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

[David Long]

On 06/27/2016 02:57 AM, Pratyush Anand wrote:
> 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.

I had some reservations about making it a "kprobes" subdir for that 
reason but the advice I got was to simply go with Masami Hiramatsu's 
specific feedback and have a subsequent uprobes patch move these files 
to a replacement "probes" subdir.  If there ends up being yet another 
revision of this patch maybe he will consider allowing it to be called 
"probes" from the start.

> >   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

I will look at this.

> ~Pratyush

Thanks,
-dl