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

From: Stas Sergeev
Date: Thu Aug 13 2015 - 14:20:04 EST

13.08.2015 21:05, Andy Lutomirski ÐÐÑÐÑ:
On Thu, Aug 13, 2015 at 11:00 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
13.08.2015 20:17, Andy Lutomirski ÐÐÑÐÑ:
On Thu, Aug 13, 2015 at 10:13 AM, Stas Sergeev <stsp@xxxxxxx> wrote:

Ah, I see your point now.
But that's not what I mean, as it doesn't cover fs/gs, which
is what Linus is looking to revert now too (I am building the
testing kernels now).
So you obviously don't want the flag that will control all 3
things together without any lar heuristics, but I don't understand why...
Yes, your heuristic+uc_flag may work, but IMHO far from
perfection and TLS problem is not covered. I can test such
a patch but I don't understand why you don't want the flag
that will just control all things together.
The fs/gs patch doesn't change anything, so there's nothing to
control. It just renamed fields that did nothing. (It turns out they
did something back before arch_prctl existed, but there's only a
narrow range of kernels like that, and I'm not at all convinced that
those kernels are ABI-compatible with modern kernels at all. This is
all pre-git.)
The problem is that dosemu existed back then too.
It still uses these fields as a place-holders. Well, this is a
compile-time breakage only, so perhaps not as important
as the run-time one, but still, you broke it in yet another way.
Great. What exactly is DOSEMU sticking in those fields?
FS and GS of course. Saves by hands in a sighandler.

Are we now
stuck ignoring the contents in sigreturn because DOSEMU coopts them
for its own purposes?
No, its just that these fields _were_ valid in times, and so,
when kernel stopped using them, dosemu authors opted
not to change their locations but just save things by hands.
What else do you suppose could they do?
Well, compile-time breakage is another thing, it can safely
be ignored on your side I guess.
Something like this should do:

Sure, it might make sense to change TLS behavior in signals at some
point, but I don't think we're there yet. We need to deal with
fsgsbase first, and that's a *huge* can of worms.
My point is not when to fix TLS or how.
But you can get the flag ready, for now controlling only SS
and fixing the regression, but it will define the course of the
further developments. When the time will come, it will cover
also TLS, but why not to get such a flag ready now, without
yet fixing TLS?
I think that if we create a flag to change semantics, we shouldn't
introduce the flag and make it look like it works without actually
changing the semantics.
It is more about selecting the right field for such a flag.
You can select the right field now, and introduce some flag
to it, like SIG_SAVE_SS or whatever. This will fix a regression.
Then, when the TLS time will code, you'll just add SIG_SAVE_FS
flag to the same field, so that they can be ORed.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at