linux-kernel - Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

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Message-ID: <20160108124055.GD3097@leverpostej>
Date:	Fri, 8 Jan 2016 12:40:56 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	kernel-hardening@...ts.openwall.com,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stuart Yoder <stuart.yoder@...escale.com>,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <marc.zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

On Fri, Jan 08, 2016 at 01:38:54PM +0100, Ard Biesheuvel wrote:
> On 8 January 2016 at 13:36, Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Dec 30, 2015 at 04:26:11PM +0100, Ard Biesheuvel wrote:
> >> +
> >> +     /*
> >> +      * Iterate over each entry in the relocation table, and apply the
> >> +      * relocations in place.
> >> +      */
> >> +     adr_l   x8, __dynsym_start              // start of symbol table
> >> +     adr_l   x9, __reloc_start               // start of reloc table
> >> +     adr_l   x10, __reloc_end                // end of reloc table
> >> +
> >> +0:   cmp     x9, x10
> >> +     b.hs    2f
> >> +     ldp     x11, x12, [x9], #24
> >> +     ldr     x13, [x9, #-8]
> >> +     cmp     w12, #R_AARCH64_RELATIVE
> >> +     b.ne    1f
> >> +     add     x13, x13, x23                   // relocate
> >> +     str     x13, [x11, x23]
> >> +     b       0b
> >> +
> >> +1:   cmp     w12, #R_AARCH64_ABS64
> >> +     b.ne    0b
> >> +     add     x12, x12, x12, lsl #1           // symtab offset: 24x top word
> >> +     add     x12, x8, x12, lsr #(32 - 3)     // ... shifted into bottom word
> >> +     ldrsh   w14, [x12, #6]                  // Elf64_Sym::st_shndx
> >> +     ldr     x15, [x12, #8]                  // Elf64_Sym::st_value
> >> +     cmp     w14, #-0xf                      // SHN_ABS (0xfff1) ?
> >> +     add     x14, x15, x23                   // relocate
> >> +     csel    x15, x14, x15, ne
> >> +     add     x15, x13, x15
> >> +     str     x15, [x11, x23]
> >> +     b       0b
> >
> > We need to clean each of these relocated instructions to the PoU to be
> > visible for I-cache fetches.
> >
> > As this is normal-cacheable we can post the maintenance with a DC CVAU
> > immediately after the store (no barriers necessary), and rely on the DSB
> > at 2f to complete all of those.
> >
> 
> Dynamic relocations never apply to instructions, so i don't think that
> is necessary here.

Ah, yes. I was being thick. Sorry for the noise!

Thanks,
Mark.