Re: [PATCH v2] x86: Remove compat vdso support

Andy Lutomirski
2014-03-11T02:38:12Z

On Mon, Mar 10, 2014 at 6:39 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Mar 10, 2014 at 6:03 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> This is a bit of an abuse of the no-breaking-userspace policy.
>
> No it's not, because it won't be applied.
>
> You need to fix it.
>
> I'm not sure what goes wrong, since it *looks* like you handle the
> "vdso_enabled" thing correctly, so I find it surprising that you say
> that
>
> echo 0 >/proc/sys/abi/vsyscall32
>
> makes it work, since it should be zero already, and that echo should
> be a no-op. But maybe I'm missing something.
>
> Maybe you can just fake the boot parameter and fix the OpenSuSE
> breakage that way (presumably that "init" sees it if it's some
> user-space setup thing), but I'd like to know why that "echo 0" works,
> but just initializing it to zero does not?

It does. My patch breaks OpenSuSE 9 when
CONFIG_ENABLE_VDSO32_BY_DEFAULT=y unless it's overridden by sysctl or
boot option.

The behavior with my patch is:

If ENABLE_VDSO32_BY_DEFAULT (which is the default), then OpenSuSE 9
breaks. Everything else works. Booting with vdso=0, vdso=2,
vdso32=0, or vdso32=2, or setting abi.vsyscall32=0 will switch to the
no-vDSO behavior.

If !ENABLE_VDSO32_BY_DEFAULT, then OpenSuSE 9 breaks and other 32-bit
code runs without a vDSO, which slows it down a bit.

I did that because I seem to remember that it's not so bad to break
small amounts of userspace as long as there's a backwards
compatibility path (there are plenty of kernel options that turn on
"legacy" things needed for small numbers of users).

If this is not okay, then I can redo the patch, leaving it with
CONFIG_COMPAT_VDSO as the option name, so that anyone with a working
config will keep working if they run 'make oldconfig' (as opposed to
being prompted). If I do that, I'd still prefer to make the
non-compatible version be the default, since it's the right choice for
the vast majority of users. Currently CONFIG_COMPAT_VDSO is default
y, which seems like an odd choice to me.

--Andy