linux-kernel - Re: [PATCH 1/3] arm64: clean up trampoline vector loads

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Date:   Mon, 23 Mar 2020 12:14:37 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Rémi Denis-Courmont <remi@...lab.net>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, james.morse@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] arm64: clean up trampoline vector loads

On Mon, Mar 23, 2020 at 02:08:53PM +0200, Rémi Denis-Courmont wrote:
> Le maanantaina 23. maaliskuuta 2020, 14.07.00 EET Mark Rutland a écrit :
> > On Thu, Mar 19, 2020 at 11:14:05AM +0200, Rémi Denis-Courmont wrote:
> > > From: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> > > 
> > > This switches from custom instruction patterns to the regular large
> > > memory model sequence with ADRP and LDR. In doing so, the ADD
> > > instruction can be eliminated in the SDEI handler, and the code no
> > > longer assumes that the trampoline vectors and the vectors address both
> > > start on a page boundary.
> > > 
> > > Signed-off-by: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> > > ---
> > > 
> > >  arch/arm64/kernel/entry.S | 9 ++++-----
> > >  1 file changed, 4 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > > index e5d4e30ee242..24f828739696 100644
> > > --- a/arch/arm64/kernel/entry.S
> > > +++ b/arch/arm64/kernel/entry.S
> > > @@ -805,9 +805,9 @@ alternative_else_nop_endif
> > > 
> > >  2:
> > >  	tramp_map_kernel	x30
> > >  
> > >  #ifdef CONFIG_RANDOMIZE_BASE
> > > 
> > > -	adr	x30, tramp_vectors + PAGE_SIZE
> > > +	adrp	x30, tramp_vectors + PAGE_SIZE
> > > 
> > >  alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
> > > 
> > > -	ldr	x30, [x30]
> > > +	ldr	x30, [x30, #:lo12:__entry_tramp_data_start]
> > 
> > I think this is busted for !4K kernels once we reduce the alignment of
> > __entry_tramp_data_start.
> > 
> > The ADRP gives us a 64K aligned address (with bits 15:0 clear). The lo12
> > relocation gives us bits 11:0, so we haven't accounted for bits 15:12.
> 
> IMU, ADRP gives a 4K aligned value, regardless of MMU (TCR) settings.

Sorry, I had erroneously assumed tramp_vectors was page aligned. The
issue still stands -- we haven't accounted for bits 15:12, as those can
differ between tramp_vectors and __entry_tramp_data_start.

Thanks,
Mark.