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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Sep 26, 2016 at 06:33:59PM +0530, Pratyush Anand wrote:
> 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@arm.com> 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.

For the time being, I think the default is_trap_insn() check is still
useful on arm64. The problem gets trickier when we add AArch32 support
as it may return 'true' on an AArch32 instruction that matches the
AArch64 BRK (or vice-versa). That's when we need to either pass the mm
to is_trap_insn() or simply return false and always perform the check in
the arch_uprobe_analyze_insn() (which should, in addition, check for the
trap instruction).

There is also the is_trap_at_addr() function which uses is_trap_insn().
I haven't checked the call paths here, are there any implications if
is_trap_insn() always returns false?

-- 
Catalin