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

From: Pratyush Anand
Date: Tue Sep 27 2016 - 11:04:16 EST

On Tuesday 27 September 2016 07:21 PM, Catalin Marinas wrote:
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.

I have already sent V2 with arm64 is_trap_insn() :(

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).

Yes, I agree that we will have to modify is_trap_insn() for supporting aarch32 task tracing.

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?

I had looked into it and also tested that a tracepoint at an application having a same instruction as that of "uprobe break instruction" ie "BRK #0x5" is rejected. So, I think a false positive return from is_tarp_insn() is still OK.