linux-kernel - Re: VDSO unmap and remap support for additional architectures

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Message-ID: <2ce7203f-305c-6edf-0ef9-448c141cb103@kernel.org>
Date:	Thu, 28 Apr 2016 11:53:22 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Christopher Covington <cov@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>, criu@...nvz.org,
	Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
	Will Deacon <Will.Deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, dsafonov@...tuozzo.com
Subject: Re: VDSO unmap and remap support for additional architectures

On 04/28/2016 08:18 AM, Christopher Covington wrote:
> Please take a look at the following prototype of sharing the PowerPC
> VDSO unmap and remap code with other architectures. I've only hooked
> up arm64 to begin with. If folks think this is a reasonable approach I
> can work on 32 bit ARM as well. Not hearing back from an earlier
> request for guidance [1], I simply dove in and started hacking.
> Laurent's test case [2][3] is a compelling illustration of whether VDSO
> remap works or not on a given architecture.

I think there's a much nicer way:

https://lkml.kernel.org/r/1461584223-9418-1-git-send-email-dsafonov@virtuozzo.com

Could arm64 and ppc use this approach?  These arch_xyz hooks are gross.

Also, at some point, possibly quite soon, x86 will want a way for user code to ask the kernel to map a specific vdso variant at a specific address.  Could we perhaps add a new pair of syscalls:

struct vdso_info {
    unsigned long space_needed_before;
    unsigned long space_needed_after;
    unsigned long alignment;
};

long vdso_get_info(unsigned int vdso_type, struct vdso_info *info);

long vdso_remap(unsigned int vdso_type, unsigned long addr, unsigned int flags);

#define VDSO_X86_I386 0
#define VDSO_X86_64 1
#define VDSO_X86_X32 2
// etc.

vdso_remap will map the vdso of the chosen type such at AT_SYSINFO_EHDR lines up with addr.  It will use up to space_needed_before bytes before that address and space_needed_after after than address.  It will also unmap the old vdso (or maybe only do that if some flag is set).

On x86, mremap is *not* sufficient for everything that's needed, because some programs will need to change the vdso type.

--Andy