Subject: [PATCH v7 5/7] arm64: Add trampoline code for kretprobes

From: [email-masked] (David Long)
Date: Wed, 17 Jun 2015 14:37:00 -0400

On 06/16/15 11:09, William Cohen wrote:
> On 06/15/2015 03:07 PM, David Long wrote:
>> From: William Cohen <[email-masked]>
>>
>> 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 roginal saved caller
>
> s/roginal/original
>

Hopefully no one wants a v8 if it turns out only to be to fix this typo 
in the commit description.  I'll keep this in mind if a respin is needed 
for other reasons.

>> 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 <[email-masked]>
>> Signed-off-by: David A. Long <[email-masked]>
>> ---
>>   arch/arm64/include/asm/kprobes.h  |  1 +
>>   arch/arm64/kernel/kprobes-arm64.h | 41 +++++++++++++++++++++++++++++++++++++++
>>   arch/arm64/kernel/kprobes.c       | 26 +++++++++++++++++++++++++
>>   3 files changed, 68 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
>> index af31c4d..d081f49 100644
>> --- a/arch/arm64/include/asm/kprobes.h
>> +++ b/arch/arm64/include/asm/kprobes.h
>> @@ -58,5 +58,6 @@ 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);
>>
>>   #endif /* _ARM_KPROBES_H */
>> diff --git a/arch/arm64/kernel/kprobes-arm64.h b/arch/arm64/kernel/kprobes-arm64.h
>> index ff8a55f..bdcfa62 100644
>> --- a/arch/arm64/kernel/kprobes-arm64.h
>> +++ b/arch/arm64/kernel/kprobes-arm64.h
>> @@ -27,4 +27,45 @@ extern kprobes_pstate_check_t * const kprobe_condition_checks[16];
>>   enum kprobe_insn __kprobes
>>   arm_kprobe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi);
>>
>> +#define SAVE_REGS_STRING\
>> +	"	stp x0, x1, [sp, #16 * 0]\n"	\
>> +	"	stp x2, x3, [sp, #16 * 1]\n"	\
>> +	"	stp x4, x5, [sp, #16 * 2]\n"	\
>> +	"	stp x6, x7, [sp, #16 * 3]\n"	\
>> +	"	stp x8, x9, [sp, #16 * 4]\n"	\
>> +	"	stp x10, x11, [sp, #16 * 5]\n"	\
>> +	"	stp x12, x13, [sp, #16 * 6]\n"	\
>> +	"	stp x14, x15, [sp, #16 * 7]\n"	\
>> +	"	stp x16, x17, [sp, #16 * 8]\n"	\
>> +	"	stp x18, x19, [sp, #16 * 9]\n"	\
>> +	"	stp x20, x21, [sp, #16 * 10]\n"	\
>> +	"	stp x22, x23, [sp, #16 * 11]\n"	\
>> +	"	stp x24, x25, [sp, #16 * 12]\n"	\
>> +	"	stp x26, x27, [sp, #16 * 13]\n"	\
>> +	"	stp x28, x29, [sp, #16 * 14]\n"	\
>> +	"	str x30,   [sp, #16 * 15]\n"    \
>> +	"	mrs x0, nzcv\n"			\
>> +	"	str x0, [sp, #8 * 33]\n"
>> +
>> +
>> +#define RESTORE_REGS_STRING\
>> +	"	ldr x0, [sp, #8 * 33]\n"	\
>> +	"	msr nzcv, x0\n"			\
>> +	"	ldp x0, x1, [sp, #16 * 0]\n"	\
>> +	"	ldp x2, x3, [sp, #16 * 1]\n"	\
>> +	"	ldp x4, x5, [sp, #16 * 2]\n"	\
>> +	"	ldp x6, x7, [sp, #16 * 3]\n"	\
>> +	"	ldp x8, x9, [sp, #16 * 4]\n"	\
>> +	"	ldp x10, x11, [sp, #16 * 5]\n"	\
>> +	"	ldp x12, x13, [sp, #16 * 6]\n"	\
>> +	"	ldp x14, x15, [sp, #16 * 7]\n"	\
>> +	"	ldp x16, x17, [sp, #16 * 8]\n"	\
>> +	"	ldp x18, x19, [sp, #16 * 9]\n"	\
>> +	"	ldp x20, x21, [sp, #16 * 10]\n"	\
>> +	"	ldp x22, x23, [sp, #16 * 11]\n"	\
>> +	"	ldp x24, x25, [sp, #16 * 12]\n"	\
>> +	"	ldp x26, x27, [sp, #16 * 13]\n"	\
>> +	"	ldp x28, x29, [sp, #16 * 14]\n"	\
>> +	"	ldr x30,   [sp, #16 * 15]\n"
>> +
>>   #endif /* _ARM_KERNEL_KPROBES_ARM64_H */
>> diff --git a/arch/arm64/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
>> index 6255814..570218c 100644
>> --- a/arch/arm64/kernel/kprobes.c
>> +++ b/arch/arm64/kernel/kprobes.c
>> @@ -560,6 +560,32 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
>>   	return 0;
>>   }
>>
>> +/*
>> + * When a retprobed function returns, this code saves registers and
>> + * calls trampoline_handler() runs, which calls the kretprobe's handler.
>> + */
>> +static void __used __kprobes kretprobe_trampoline_holder(void)
>> +{
>> +	asm volatile (".global kretprobe_trampoline\n"
>> +			"kretprobe_trampoline:\n"
>> +			"sub sp, sp, %0\n"
>> +			SAVE_REGS_STRING
>> +			"mov x0, sp\n"
>> +			"bl trampoline_probe_handler\n"
>> +			/* Replace trampoline address in lr with actual
>> +			   orig_ret_addr return address. */
>> +			"str x0, [sp, #16 * 15]\n"
>> +			RESTORE_REGS_STRING
>> +			"add sp, sp, %0\n"
>> +			"ret\n"
>> +		      : : "I"(sizeof(struct pt_regs)) : "memory");
>> +}
>> +
>> +static void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
>> +{
>> +	return (void *) 0;
>> +}
>> +
>>   int __init arch_init_kprobes(void)
>>   {
>>   	return 0;
>>
>