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

William Cohen
2016-03-02T21:20:53Z

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.

>
>> 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.

>
>> +}
>> +
>> 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>
>
>> +
>> + .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?

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

>
>> + 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.

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.

>
>> + 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.

>> +
>> + 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