linux-kernel - RE: [PATCH 2/4] arm64: implement support for static call trampolines

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Date:   Sat, 25 Sep 2021 17:46:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mark Rutland' <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        Quentin Perret <qperret@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: RE: [PATCH 2/4] arm64: implement support for static call trampolines

From: Mark Rutland
> Sent: 21 September 2021 17:28
> 
> On Tue, Sep 21, 2021 at 05:55:11PM +0200, Ard Biesheuvel wrote:
> > On Tue, 21 Sept 2021 at 17:33, Mark Rutland <mark.rutland@....com> wrote:
> > >
> > > On Tue, Sep 21, 2021 at 04:44:56PM +0200, Ard Biesheuvel wrote:
> > > > On Tue, 21 Sept 2021 at 09:10, Peter Zijlstra <peterz@...radead.org> wrote:
> > ...
...
> > >
> > > I think so, yes. We can do sligntly better with an inline literal pool
> > > and a PC-relative LDR to fold the ADRP+LDR, e.g.
> > >
> > >         .align 3
> > > tramp:
> > >         BTI     C
> > >         {B <func> | RET | NOP}
> > >         LDR     X16, 1f
> > >         BR      X16
> > > 1:      .quad   <literal>
> > >
> > > Since that's in the .text, it's RO for regular accesses anyway.
> > >
> >
> > I tried to keep the literal in .rodata to avoid inadvertent gadgets
> > and/or anticipate exec-only mappings of .text, but that may be a bit
> > overzealous.
> 
> I think that in practice the risk of gadgetisation is minimal, and
> having it inline means we only need to record a single address per
> trampoline, so there's less risk that we get the patching wrong.

But doesn't that mean that it is almost certainly a data cache miss?
You really want an instruction that reads the constant from the I-cache.
Or at least be able to 'bunch together' the constants so they
stand a chance of sharing a D-cache line.

	David

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