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RE: [RFC PATCH 1/1] x86/traps: Get rid of exception handlers' second argument error code
- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- From: "Li, Xin3" <xin3.li@xxxxxxxxx>
- Date: Fri, 4 Aug 2023 17:35:11 +0000
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- Cc: "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "Lutomirski, Andy" <luto@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "x86@xxxxxxxxxx" <x86@xxxxxxxxxx>, "H . Peter Anvin" <hpa@xxxxxxxxx>, "Gross, Jurgen" <jgross@xxxxxxxx>, "Ostrovsky, Boris" <boris.ostrovsky@xxxxxxxxxx>
- Delivery-date: Fri, 04 Aug 2023 17:35:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHZxq1luoPee3LE20qWOZPEmJublq/Z6zyAgAB3ZVA=
- Thread-topic: [RFC PATCH 1/1] x86/traps: Get rid of exception handlers' second argument error code
> > The IDT event delivery of X86_TRAP_DF, X86_TRAP_TS, X86_TRAP_NP,
> > X86_TRAP_SS, X86_TRAP_GP, X86_TRAP_AC and X86_TRAP_CP pushes an error
> > code into the orig_ax member of the pt_regs structure, and the error
> > code is passed as the second argument of their C-handlers, although
> > the pt_regs structure is already passed as the first argument.
> >
> > The asm entry code of such faults does the following
> >
> > movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
> > movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
> >
> > to set the orig_ax member to -1 just before calling the C-handler.
> >
> > In addition, the IRQ entry code uses the second error code argument as
> > its IRQ vector, as the IRQ asm entry code pushes its IRQ vector into
> > the orig_ax member.
> >
> > The commit d99015b1abbad ("x86: move entry_64.S register saving out of
> > the macros") introduced the changes to set orig_ax to -1, but I can't
> > see why it's required. Our tests on x86_64 and x86_32 seem fine if
> > orig_ax is left unchanged instead of set to -1.
>
> That means that SYSCALL_NUM(regs) get to be garbage; or something like that.
I find SYSCALL_NUM(regs) in tools/testing/selftests/seccomp/seccomp_bpf.c,
but nothing obvious to me.
I think it's clear that once exceptions and IRQs are handled, the original
context will be fully recovered in a normal case.
Is it related to preemption after such a event?
I must have missed something; can you please elaborate it?
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