Re: [PATCH 5/5] arm64: Add uprobe support

From: Catalin Marinas
Date: Mon Sep 26 2016 - 07:02:32 EST

On Sun, Sep 25, 2016 at 10:32:28PM +0530, Pratyush Anand wrote:
> On Fri, Sep 23, 2016 at 6:35 PM, Catalin Marinas
> <catalin.marinas@xxxxxxx> wrote:
> > On Fri, Sep 23, 2016 at 09:42:30AM +0530, Pratyush Anand wrote:
> >> On 22/09/2016:05:50:30 PM, Catalin Marinas wrote:
> >> > On Thu, Sep 22, 2016 at 08:53:28AM +0530, Pratyush Anand wrote:
> >> > > On 21/09/2016:06:04:04 PM, Catalin Marinas wrote:
> >> > As a quick workaround you could check mm->task_size > TASK_SIZE_32 in
> >> > the arch_uprobe_analyze_insn() function.
> >>
> >> It would be doable. TASK_SIZE_32 is defined only for COMPAT. So, may be I can
> >> return -EINVAL when mm->task_size < TASK_SIZE_64.
> >
> > That's just a temporary workaround. If we ever merge ILP32, this test
> > would no longer be enough (as the ISA is AArch64 but with TASK_SIZE_32).
> OK.. So what about doing something similar what x86 is doing.
> We can have a flag for task Type in arch specific mm_context_t. We
> also set this flag in COMPAT_SET_PERSONALITY() along with setting
> thread_info flag, and we clear them in SET_PERSONALITY().

This looks like a better approach.

> > Looking at prepare_uprobe(), we have a weak is_trap_insn() function.
> > This check is meaningless without knowing which instruction set we
> > target. A false positive here, however, is not that bad as we wouldn't
> > end up inserting the wrong breakpoint in the executable. But it looks to
> > me like the core uprobe code needs to pass some additional information
> > like the type of task or ELF format to the arch code to make a useful
> > choice of breakpoint type.
> It seems that 'strtle r0, [r0], #160' would have the closest matching
> aarch32 instruction wrt BRK64_OPCODE_UPROBES(0xd42000A0). But that too
> seems a bad instruction. So, may be we can use still weak
> is_trap_insn().

Even if the is_trap_insn() check passes, we would reject the probe in
arch_uprobe_analyze_insn() immediately after based on the mm type check,
so not too bad.

If we add support for probing 32-bit tasks, I would rather have
is_trap_insn() take the mm_struct as argument so that a non-weak
implementation can check for the correct encoding.