linux-kernel - Re: [PATCH v8 2/8] powerpc/vdso: Remove __kernel_datapage_offset and simplify __get

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Message-ID: <542145eb-7d90-0444-867e-c9cbb6bdd8e3@gmail.com>
Date:   Mon, 28 Sep 2020 16:08:03 +0100
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, nathanl@...ux.ibm.com,
        linux-arch <linux-arch@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        open list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v8 2/8] powerpc/vdso: Remove __kernel_datapage_offset and
 simplify __get_datapage()

On 9/27/20 8:43 AM, Christophe Leroy wrote:
> 
> 
> Le 21/09/2020 à 13:26, Will Deacon a écrit :
>> On Fri, Aug 28, 2020 at 12:14:28PM +1000, Michael Ellerman wrote:
>>> Dmitry Safonov <0x7f454c46@...il.com> writes:
[..]
>>>> I'll cook a patch for vm_special_mapping if you don't mind :-)
>>>
>>> That would be great, thanks!
>>
>> I lost track of this one. Is there a patch kicking around to resolve
>> this,
>> or is the segfault expected behaviour?
>>
> 
> IIUC dmitry said he will cook a patch. I have not seen any patch yet.

Yes, sorry about the delay - I was a bit busy with xfrm patches.

I'll send patches for .close() this week, working on them now.

> AFAIKS, among the architectures having VDSO sigreturn trampolines, only
> SH, X86 and POWERPC provide alternative trampoline on stack when VDSO is
> not there.
> 
> All other architectures just having a VDSO don't expect VDSO to not be
> mapped.
> 
> As far as nowadays stacks are mapped non-executable, getting a segfaut
> is expected behaviour. However, I think we should really make it
> cleaner. Today it segfaults because it is still pointing to the VDSO
> trampoline that has been unmapped. But should the user map some other
> code at the same address, we'll run in the weed on signal return instead
> of segfaulting.

+1.

> So VDSO unmapping should really be properly managed, the reference
> should be properly cleared in order to segfault in a controllable manner.
> 
> Only powerpc has a hook to properly clear the VDSO pointer when VDSO is
> unmapped.

Thanks,
         Dmitry