Re: [PATCH] arm64: kprobes: Use BRK instead of single-step when executing instructions out-of-line

From: Will Deacon <will@kernel.org>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: catalin.marinas@arm.com, Masami Hiramatsu <mhiramat@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: kprobes: Use BRK instead of single-step when executing instructions out-of-line
Date: Tue, 3 Nov 2020 09:23:16 +0000	[thread overview]
Message-ID: <20201103092315.GC4403@willie-the-truck> (raw)
In-Reply-To: <20201103091305.GA6723@myrica>

On Tue, Nov 03, 2020 at 10:13:05AM +0100, Jean-Philippe Brucker wrote:
> On Mon, Nov 02, 2020 at 10:52:55PM +0900, Masami Hiramatsu wrote:
> > > > -	patch_text(p->addr, p->opcode);
> > > > +	patch_text(p->addr, p->opcode, 0);
> > > 
> > > I don't think patch_text() is offering an awful lot to these callers. Why
> > > don't we just call aarch64_insn_patch_text() directly, which I think will
> > > make the code easier to read too?
> > 
> > Agreed. I guess it's just because histrically used.
> 
> Yes it's easy to remove, sending a v2
> 
> > 
> > > >  void __kprobes arch_remove_kprobe(struct kprobe *p)
> > > > @@ -163,20 +167,14 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
> > > >  }
> > > >  
> > > >  /*
> > > > - * Interrupts need to be disabled before single-step mode is set, and not
> > > > - * reenabled until after single-step mode ends.
> > > > - * Without disabling interrupt on local CPU, there is a chance of
> > > > - * interrupt occurrence in the period of exception return and  start of
> > > > - * out-of-line single-step, that result in wrongly single stepping
> > > > - * into the interrupt handler.
> > > > + * Keep interrupts disabled while executing the instruction out-of-line. Since
> > > > + * the kprobe state is per-CPU, we can't afford to be migrated.
> > > >   */
> > > >  static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
> > > >  						struct pt_regs *regs)
> > > >  {
> > > >  	kcb->saved_irqflag = regs->pstate & DAIF_MASK;
> > > >  	regs->pstate |= PSR_I_BIT;
> > > > -	/* Unmask PSTATE.D for enabling software step exceptions. */
> > > > -	regs->pstate &= ~PSR_D_BIT;
> > > 
> > > Can we set the D bit now? Would be one less thing to worry about.
> > 
> > Hmm, I think we just ignore D bit above. If someone (kgdb?) is set D flag
> > then there may be no reason to prohibit it on the trampoline.
> > (Of course they must handle this case)
> 
> Masking debug just for the trampoline would simplify reasoning about
> nesting exceptions. Although it prevents from debugging kprobes using
> KGDB, it seems reasonable to me because I don't think we can provide
> guarantees that KGDB works reliably with kprobes. For example it doesn't
> seem like a watchpoint would fire if the instruction performing the access
> is simulated by kprobe.

Yes, let's just set all of DAIF during the trampoline. Also, while I've got
you, If you get a chance, I'd appreciate any feedback on my proposal for
reworking our debug exception handling altogether:

https://lore.kernel.org/r/20200626095551.GA9312@willie-the-truck

Will

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