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SubjectRe: [PATCH 4/5] stack overflow safe kdump (2.6.15-i386) - nmi handler and trap vector replacement
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On Tue, 2006-01-17 at 21:07 -0500, Vivek Goyal wrote:
> On Mon, Jan 16, 2006 at 10:25:04PM +0900, Fernando Luis Vazquez Cao wrote:
> > In the nmi path, we have the problem that both nmi_enter and nmi_exit in
> > do_nmi (see code below) make extensive use of "current" (which might be
> > invalid) indirectly (specially through the kernel preemption code).
> > Create a new nmi trap handler robust against stack overflows and use it
> > on the crash dump path.
> >
> > ---
> > diff -urNp linux-2.6.15/arch/i386/kernel/crash.c
> > linux-2.6.15-sov/arch/i386/kernel/crash.c
> > --- linux-2.6.15/arch/i386/kernel/crash.c 2006-01-16 20:29:50.000000000
> > +0900
> > +++ linux-2.6.15-sov/arch/i386/kernel/crash.c 2006-01-16
> > 20:33:55.000000000 +0900
> > @@ -22,6 +22,7 @@
> > #include <asm/nmi.h>
> > #include <asm/hw_irq.h>
> > #include <asm/apic.h>
> > +#include <mach_traps.h>
> > #include <mach_ipi.h>
> >
> >
> > @@ -142,6 +143,7 @@ static int crash_nmi_callback(struct pt_
> > if (cpu == crashing_cpu)
> > return 1;
> > local_irq_disable();
> > + disable_nmi();
> >
> > if (!user_mode(regs)) {
> > crash_setup_regs(&fixed_regs, regs);
> > @@ -167,13 +169,18 @@ static void smp_send_nmi_allbutself(void
> > send_IPI_allbutself(APIC_DM_NMI);
> > }
> >
> > +static void set_crash_nmi(void)
> > +{
> > + set_crash_nmi_callback(crash_nmi_callback);
> > +}
> > +
>
> Is it required to be a separate function? Probably can call
> set_crash_nmi_callback() directly.
Thanks for the catch.

