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

[Pratyush Anand]

On 26/09/2016:12:01:59 PM, Catalin Marinas wrote:
> 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.

OK..I will have an always returning false from arm64 is_trap_insn() in v2.

> 
> 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.

Yes, for 32 bit task we would need mm_struct as arg in is_trap_insn() as well as
in is_swbp_insn(). We would also need to have arm64 specific set_swbp().

Thanks for all your input. It was helpful. I will send V2 soon.

~Pratyush