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

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Message-ID: <87edjmz864.fsf@all.your.base.are.belong.to.us>
Date:   Tue, 29 Aug 2023 08:14:59 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Nam Cao <namcaov@...il.com>
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()

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.