> > static void nmi_shootdown_cpus(void)
> > {
> > unsigned long msecs;
> >
> > atomic_set(&waiting_for_crash_ipi, num_online_cpus() - 1);
> > - /* Would it be better to replace the trap vector here? */
> > - set_nmi_callback(crash_nmi_callback);
> > + /* Set the nmi handler appropriately for the crash case */
> > + set_crash_nmi();
> > /* Ensure the new callback function is set before sending
> > * out the NMI
> > */
> > diff -urNp linux-2.6.15/arch/i386/kernel/entry.S
> > linux-2.6.15-sov/arch/i386/kernel/entry.S
> > --- linux-2.6.15/arch/i386/kernel/entry.S 2006-01-03 12:21:10.000000000
> > +0900
> > +++ linux-2.6.15-sov/arch/i386/kernel/entry.S 2006-01-16
> > 21:52:24.000000000 +0900
> > @@ -590,6 +590,41 @@ nmi_16bit_stack:
> > .long 1b,iret_exc
> > .previous
> >
> > +ENTRY(crash_nmi)
> > + pushl %eax
> > + movl %ss, %eax
> > + cmpw $__ESPFIX_SS, %ax
> > + popl %eax
> > + je crash_nmi_16bit_stack
> > + pushl %eax
> > + SAVE_ALL
> > + xorl %edx,%edx # zero error code
> > + movl %esp,%eax # pt_regs pointer
> > + call do_crash_nmi
> > + jmp restore_all
> > +crash_nmi_16bit_stack:
> > + /* create the pointer to lss back */
> > + pushl %ss
> > + pushl %esp
> > + movzwl %sp, %esp
> > + addw $4, (%esp)
> > + /* copy the iret frame of 12 bytes */
> > + .rept 3
> > + pushl 16(%esp)
> > + .endr
> > + pushl %eax
> > + SAVE_ALL
> > + FIXUP_ESPFIX_STACK # %eax == %esp
> > + xorl %edx,%edx # zero error code
> > + call do_crash_nmi
> > + RESTORE_REGS
> > + lss 12+4(%esp), %esp # back to 16bit stack
> > +1: iret
> > +.section __ex_table,"a"
> > + .align 4
> > + .long 1b,iret_exc
> > +.previous
> > +
> > KPROBE_ENTRY(int3)
> > pushl $-1 # mark this as an int
> > SAVE_ALL
> > diff -urNp linux-2.6.15/arch/i386/kernel/traps.c
> > linux-2.6.15-sov/arch/i386/kernel/traps.c
> > --- linux-2.6.15/arch/i386/kernel/traps.c 2006-01-03 12:21:10.000000000
> > +0900
> > +++ linux-2.6.15-sov/arch/i386/kernel/traps.c 2006-01-16
> > 20:51:31.000000000 +0900
> > @@ -74,6 +74,7 @@ struct desc_struct idt_table[256] __attr
> > asmlinkage void divide_error(void);
> > asmlinkage void debug(void);
> > asmlinkage void nmi(void);
> > +asmlinkage void crash_nmi(void);
> > asmlinkage void int3(void);
> > asmlinkage void overflow(void);
> > asmlinkage void bounds(void);
> > @@ -641,23 +642,37 @@ static int dummy_nmi_callback(struct pt_
> > }
> >
> > static nmi_callback_t nmi_callback = dummy_nmi_callback;
> > -
> > -fastcall void do_nmi(struct pt_regs * regs, long error_code)
> > +
> > +static fastcall unsigned int __do_nmi(struct pt_regs * regs, long
> > error_code)
> > {
> > int cpu;
> >
> > - nmi_enter();
> > -
> > - cpu = smp_processor_id();
> > + cpu = safe_smp_processor_id();
> >
> > ++nmi_count(cpu);
> >
> > if (!rcu_dereference(nmi_callback)(regs, cpu))
> > default_do_nmi(regs);
> >
> > + return 0;
> > +}
> > +
> > +#define _do_nmi(regs, error_code, nmih) nmih(regs, error_code);
> > +
> > +fastcall void do_nmi(struct pt_regs * regs, long error_code)
> > +{
> > + nmi_enter();
> > +
> > + _do_nmi(regs, error_code, __do_nmi);
> > +
> > nmi_exit();
> > }
> >
> > +fastcall void do_crash_nmi(struct pt_regs * regs, long error_code)
> > +{
> > + _do_nmi(regs, error_code, __do_nmi);
> > +}
> > +
> > void set_nmi_callback(nmi_callback_t callback)
> > {
> > rcu_assign_pointer(nmi_callback, callback);
> > @@ -670,6 +685,17 @@ void unset_nmi_callback(void)
> > }
> > EXPORT_SYMBOL_GPL(unset_nmi_callback);
> >
> > +void set_crash_nmi_callback(nmi_callback_t callback)
> > +{
> > + /* XXX Do we need to do this atomically? */
> > + disable_nmi();
> > + unset_nmi_callback();
> > + /* Replace the trap vector */
> > + set_intr_gate(2,&crash_nmi);
> > + rcu_assign_pointer(nmi_callback, callback);
> > +}
> > +EXPORT_SYMBOL_GPL(set_crash_nmi_callback);
> > +
>
> Why do we need to export this symbol?
This is a vestige of the code I used for testing. Sure, we do not need
to export this symbol.

> In fact, probably can get rid of
> set_crash_nmi_callback(). After the system crash and replacing the trap
> vector, proably there is no point in keeping mutiple type of callback
> things. Action is more or less decided, that is save registers and halt
> upon an NMI.
I implemented the callback mechanism for flexibility. I think that on
the event of a system crash we may want to do something different than
booting a crash dump capturing kernel: failover, just shutting down the
system, etc.

> > #ifdef CONFIG_KPROBES
> > fastcall void __kprobes do_int3(struct pt_regs *regs, long error_code)
> > {
> > diff -urNp linux-2.6.15/include/asm-i386/mach-default/mach_traps.h
> > linux-2.6.15-sov/include/asm-i386/mach-default/mach_traps.h
> > --- linux-2.6.15/include/asm-i386/mach-default/mach_traps.h 2006-01-03
> > 12:21:10.000000000 +0900
> > +++ linux-2.6.15-sov/include/asm-i386/mach-default/mach_traps.h
> > 2006-01-16 20:33:55.000000000 +0900
> > @@ -20,6 +20,11 @@ static inline unsigned char get_nmi_reas
> > return inb(0x61);
> > }
> >
> > +static inline void disable_nmi(void)
> > +{
> > + outb(0x8f, 0x70);
> > +}
> > +
>
> Will it disable NMIs originating from LAPIC?
I think that, at least, this disables NMIs originating from "external"
sources (NMI watchdog, etc). I am not sure if it applies to NMIs
originating from the LAPIC too. Anyway, the former are the interrupts I
want to prevent from happening. Eric, could you comment on this?

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