linux-kernel - Re: [PATCH] riscv: provide riscv-specific is_trap

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Date:   Tue, 29 Aug 2023 20:54:52 +0200
From:   Nam Cao <namcaov@...il.com>
To:     Björn Töpel <bjorn@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        guoren@...nel.org
Subject: Re: [PATCH] riscv: provide riscv-specific is_trap_insn()

On Tue, Aug 29, 2023 at 08:14:59AM +0200, Björn Töpel wrote:
> Nam Cao <namcaov@...il.com> 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@...il.com> 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.

Best regards,
Nam