Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 11 Dec 2017 10:31:14 -0800 | Subject | Re: [PATCH] Fix resume on x86-32 machines |
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On Mon, Dec 11, 2017 at 6:22 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote: > On Sunday, December 10, 2017 10:58:23 PM CET Andy Lutomirski wrote: >> >> I'm guessing that the real issue is that 32-bit needs %fs restored early for TLS. > > I *think* you are right. > > Anyway, that should be easy enough to verify. > > Pavel, can you please check if the below change works too?
So Jarkko confirmed this works for him, but the more I look at this crap, the less I like it.
Why do we save fs/ds/es/ss at all on x86-32? Don't they all have fixed values in the kernel, with %fs being __KERNEL_PERCPU, and the others being __USER_DS?
Nothing else can possibly be valid, as far as I can tell.
I think we actually leave the user-space percpu segment in %gs (or the stack canary base), so that one we should actually save/restore, but I'm getting the feeling that we should just reset the other segment registers to known values on 32-bit.
Also, why does the 32-bit code do
loadsegment(es, ctxt->es);
but the 64-bit code does
asm volatile ("movw %0, %%es" :: "r" (ctxt->es));
And look at that confusion between MSR_GS_BASE and MSR_KERNEL_GS_BASE all within the 64-bit case.
In particular, note how we reload the %gs segment in between the two - wouldn't that mess with the currently active gs base if %gs can be non-zero?
Christ, what a mess.
So I think that whole sequence is garbage. It has been written as some kind of "save and restore registers", but that's not what it really then does - or what it should do.
It should make sure to restore a sane kernel state, not some random register state.
And the 32-bit and 64-bit code really should strive to be at least _sanely_ different, not this randomly and insanely different mess.
But yes, Rafael's patch looks like the minimal one-liner. But I think we should do the %gs load early too for the 32-bit stack canary case, kind of like we need to do %fs for percpu base.
Linus
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