Re: [PATCH] riscv: provide riscv-specific is_trap_insn()

[Björn Töpel]

Nam Cao <namcaov@xxxxxxxxx> writes:

> On Tue, Aug 29, 2023 at 08:14:59AM +0200, Björn Töpel wrote:
>> Nam Cao <namcaov@xxxxxxxxx> writes:
>> 
>> > On Mon, Aug 28, 2023 at 03:31:15PM +0200, Nam Cao wrote:
>> >> On Mon, Aug 28, 2023 at 02:48:06PM +0200, Björn Töpel wrote:
>> >> > Nam Cao <namcaov@xxxxxxxxx> writes:
>> >> > 
>> >> > > uprobes expects is_trap_insn() to return true for any trap instructions,
>> >> > > not just the one used for installing uprobe. The current default
>> >> > > implementation only returns true for 16-bit c.ebreak if C extension is
>> >> > > enabled. This can confuse uprobes if a 32-bit ebreak generates a trap
>> >> > > exception from userspace: uprobes asks is_trap_insn() who says there is no
>> >> > > trap, so uprobes assume a probe was there before but has been removed, and
>> >> > > return to the trap instruction. This cause an infinite loop of entering
>> >> > > and exiting trap handler.
>> >> > >
>> >> > > Instead of using the default implementation, implement this function
>> >> > > speficially for riscv which checks for both ebreak and c.ebreak.
>> >> > 
>> >> > I took this for a spin, and it indeed fixes this new hang! Nice!
>> >> 
>> >> Great! Thanks for testing it.
>> >>  
>> >> > However, when I tried setting an uprobe on the ebreak instruction
>> >> > (offset 0x118) from your example [1], the probe does not show up in the
>> >> > trace buffer.
>> >> > 
>> >> > Any ideas?
>> >> 
>> >> >From my understanding, both uprobes and kprobes refuse to install break points
>> >> into existing trap instructions. Otherwise, we may conflict with something else
>> >> that is also using trap instructions.
>> >
>> > I just realize you probably ask this because uprobe can still be installed before
>> > applying the patch. But I think that is another bug that my patch also
>> > accidentally fix: uprobes should not install breakpoint into ebreak instructions,
>> > but it incorrectly does so because it does not even know about the existence of
>> > 32-bit ebreak.
>> 
>> FWIW, I can still install the uprobe at an ebreak with you patch. It's
>> not hit, but succeeds to install.
>
> It seems uprobes install failures are completely silent (see uprobe_mmap() in
> kernel/events/uprobes.c). So I think although uprobes install seems fine, it
> actually is not.

Huh, so there's no check if the instruction is a valid one at event
register point?