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

[Pratyush Anand]

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.

~Pratyush