linux-kernel - Re: [PATCH v5 1/2] xen/PVH: Set up GS segment for stack canary

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Message-Id: <5B058BA002000078001C523F@prv1-mh.provo.novell.com>
Date:   Wed, 23 May 2018 09:41:20 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:     <brgerst@...il.com>, "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] xen/PVH: Set up GS segment for stack canary

>>> On 23.05.18 at 16:30, <boris.ostrovsky@...cle.com> wrote:
> @@ -98,6 +101,12 @@ ENTRY(pvh_start_xen)
>  	/* 64-bit entry point. */
>  	.code64
>  1:
> +	/* Set base address in stack canary descriptor. */
> +	mov $MSR_GS_BASE,%ecx
> +	mov $_pa(canary), %rax
> +	xor %rdx, %rdx

Why rax and rdx instead of eax and edx? In the former case, the
relocation produced might confuse whatever entity processing it
(it'll have a sign-extended 32-bit quantity to deal with, which
wouldn't allow representing an address in the [2Gb, 4Gb) range).
In the latter case, while surely neither performance nor code size
matter much here, it's still a bad precedent (people copy-and-paste
code all the time): Zero-ing of registers should generally use the
32-bit forms of the insn. Gas has actually gained an optimization
mode recently (upon request from Linus and the x86 maintainers)
to silently "repair" such inefficiencies.

Jan