linux-kernel - Re: [patch 01/10] x86/fpu/signal: Clarify exception handling in restore_fpregs_from

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Date:   Mon, 30 Aug 2021 21:30:59 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [patch 01/10] x86/fpu/signal: Clarify exception handling in
 restore_fpregs_from_user()

On Mon, Aug 30, 2021 at 02:26:12PM -0700, Linus Torvalds wrote:
> On Mon, Aug 30, 2021 at 2:07 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Incidentally, why do we bother with negation in those?  Why not have
> > user_insn(), XSTATE_OP() and kernel_insn_err() return 0 or trap number...
> 
> I really wish we didn't have that odd _ASM_EXTABLE_FAULT/
> ex_handler_fault() special case at all.
> 
> It's *very* confusing, and it actually seems to be mis-used. It looks
> like the "copy_mc_fragile" code uses it by mistake, and doesn't
> actually want that "modify %%rax" behavior of that exception handler
> AT ALL.
> 
> If I read that code correctly, it almost by mistake doesn't actually
> care, and will overwrite %%rax with the right result, but it doesn't
> look like the "fault code in %eax" was ever *intentional*. There's no
> mention of it.
> 
> Maybe I'm misreading that code, but I look at it and just go "Whaa?"
> 
> The code in user_insn() clearly *does* use that fault number (and, as
> you say, inverts it for some reason), but I wonder how much it really
> cares? Could we get rid of it, and just set a fixed error code?
> 
> I only checked one user, but that one didn't actually care which fault
> it was, it only cared about fault-vs-no-fault.

There's a place where we care about #PF vs. #MC (see upthread)...