Re: [PATCH v10 7/9] arm64: Add trampoline code for kretprobes

David Long
2016-03-08T05:43:08Z

On 03/02/2016 04:20 PM, William Cohen wrote:
> On 03/01/2016 01:19 PM, Marc Zyngier wrote:
>> On 01/03/16 02:57, David Long wrote:
>>> From: William Cohen <wcohen@redhat.com>
>>>
>>> The trampoline code is used by kretprobes to capture a return from a probed
>>> function. This is done by saving the registers, calling the handler, and
>>> restoring the registers. The code then returns to the original saved caller
>>> return address. It is necessary to do this directly instead of using a
>>> software breakpoint because the code used in processing that breakpoint
>>> could itself be kprobe'd and cause a problematic reentry into the debug
>>> exception handler.
>>>
>>> Signed-off-by: William Cohen <wcohen@redhat.com>
>>> Signed-off-by: David A. Long <dave.long@linaro.org>
>>> ---
>>> arch/arm64/include/asm/kprobes.h | 2 +
>>> arch/arm64/kernel/Makefile | 1 +
>>> arch/arm64/kernel/asm-offsets.c | 22 +++++++++++
>>> arch/arm64/kernel/kprobes.c | 5 +++
>>> arch/arm64/kernel/kprobes_trampoline.S | 67 ++++++++++++++++++++++++++++++++++
>>> 5 files changed, 97 insertions(+)
>>> create mode 100644 arch/arm64/kernel/kprobes_trampoline.S
>>>
>>> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
>>> index 79c9511..61b4915 100644
>>> --- a/arch/arm64/include/asm/kprobes.h
>>> +++ b/arch/arm64/include/asm/kprobes.h
>>> @@ -56,5 +56,7 @@ int kprobe_exceptions_notify(struct notifier_block *self,
>>> unsigned long val, void *data);
>>> int kprobe_breakpoint_handler(struct pt_regs *regs, unsigned int esr);
>>> int kprobe_single_step_handler(struct pt_regs *regs, unsigned int esr);
>>> +void kretprobe_trampoline(void);
>>> +void __kprobes *trampoline_probe_handler(struct pt_regs *regs);
>>>
>>> #endif /* _ARM_KPROBES_H */
>>> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
>>> index 08325e5..f192b7d 100644
>>> --- a/arch/arm64/kernel/Makefile
>>> +++ b/arch/arm64/kernel/Makefile
>>> @@ -37,6 +37,7 @@ arm64-obj-$(CONFIG_CPU_IDLE) += cpuidle.o
>>> arm64-obj-$(CONFIG_JUMP_LABEL) += jump_label.o
>>> arm64-obj-$(CONFIG_KGDB) += kgdb.o
>>> arm64-obj-$(CONFIG_KPROBES) += kprobes.o kprobes-arm64.o \
>>> + kprobes_trampoline.o \
>>> probes-simulate-insn.o
>>> arm64-obj-$(CONFIG_EFI) += efi.o efi-entry.stub.o
>>> arm64-obj-$(CONFIG_PCI) += pci.o
>>> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
>>> index fffa4ac6..460b54c 100644
>>> --- a/arch/arm64/kernel/asm-offsets.c
>>> +++ b/arch/arm64/kernel/asm-offsets.c
>>> @@ -50,6 +50,28 @@ int main(void)
>>> DEFINE(S_X5, offsetof(struct pt_regs, regs[5]));
>>> DEFINE(S_X6, offsetof(struct pt_regs, regs[6]));
>>> DEFINE(S_X7, offsetof(struct pt_regs, regs[7]));
>>> + DEFINE(S_X8, offsetof(struct pt_regs, regs[8]));
>>> + DEFINE(S_X9, offsetof(struct pt_regs, regs[9]));
>>> + DEFINE(S_X10, offsetof(struct pt_regs, regs[10]));
>>> + DEFINE(S_X11, offsetof(struct pt_regs, regs[11]));
>>> + DEFINE(S_X12, offsetof(struct pt_regs, regs[12]));
>>> + DEFINE(S_X13, offsetof(struct pt_regs, regs[13]));
>>> + DEFINE(S_X14, offsetof(struct pt_regs, regs[14]));
>>> + DEFINE(S_X15, offsetof(struct pt_regs, regs[15]));
>>> + DEFINE(S_X16, offsetof(struct pt_regs, regs[16]));
>>> + DEFINE(S_X17, offsetof(struct pt_regs, regs[17]));
>>> + DEFINE(S_X18, offsetof(struct pt_regs, regs[18]));
>>> + DEFINE(S_X19, offsetof(struct pt_regs, regs[19]));
>>> + DEFINE(S_X20, offsetof(struct pt_regs, regs[20]));
>>> + DEFINE(S_X21, offsetof(struct pt_regs, regs[21]));
>>> + DEFINE(S_X22, offsetof(struct pt_regs, regs[22]));
>>> + DEFINE(S_X23, offsetof(struct pt_regs, regs[23]));
>>> + DEFINE(S_X24, offsetof(struct pt_regs, regs[24]));
>>> + DEFINE(S_X25, offsetof(struct pt_regs, regs[25]));
>>> + DEFINE(S_X26, offsetof(struct pt_regs, regs[26]));
>>> + DEFINE(S_X27, offsetof(struct pt_regs, regs[27]));
>>> + DEFINE(S_X28, offsetof(struct pt_regs, regs[28]));
>>> + DEFINE(S_X29, offsetof(struct pt_regs, regs[29]));
>>
>> Do we need all of these? Specially considering that we're only using the
>> even ones? You may want to consider something like
>> arch/arm64/kvm/hyp/entry.S does.
>
> Following what arch/arm64/kvm/hyp/entry.S does would make the patch a bit smaller.
>

I have cut the defines in half and duplicated the entry.S style using
macros.

