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

From: [email-masked] (William Cohen)
Date: Wed, 2 Mar 2016 16:20:47 -0500

On 03/01/2016 01:19 PM, Marc Zyngier wrote:
> On 01/03/16 02:57, 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 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 <[email-masked]>
>> Signed-off-by: David A. Long <[email-masked]>
>> ---
>>  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