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    SubjectRe: [PATCH RFC 4/6] arm64: Add kernel return probes support(kretprobes)
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    On 8 November 2013 22:34, Will Deacon <will.deacon@arm.com> wrote:
    > 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.
    OK.
    >
    >> 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?).
    Not sure, I did not see this being implemented in audit(audit adds for
    'syscallno', not for return value in x0)
    I can rebase my code if this change is implemented and queued in other patchset.

    >
    > Will
    > --
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