Re: [PATCH v14 04/10] arm64: Kprobes with single stepping support

[Pratyush Anand]

Hi David,

On 26/06/2016:11:06:47 PM, David Long wrote:
> From: Sandeepa Prabhu <sandeepa.s.prabhu@xxxxxxxxx>
> 
> Add support for basic kernel probes(kprobes) and jump probes
> (jprobes) for ARM64.
> 
> Kprobes utilizes software breakpoint and single step debug
> exceptions supported on ARM v8.
> 
> A software breakpoint is placed at the probe address to trap the
> kernel execution into the kprobe handler.
> 
> ARM v8 supports enabling single stepping before the break exception
> return (ERET), with next PC in exception return address (ELR_EL1). The
> kprobe handler prepares an executable memory slot for out-of-line
> execution with a copy of the original instruction being probed, and
> enables single stepping. The PC is set to the out-of-line slot address
> before the ERET. With this scheme, the instruction is executed with the
> exact same register context except for the PC (and DAIF) registers.
> 
> Debug mask (PSTATE.D) is enabled only when single stepping a recursive
> kprobe, e.g.: during kprobes reenter so that probed instruction can be
> single stepped within the kprobe handler -exception- context.
> The recursion depth of kprobe is always 2, i.e. upon probe re-entry,
> any further re-entry is prevented by not calling handlers and the case
> counted as a missed kprobe).
> 
> Single stepping from the x-o-l slot has a drawback for PC-relative accesses
> like branching and symbolic literals access as the offset from the new PC
> (slot address) may not be ensured to fit in the immediate value of
> the opcode. Such instructions need simulation, so reject
> probing them.
> 
> Instructions generating exceptions or cpu mode change are rejected
> for probing.
> 
> Exclusive load/store instructions are rejected too.  Additionally, the
> code is checked to see if it is inside an exclusive load/store sequence
> (code from Pratyush).
> 
> System instructions are mostly enabled for stepping, except MSR/MRS
> accesses to "DAIF" flags in PSTATE, which are not safe for
> probing.
> 
> Thanks to Steve Capper and Pratyush Anand for several suggested
> Changes.
> 
> Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@xxxxxxxxx>
> Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
> Signed-off-by: Pratyush Anand <panand@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig                      |   1 +
>  arch/arm64/include/asm/debug-monitors.h |   5 +
>  arch/arm64/include/asm/insn.h           |   2 +
>  arch/arm64/include/asm/kprobes.h        |  60 ++++
>  arch/arm64/include/asm/probes.h         |  34 +++
>  arch/arm64/include/asm/ptrace.h         |   1 +
>  arch/arm64/kernel/Makefile              |   2 +-
>  arch/arm64/kernel/debug-monitors.c      |  16 +-
>  arch/arm64/kernel/kprobes/Makefile      |   1 +
>  arch/arm64/kernel/kprobes/decode-insn.c | 143 +++++++++
>  arch/arm64/kernel/kprobes/decode-insn.h |  34 +++
>  arch/arm64/kernel/kprobes/kprobes.c     | 525 ++++++++++++++++++++++++++++++++
>  arch/arm64/kernel/vmlinux.lds.S         |   1 +
>  arch/arm64/mm/fault.c                   |  26 ++
>  14 files changed, 848 insertions(+), 3 deletions(-)
>  create mode 100644 arch/arm64/include/asm/kprobes.h
>  create mode 100644 arch/arm64/include/asm/probes.h
>  create mode 100644 arch/arm64/kernel/kprobes/Makefile
>  create mode 100644 arch/arm64/kernel/kprobes/decode-insn.c
>  create mode 100644 arch/arm64/kernel/kprobes/decode-insn.h

Can we rename kernel/kprobes as kernel/probes? uprobes code will use
decode-insn.c and further simulate-insn.c as well. So, I would like to place my
uprobes.c in same directory.

>  create mode 100644 arch/arm64/kernel/kprobes/kprobes.c
> 

[...]

> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index 6c0c7d3..c7bbeed 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -209,6 +209,7 @@ struct task_struct;
>  int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task);
>  
>  #define instruction_pointer(regs)	((unsigned long)(regs)->pc)
> +#define instruction_pointer_set(regs, value)	((regs)->pc = ((u64) (value)))

IIRC, Will Daecon had asked to include asm-generic/ptrace.h into asm/ptrace.h when
I had done similar changes for uprobe needs. May be you can pick patch from
my uprobe tree.

https://github.com/pratyushanand/linux/commit/bb3e114797c2888ed8ad528cca20e569dd2d818e

~Pratyush