Re: [PATCH RFC 4/6] arm64: Add kernel return probes support(kretprobes)

[Will Deacon <will.deacon@arm.com>]

On Thu, Oct 17, 2013 at 12:17:49PM +0100, Sandeepa Prabhu wrote:
> AArch64 ISA does not instructions to pop PC register value
> from stack(like ARM v7 has ldmia {...,pc}) without using
> one of the general purpose registers. This means return probes
> cannot return to the actual return address directly without
> modifying register context, and without trapping into debug exception.
> 
> So like many other architectures, we prepare a global routine
> with NOPs, which serve as trampoline to hack away the
> function return address, by placing an extra kprobe on the
> trampoline entry.
> 
> The pre-handler of this special trampoline' kprobe execute return
> probe handler functions and restore original return address in ELR_EL1,
> this way, saved pt_regs still hold the original register context to be
> carried back to the probed kernel function.
> 
> Signed-off-by: Sandeepa Prabhu <sandeepa.prabhu@linaro.org>
> ---
>  arch/arm64/Kconfig               |   1 +
>  arch/arm64/include/asm/kprobes.h |   1 +
>  arch/arm64/include/asm/ptrace.h  |   5 ++
>  arch/arm64/kernel/kprobes.c      | 125 ++++++++++++++++++++++++++++++++++++++-
>  4 files changed, 129 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 2e89059..73eff55 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -28,6 +28,7 @@ config ARM64
>  	select HAVE_MEMBLOCK
>  	select HAVE_PERF_EVENTS
>  	select HAVE_KPROBES if !XIP_KERNEL
> +	select HAVE_KRETPROBES if (HAVE_KPROBES)

Don't need the brackets.

>  	select IRQ_DOMAIN
>  	select MODULES_USE_ELF_RELA
>  	select NO_BOOTMEM
> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
> index 9b491d0..eaca849 100644
> --- a/arch/arm64/include/asm/kprobes.h
> +++ b/arch/arm64/include/asm/kprobes.h
> @@ -55,5 +55,6 @@ void arch_remove_kprobe(struct kprobe *);
>  int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
>  int kprobe_exceptions_notify(struct notifier_block *self,
>  			     unsigned long val, void *data);
> +void kretprobe_trampoline(void);
>  
>  #endif /* _ARM_KPROBES_H */
> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index 89f1727..58b2589 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -166,6 +166,11 @@ static inline int valid_user_regs(struct user_pt_regs *regs)
>  #define instruction_pointer(regs)	(regs)->pc
>  #define stack_pointer(regs)		((regs)->sp)
>  
> +static inline long regs_return_value(struct pt_regs *regs)
> +{
> +	return regs->regs[0];
> +}

This is also being implemented by another patch series (I think the audit
stuff?).

Will