >   | #define SYM_FUNC_ALIAS(alias, name)                                     \
> >   |         SYM_START_AT(alias, .L____sym_entry__##name, SYM_L_GLOBAL)      \
> >   |         SYM_END_AT(alias, .L____sym_end__##name, SYM_T_FUNC)
> >
> > Note that:
> >
> > * SYM_FUNC_START() uses SYM_START(), which uses SYM_ENTRY_AT()
> > * SYM_FUNC_END() uses SYM_END(), which uses SYM_END_AT()
> >
> > ... so the definition of tha alias is ultimately structurally identical to the
> > definition of the canoncial name, at least for now.
> >
> 
> Ah right, apologies for not looking more carefully - I assumed the
> changed placement implied that the aliases had zero size.

NP; I didn't make that clear in the cover letter, and now it's written up and
archived for future reference. :)

> And ultimately, I don't think there is an obviously correct answer
> anyway, it's just the [apparently non-existent] change in behavior I
> was curious about.

FWIW, I hadn't really considered whether we actually need that for the aliases;
it was jsut the path of least resistance implementation-wise, and I'd like to
be able to use the local labls for the bounds in future for other annotations
and sanity-checks.

Thanks,
Mark.