>>
>>> DEFINE(S_LR, offsetof(struct pt_regs, regs[30]));
>>> DEFINE(S_SP, offsetof(struct pt_regs, sp));
>>> #ifdef CONFIG_COMPAT
>>> diff --git a/arch/arm64/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
>>> index ffc5affd..98f4fe5 100644
>>> --- a/arch/arm64/kernel/kprobes.c
>>> +++ b/arch/arm64/kernel/kprobes.c
>>> @@ -532,6 +532,11 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
>>> return 1;
>>> }
>>>
>>> +void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
>>> +{
>>> + return (void *) 0;
>>
>> Something wrong with NULL? ;-)
>
> Nothing wrong with using NULL.
>

Changed.

>>
>>> +}
>>> +
>>> int __init arch_init_kprobes(void)
>>> {
>>> return 0;
>>> diff --git a/arch/arm64/kernel/kprobes_trampoline.S b/arch/arm64/kernel/kprobes_trampoline.S
>>> new file mode 100644
>>> index 0000000..5a336cf
>>> --- /dev/null
>>> +++ b/arch/arm64/kernel/kprobes_trampoline.S
>>> @@ -0,0 +1,67 @@
>>> +/*
>>> + * trampoline entry and return code for kretprobes.
>>> + */
>>> +
>>> +#include <linux/linkage.h>
>>> +#include <generated/asm-offsets.h>
>>
>> #include <asm/asm-offsets.h>
>>

Fixed.

>>> +
>>> + .text
>>> +
>>> +ENTRY(kretprobe_trampoline)
>>> +
>>> + sub sp, sp, #S_FRAME_SIZE
>>> +
>>> + stp x0, x1, [sp, #S_X0]
>>> + stp x2, x3, [sp, #S_X2]
>>> + stp x4, x5, [sp, #S_X4]
>>> + stp x6, x7, [sp, #S_X6]
>>> + stp x8, x9, [sp, #S_X8]
>>> + stp x10, x11, [sp, #S_X10]
>>> + stp x12, x13, [sp, #S_X12]
>>> + stp x14, x15, [sp, #S_X14]
>>> + stp x16, x17, [sp, #S_X16]
>>> + stp x18, x19, [sp, #S_X18]
>>> + stp x20, x21, [sp, #S_X20]
>>> + stp x22, x23, [sp, #S_X22]
>>> + stp x24, x25, [sp, #S_X24]
>>> + stp x26, x27, [sp, #S_X26]
>>> + stp x28, x29, [sp, #S_X28]
>>> + str x30, [sp, #S_LR]
>>
>> Might as well call it LR?

Ah, that exposed the fact there's a missing include of asm/assembler.h.
All fixed.

>
> Given the code is going to overwrite lr with the value returned by trampoline_probe_handler might eliminate the store to #S_LR above.
>

It's not only about what's restored though, it's about what the
trace/user code is told the register contents are.

>>
>>> + add x0, sp, #S_FRAME_SIZE
>>> + str x0, [sp, #S_SP]
>>> + mrs x0, nzcv
>>> + mrs x1, daif
>>> + orr x0, x0, x1
>>> + /* There seems no easy way to get the mode field so make one up */
>>> + add x0, x0, #5
>>
>> Do you mean something like CurrentEL? You could also save SPSel whilst
>> you're at it.

OK, I've retrieved CurrentEL and SPSel and orr'd them into the saved PSTATE.

>
> Looking at the my patch again I am wondering if the patch could skip storing daif and the mode field. The nzcv bit are the only thing restored.
>

Again, want to have realistic saved register contents.

>>
>>> + str x0, [sp, #S_PSTATE]
>>> +
>>> + mov x0, sp
>>> + bl trampoline_probe_handler
>>> + /* Replace trampoline address in lr with actual
>>> + orig_ret_addr return address. */
>>> + str x0, [sp, #S_LR]
>>
>> Why do you need to store it on the stack? You could do a "mov lr, x0",
>> and drop the last load of the sequence below...
>
> Ah, yes, that would save a store/load pair.
>

Done.

>>> +
>>> + ldr x0, [sp, #S_PSTATE]
>>> + msr nzcv, x0
>>> + ldp x0, x1, [sp, #S_X0]
>>> + ldp x2, x3, [sp, #S_X2]
>>> + ldp x4, x5, [sp, #S_X4]
>>> + ldp x6, x7, [sp, #S_X6]
>>> + ldp x8, x9, [sp, #S_X8]
>>> + ldp x10, x11, [sp, #S_X10]
>>> + ldp x12, x13, [sp, #S_X12]
>>> + ldp x14, x15, [sp, #S_X14]
>>> + ldp x16, x17, [sp, #S_X16]
>>> + ldp x18, x19, [sp, #S_X18]
>>> + ldp x20, x21, [sp, #S_X20]
>>> + ldp x22, x23, [sp, #S_X22]
>>> + ldp x24, x25, [sp, #S_X24]
>>> + ldp x26, x27, [sp, #S_X26]
>>> + ldp x28, x29, [sp, #S_X28]
>>> + ldr x30, [sp, #S_LR]
>>> +
>>> + add sp, sp, #S_FRAME_SIZE
>>> + ret
>>> +
>>> +ENDPROC(kretprobe_trampoline)
>>>
>>
>> Thanks,
>>
>> M.
>>
>
> Thanks,
>
> -Will
>

-dl