Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 12 Aug 2015 09:19:30 -0700 | Subject | Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu |
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On Wed, Aug 12, 2015 at 1:02 AM, Stas Sergeev <stsp@list.ru> wrote: > 12.08.2015 03:38, Andy Lutomirski пишет: >> >> On Tue, Aug 11, 2015 at 5:17 PM, Stas Sergeev <stsp@list.ru> wrote: >>> >>> Hi guys, I wonder how easily the include/uapi/* is being >>> changed these days. >>> The patch: >>> >>> http://lkml.kernel.org/r/405594361340a2ec32f8e2b115c142df0e180d8e.1426193719.git.luto@kernel.org >>> breaks dosemu (and perhaps everyone else who used >>> to restore the segregs by hands). And the fix involves >>> both autoconf magic and run-time magic, so it is not even >>> trivial. >>> I realize this patch may be good to have in general, but >>> breaking userspace without a single warning is a bit >>> discouraging. Seems like the old "we don't break userspace" >>> rule have gone. >> >> I didn't anticipate any breakage. I could have been wrong. > > You changed include/uapi/*, which is obviously an asking > for problems. I applied the following changes to my local > git tree to get dosemu working again:
To be fair, I renamed a field that used to be padding. The UAPI has to change on occasion -- it's just not supposed to break things.
> https://github.com/stsp/dosemu2/commit/48b2a13a49a9fe1a456cd77df6b9a1feec675a01
Maybe I'm still missing something, but this seems like it should be unnecessary. What goes wrong without it?
The new ss field serves two purposes: it stores the old ss (dosemu needs that on new kernels and would benefit in general) and it stores the new post-sigreturn ss (dosemu doesn't currently have any use for that because of the iret trampoline trick).
But maybe you're doing this to make the next patch work.
> https://github.com/stsp/dosemu2/commit/7898ac60d5e569964127d6cc48f592caecd20b81
So the problem is that dosemu was actually hacking around the old buggy behavior and thus relying on it. Grr.
>> We might still be able to require a new sigcontext flag to be set and >> to forcibly return to __USER_DS if the flag is set regardless of the >> ss value in sigcontext when sigreturn is called, if that is indeed the >> problem with DOSEMU. But I'm not actually sure that that's the >> problem. > > Well, the flag would be an ideal solution in an ideal world, > but in our world I don't know the current relevance of dosemu, > and whether or not it worth a new flag to add.
It wouldn't even help here, because the breakage isn't caused by incompatible sigcontext formats -- it's caused by dosemu's reliance on ss being preserved across signal delivery (even if it wasn't preserved on the way back).
> >> In fact, DOSEMU contains this: >> >> /* set up a frame to get back to DPMI via iret. The kernel does not >> save >> %ss, and the SYSCALL instruction in sigreturn() destroys it. >> >> IRET pops off everything in 64-bit mode even if the privilege >> does not change which is nice, but clobbers the high 48 bits >> of rsp if the DPMI client uses a 16-bit stack which is not so >> nice (see EMUfailure.txt). Setting %rsp to 0x100000000 so that >> bits 16-31 are zero works around this problem, as DPMI code >> can't see bits 32-63 anyway. >> */ >> >> So, if DOSEMU were to realize that both sigreturnissues it's >> complaining about are fixed in recent kernels, it could sigreturn >> directly back to any state. > > Good, but have you added any flag for dosemu to even know > it can do this? Unless I am mistaken, you didn't. So the fix you > suggest, is not easy to detect and make portable with the older > kernels. Any suggestions? >
You could probe for it directly: raise a signal, change the saved ss and see what's in ss after sigreturn.
Let me see if I can come up with a clean kernel fix.
--Andy
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