Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

From: Andy Lutomirski
Date: Wed Aug 12 2015 - 16:29:21 EST

On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 12.08.2015 23:01, Andy Lutomirski ÐÐÑÐÑ:
>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>> 12.08.2015 22:20, Andy Lutomirski ÐÐÑÐÑ:
>>>> current kernels, it stays switched. If we change this, it won't stay
>>>> switched. Even ignoring old ABI, it's not really clear to me what the
>>>> right thing to do is.
>>> There can be the following cases:
>>> - switch_userspace_thread() switches fs to non-zero selector
>>> - switch_userspace_thread() switches the fs base via syscall
>>> - switch_userspace_thread() switches fs in sigcontext
>>> - switch_userspace_thread() switches fs_base in sigcontext (???)
>>> What exactly case do you have in mind?
>>> I'd say, the way x86_32 is doing things - is good, but the
>>> bases... perhaps, in ideal world, they should be a part of
>>> the sigcontext as well?
>> Any of the above. What do you want the kernel to do, and how does the
>> kernel know you want to do that? The kernel has to pick *some*
>> semantics here.
> Assuming the bases are made the part of a sigcontext,
> I'd say there would be no ambiguities remained at all:
> whatever you change in a sigcontext, will be "applied" by
> the sigreturn(). Whatever you put in the registers
> (either segregs or MSRs), is valid until sigreturn(), then
> forgotten forever.
> The mess only comes in when some things are part of
> sigcontext and some are not. But if you have _all_ things
> accessable in sigcontext, then the user has a way of expressing
> his needs very clearly: he'll either touch sigcontext or direct
> values, depending on what he need.
> Is this right?

Maybe, except that doing this might break existing code (Wine and Java
come to mind). I'm not really sure.

Anyway, can you give this and its parent a try:

If they fix the problem for you, I'll improve the test cases and send
them to -stable.


Andy Lutomirski
AMA Capital Management, LLC
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