linux-kernel - Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

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Date:   Mon, 21 Nov 2016 14:17:39 -0800
From:   hpa@...or.com
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Brian Gerst <brgerst@...il.com>,
        Andy Lutomirski <luto@...nel.org>, tedheadster@...il.com,
        George Spelvin <linux@...izon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

On November 21, 2016 1:21:35 PM PST, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Mon, Nov 21, 2016 at 10:26 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 11/21/16 10:00, Linus Torvalds wrote:
>>>
>>> I'd much rather we go back to just making the "cs" entry explicitly
>>> 16-bit, and have a separate padding entry, the way we used to long
>>> long ago.
>>>
>>
>> I would agree 100% with this.
>
>We _used_ to do it like this in some places (signal stack, other
>places):
>
>    unsigned short  cs, __csh;
>
>and
>
>    int xcs;
>
>in others (pt_regs, for example).
>
>You still see that "xcs" thing in the x86 uapi/asm/ptrace.h file, but
>that's what our native pt_regs used to look like). And we still have
>that "cs+__cs" thing in at least 'struct user_regs_struct32'. But our
>"struct pt_regs" gas lost it.
>
>I wonder why we broke that. I suspect it happened when we merged the
>64-bit and 32-bit files, but I was too lazy to try to pinpoint it.
>
>And I do think the original i386 model was better - exactly because it
>didn't access undefined state when you just accessed "cs". Either you
>had to know about it and it wasn't called 'cs' ("xcs") or you had that
>high/low separation.
>
>Of course, what might be better yet is to use an anonymous union, so
>that you can do both of the above for all the cases (ie access it both
>as a trustworthy low 16 bits, _and_ as a single 32-bit piece of
>information).
>
>We use anonymous unions all over now, we used to not do it because of
>compiler limitations.
>
>With an anonymous union, we could do soemthing like
>
>    union {
>        unsigned int xcs;
>        unsigned short cs;
>    }
>
>and so easily access either the reliable part (cs) or the full word
>(xcs) without masking or having to play games.
>
>[ In fact, I think we could try to make the "cs" member in that union
>be marked "const", which should mean that we'd get warnings if
>somebody were to try to assign just the half-word (so you'd always
>have to *assign* to "xcs", but you'd be able to read "cs").
>
>  I think that has made it from C++ to C. I'm not sure that's
>somethign we can/should use, but it sounds potentially useful for
>these kinds of cases ]
>
>              Linus

Now, segment loads have always ignored the top 32 bits; it's an issue when examined by other kinds of code.
-- 
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