From: David Laight <David.Laight@ACULAB.COM>
To: 'Ard Biesheuvel' <ardb@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <frederic@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
James Morse <james.morse@arm.com>,
"Quentin Perret" <qperret@google.com>,
Catalin Marinas <catalin.marinas@arm.com>,
"Will Deacon" <will@kernel.org>,
Mark Rutland <mark.rutland@arm.com>
Subject: RE: [PATCH 2/4] arm64: implement support for static call trampolines
Date: Mon, 25 Oct 2021 15:03:07 +0000 [thread overview]
Message-ID: <cf2bf7a61a0a4fca8425b96e139d71e2@AcuMS.aculab.com> (raw)
In-Reply-To: <CAMj1kXEKASsYJMHHNA=uNGTnLMoXO_4BP0--1k7cEfZZupdsog@mail.gmail.com>
From: Ard Biesheuvel
> Sent: 25 October 2021 15:55
>
> On Mon, 25 Oct 2021 at 16:47, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Oct 25, 2021 at 04:19:16PM +0200, Peter Zijlstra wrote:
> > > On Mon, Oct 25, 2021 at 04:08:37PM +0200, Ard Biesheuvel wrote:
> >
> > > > > Ooohh, but what if you go from !func to NOP.
> > > > >
> > > > > assuming:
> > > > >
> > > > > .literal = 0
> > > > > BTI C
> > > > > RET
> > > > >
> > > > > Then
> > > > >
> > > > > CPU0 CPU1
> > > > >
> > > > > [S] literal = func [I] NOP
> > > > > [S] insn[1] = NOP [L] x16 = literal (NULL)
> > > > > b x16
> > > > > *BANG*
> > > > >
> > > > > Is that possible? (total lack of memory ordering etc..)
> > > > >
> > > >
> > > > The CBZ will branch to the RET instruction if x16 == 0x0, so this
> > > > should not happen.
> > >
> > > Oooh, I missed that :/ I was about to suggest writing the address of a
> > > bare 'ret' trampoline instead of NULL into the literal.
> >
> > Perhaps a little something like so.. Shaves 2 instructions off each
> > trampoline.
> >
> > --- a/arch/arm64/include/asm/static_call.h
> > +++ b/arch/arm64/include/asm/static_call.h
> > @@ -11,9 +11,7 @@
> > " hint 34 /* BTI C */ \n" \
> > insn " \n" \
> > " ldr x16, 0b \n" \
> > - " cbz x16, 1f \n" \
> > " br x16 \n" \
> > - "1: ret \n" \
> > " .popsection \n")
> >
> > #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \
> > --- a/arch/arm64/kernel/patching.c
> > +++ b/arch/arm64/kernel/patching.c
> > @@ -90,6 +90,11 @@ int __kprobes aarch64_insn_write(void *a
> > return __aarch64_insn_write(addr, &i, AARCH64_INSN_SIZE);
> > }
> >
> > +asm("__static_call_ret: \n"
> > + " ret \n")
> > +
>
> This breaks BTI as it lacks the landing pad, and it will be called indirectly.
>
> > +extern void __static_call_ret(void);
> > +
>
> Better to have an ordinary C function here (with consistent linkage),
> but we need to take the address in a way that works with Clang CFI.
>
> As the two additional instructions are on an ice cold path anyway, I'm
> not sure this is an obvious improvement tbh.
If my sums are correct the code block is exactly 32 bytes.
So no point saving an instruction.
But you could have:
.long 1f
label:
bti c
nop/branch
ldr x16, 0b
br x16
1: bti c
ret
That is all self-contained.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2021-10-25 15:03 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-25 12:20 [PATCH 0/4] arm64: Support dynamic preemption v2 Frederic Weisbecker
2021-10-25 12:20 ` [PATCH 1/4] sched/preempt: Prepare for supporting !CONFIG_GENERIC_ENTRY dynamic preemption Frederic Weisbecker
2021-10-25 12:21 ` [PATCH 2/4] arm64: implement support for static call trampolines Frederic Weisbecker
2021-10-25 13:56 ` Peter Zijlstra
2021-10-25 14:08 ` Ard Biesheuvel
2021-10-25 14:19 ` Peter Zijlstra
2021-10-25 14:44 ` Peter Zijlstra
2021-10-25 14:55 ` Ard Biesheuvel
2021-10-25 15:03 ` Peter Zijlstra
2021-10-25 15:10 ` Ard Biesheuvel
2021-10-26 10:36 ` Mark Rutland
2021-10-26 10:45 ` Peter Zijlstra
2021-10-26 11:06 ` David Laight
2021-10-27 12:47 ` Mark Rutland
2021-10-25 15:03 ` David Laight [this message]
2021-10-25 14:25 ` David Laight
2021-10-25 14:31 ` Ard Biesheuvel
2021-10-25 14:38 ` David Laight
2021-10-25 12:21 ` [PATCH 3/4] arm64: Implement IRQ exit preemption static call for dynamic preemption Frederic Weisbecker
2021-10-25 12:21 ` [PATCH 4/4] arm64: Implement HAVE_PREEMPT_DYNAMIC Frederic Weisbecker
-- strict thread matches above, loose matches on Subject: below --
2021-09-20 23:32 [PATCH 0/4] arm64: Support dynamic preemption Frederic Weisbecker
2021-09-20 23:32 ` [PATCH 2/4] arm64: implement support for static call trampolines Frederic Weisbecker
2021-09-21 7:09 ` Peter Zijlstra
2021-09-21 14:44 ` Ard Biesheuvel
2021-09-21 15:08 ` Peter Zijlstra
2021-09-21 15:33 ` Mark Rutland
2021-09-21 15:55 ` Ard Biesheuvel
2021-09-21 16:28 ` Mark Rutland
2021-09-25 17:46 ` David Laight
2021-09-27 8:58 ` Mark Rutland
2021-09-21 16:10 ` Ard Biesheuvel
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