Messages in this thread | | | Date | Thu, 13 Aug 2015 09:34:36 -0700 | Subject | Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu | From | Linus Torvalds <> |
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On Thu, Aug 13, 2015 at 9:23 AM, Andy Lutomirski <luto@amacapital.net> wrote: > On Thu, Aug 13, 2015 at 9:19 AM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: >> >> So how about this "alternate" minimal patch instead. The difference is: >> >> - we actually leave the >> >> regs->ss = __USER_DS; >> >> in __setup_rt_frame, to guarantee that when we take a signal, we do >> take it with a valid SS > > That by itself is enough to break DOSEMU. I think we may be stuck > with my hack to only replace regs->ss if the old one was invalid.
Are you sure? From the description by Stas, the problem is literally the *restoring* action of the sigcontext, and trying to restore a SS value that is no longer valid.
"The crash happens when DOS program terminates. At that point dosemu subverts the execution flow by replacing segregs and cs/ip ss/sp in sigcontext with its own. But __pad0 still has DOS SS, which crash because (presumably) the DOS LDT have been just removed"
and my "truly-minimal" patch removes all of the sigcontext games.
> You mean that we always set ss to __USER_DS on sigreturn?
No. We never touch SS at sigreturn time at all. Only when entering the signal *handler* do we reset things to a known state. The signal handler can do anything it wants, and sigreturn won't touch it (which will obviously _leave_ it as __USER_DS, but avoids the problem with sigreturn trying to load an SS that is no longer valid)
> If this regression were new in 4.2-rc, then I'd say revert first and > ask questions later, but the regression is in 4.1 as well :(
Big deal. That's why we have the "cc stable". Distributions that ship with 4.1 are still fairly few (but it's a LTS release so it will grow) but they all pick up stable kernels.
And even if they temporarily have a broken situation, it's still better to make sure that broken situation gets fixed, rather than say "oh well, too late to do anything about it now".
Linus
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