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Message-ID: <546B5C9B.5040101@redhat.com>
Date: Tue, 18 Nov 2014 09:50:03 -0500
From: William Cohen <wcohen@...hat.com>
To: David Long <dave.long@...aro.org>,
linux-arm-kernel@...ts.infradead.org,
Russell King <linux@....linux.org.uk>
CC: Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
"Jon Medhurst (Tixy)" <tixy@...aro.org>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
Ananth N Mavinakayanahalli <ananth@...ibm.com>,
Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] arm64: Add kernel return probes support(kretprobes)
On 11/18/2014 01:32 AM, David Long wrote:
> From: Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
>
> 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.
The first might read better with something like:
Unlike the ARMv7 (ldmia {...,pc}) the AArch64 ISA cannot directly pop the PC register value from the stack without using one of the general purpose registers.
-Will
>
> 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@...aro.org>
> Signed-off-by: David A. Long <dave.long@...aro.org>
> ---
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/kprobes.h | 1 +
> arch/arm64/kernel/kprobes.c | 114 ++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 115 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 256ef90..02f31b7 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -61,6 +61,7 @@ config ARM64
> select HAVE_RCU_TABLE_FREE
> select HAVE_SYSCALL_TRACEPOINTS
> select HAVE_KPROBES if !XIP_KERNEL
> + select HAVE_KRETPROBES if HAVE_KPROBES
> 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 b35d3b9..a2de3b8 100644
> --- a/arch/arm64/include/asm/kprobes.h
> +++ b/arch/arm64/include/asm/kprobes.h
> @@ -56,5 +56,6 @@ 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/kernel/kprobes.c b/arch/arm64/kernel/kprobes.c
> index 789ab61..be7c330 100644
> --- a/arch/arm64/kernel/kprobes.c
> +++ b/arch/arm64/kernel/kprobes.c
> @@ -569,6 +569,117 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
> return 0;
> }
>
> +/*
> + * Kretprobes: kernel return probes handling
> + *
> + * AArch64 mode does not support popping the PC value from the
> + * stack like on ARM 32-bit (ldmia {..,pc}), so atleast one
> + * register need to be used to achieve branching/return.
> + * It means return probes cannot return back to the original
> + * return address directly without modifying the register context.
> + *
> + * So like other architectures, we prepare a global routine
> + * with NOPs, which serve as trampoline address that hack away the
> + * function return, with the exact register context.
> + * Placing a kprobe on trampoline routine entry will trap again to
> + * execute return probe handlers and restore original return address
> + * in ELR_EL1, this way saved pt_regs still hold the original
> + * register values to be carried back to the caller.
> + */
> +static void __used kretprobe_trampoline_holder(void)
> +{
> + asm volatile (".global kretprobe_trampoline\n"
> + "kretprobe_trampoline:\n"
> + "NOP\n\t"
> + "NOP\n\t");
> +}
> +
> +static int __kprobes
> +trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
> +{
> + struct kretprobe_instance *ri = NULL;
> + struct hlist_head *head, empty_rp;
> + struct hlist_node *tmp;
> + unsigned long flags, orig_ret_addr = 0;
> + unsigned long trampoline_address =
> + (unsigned long)&kretprobe_trampoline;
> +
> + INIT_HLIST_HEAD(&empty_rp);
> + kretprobe_hash_lock(current, &head, &flags);
> +
> + /*
> + * It is possible to have multiple instances associated with a given
> + * task either because multiple functions in the call path have
> + * a return probe installed on them, and/or more than one return
> + * probe was registered for a target function.
> + *
> + * We can handle this because:
> + * - instances are always inserted at the head of the list
> + * - when multiple return probes are registered for the same
> + * function, the first instance's ret_addr will point to the
> + * real return address, and all the rest will point to
> + * kretprobe_trampoline
> + */
> + hlist_for_each_entry_safe(ri, tmp, head, hlist) {
> + if (ri->task != current)
> + /* another task is sharing our hash bucket */
> + continue;
> +
> + if (ri->rp && ri->rp->handler) {
> + __this_cpu_write(current_kprobe, &ri->rp->kp);
> + get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
> + ri->rp->handler(ri, regs);
> + __this_cpu_write(current_kprobe, NULL);
> + }
> +
> + orig_ret_addr = (unsigned long)ri->ret_addr;
> + recycle_rp_inst(ri, &empty_rp);
> +
> + if (orig_ret_addr != trampoline_address)
> + /*
> + * This is the real return address. Any other
> + * instances associated with this task are for
> + * other calls deeper on the call stack
> + */
> + break;
> + }
> +
> + kretprobe_assert(ri, orig_ret_addr, trampoline_address);
> + /* restore the original return address */
> + instruction_pointer(regs) = orig_ret_addr;
> + reset_current_kprobe();
> + kretprobe_hash_unlock(current, &flags);
> +
> + hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
> + hlist_del(&ri->hlist);
> + kfree(ri);
> + }
> +
> + kprobes_restore_local_irqflag(regs);
> +
> + /* return 1 so that post handlers not called */
> + return 1;
> +}
> +
> +void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
> + struct pt_regs *regs)
> +{
> + ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
> +
> + /* replace return addr (x30) with trampoline */
> + regs->regs[30] = (long)&kretprobe_trampoline;
> +}
> +
> +static struct kprobe trampoline = {
> + .addr = (kprobe_opcode_t *) &kretprobe_trampoline,
> + .pre_handler = trampoline_probe_handler
> +};
> +
> +int __kprobes arch_trampoline_kprobe(struct kprobe *p)
> +{
> + return p->addr == (kprobe_opcode_t *) &kretprobe_trampoline;
> +}
> +
> /* Break Handler hook */
> static struct break_hook kprobes_break_hook = {
> .esr_mask = BRK64_ESR_MASK,
> @@ -586,5 +697,6 @@ int __init arch_init_kprobes(void)
> register_break_hook(&kprobes_break_hook);
> register_step_hook(&kprobes_step_hook);
>
> - return 0;
> + /* register trampoline for kret probe */
> + return register_kprobe(&trampoline);
> }
>
--
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