linux-kernel - Re: [RFC 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context

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Message-ID: <CALCETrXo7+QvkRcsDDMM+XCZ=hyysDbK1zixVN2Wa1T-0AZwqg@mail.gmail.com>
Date:	Wed, 14 Oct 2015 09:42:58 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>, Stas Sergeev <stsp@...t.ru>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [RFC 3/4] x86/signal/64: Re-add support for SS in the 64-bit
 signal context

On Wed, Oct 14, 2015 at 9:40 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Mon, Oct 12, 2015 at 06:04:07PM -0700, Andy Lutomirski wrote:
> ...
>>
>> For the benefit of new 64-bit software that uses segmentation (new
>> versions of DOSEMU might), the new behavior can be detected with a
>> new ucontext flag UC_SIGCONTEXT_SS.
>>
>> To avoid compilation issues, __pad0 is left as an alias for ss in
>> ucontext.
>>
>> The nitty-gritty details are documented in the header file.
>>
>> Cc: Stas Sergeev <stsp@...t.ru>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: Cyrill Gorcunov <gorcunov@...il.com>
>> Cc: Pavel Emelyanov <xemul@...allels.com>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>
> Andy, so for old criu versions (prior the 1.5.1 which is Mar 2015,
> in next versions we already write proper ss into the images)
> we've been providing __pad = 0, which is ss in a new meaning,
> and the kernel will overwrite it with @user-ds after this series,
> correct? This should work for us. Stas, mind to refresh my memory,
> which ss value doesmu setups here?

That's the intent.

If you write __pad = 0, don't set UC_STRICT_RESTORE_SS, and leave cs
set to a 64-bit value, then the kernel will detect that 0 is not a
valid SS and will fix it for you.

If you do write UC_STRICT_RESTORE_SS (e.g. if you saved on a new
kernel and you restore the saved uc_flags), then you'll get a new
signal delivered.

If you're restoring a 32-bit or 16-bit context, then none of the above
applies, but I doubt that CRIU supports that anyway.

--Andy
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