linux-kernel - Re: [PATCH v2] x86: Remove compat vdso support

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Date:	Mon, 10 Mar 2014 21:10:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Stefani Seibold <stefani@...bold.net>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [PATCH v2] x86: Remove compat vdso support

On Mon, Mar 10, 2014 at 8:09 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Mar 10, 2014 at 7:37 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> It does.  My patch breaks OpenSuSE 9 when
>> CONFIG_ENABLE_VDSO32_BY_DEFAULT=y unless it's overridden by sysctl or
>> boot option.
>
> Oh, I missed that "when =y" part.
>
> But why do we then want to have that "=y" as an option at all?
>
> If the situation is that everybody is fine with that being disabled by
> default, let's just make it the default. And I'd even be ok with
> removing it as an option *entirely*.
>
> That would seem to be *much* preferable that having an option that
> nobody really wants anyway, and where the default value would break
> some users. THAT just seems completely insane.

I suspect that a lot of 32-bit Linux users want syscall and/or
sysenter, and Stefani certainly wants the fast timing that the vDSO
can provide.  Also, presumably __kernel_sigreturn serves some purpose
:)

The basic issue is that most old glibc versions (and all versions that
were ever tagged in a release) work correctly regardless of whether
there is a vDSO, and newer glibc versions (since 2004) will take
advantage of a vDSO if one exists, but OpenSuSE 9 shipped with a
creatively broken version that blows up if presented with a vDSO
that's not prelinked to its actual address.

Currently there are three options: sane vDSO, no vDSO, and OpenSuSE
9-compatible vDSO.  The latter is a mess to maintain and breaks ASLR
(even for users of modern glibc), and having a vDSO is apparently
important enough that people are willing to pay to enhance it.  The
default is OpenSuSE 9-compatible vDSO, which is IMO an odd choice.

ISTM the right solution is to make OpenSuSE 9 users turn off the vDSO
(which is a performance hit for them, but not a correctness issue) and
let everyone else have a simpler kernel that has no ASLR issues.

I'm a bit heartened by the fact that the failure mode on OpenSuSE 9 is
a rather distinctive and easily Googlable message.  Most of the hits
offer abi.vsyscall32=0 or vdso=0 as suggestions, both of which
continue to work with my patch.

--Andy
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