Re: [PATCH -next V7 1/7] riscv: ftrace: Fixup panic by disabling preemption

[Guo Ren]

On Mon, Jan 30, 2023 at 6:54 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Sat, Jan 28, 2023 at 05:37:46PM +0800, Guo Ren wrote:
> > On Thu, Jan 12, 2023 at 8:16 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > >
> > > Hi Guo,
> > >
> > > On Thu, Jan 12, 2023 at 04:05:57AM -0500, guoren@xxxxxxxxxx wrote:
> > > > From: Andy Chiu <andy.chiu@xxxxxxxxxx>
> > > >
> > > > In RISCV, we must use an AUIPC + JALR pair to encode an immediate,
> > > > forming a jump that jumps to an address over 4K. This may cause errors
> > > > if we want to enable kernel preemption and remove dependency from
> > > > patching code with stop_machine(). For example, if a task was switched
> > > > out on auipc. And, if we changed the ftrace function before it was
> > > > switched back, then it would jump to an address that has updated 11:0
> > > > bits mixing with previous XLEN:12 part.
> > > >
> > > > p: patched area performed by dynamic ftrace
> > > > ftrace_prologue:
> > > > p|      REG_S   ra, -SZREG(sp)
> > > > p|      auipc   ra, 0x? ------------> preempted
> > > >                                       ...
> > > >                               change ftrace function
> > > >                                       ...
> > > > p|      jalr    -?(ra) <------------- switched back
> > > > p|      REG_L   ra, -SZREG(sp)
> > > > func:
> > > >       xxx
> > > >       ret
> > >
> > > As mentioned on the last posting, I don't think this is sufficient to fix the
> > > issue. I've replied with more detail there:
> > >
> > >   https://lore.kernel.org/lkml/Y7%2F3hoFjS49yy52W@FVFF77S0Q05N/
> > >
> > > Even in a non-preemptible SMP kernel, if one CPU can be in the middle of
> > > executing the ftrace_prologue while another CPU is patching the
> > > ftrace_prologue, you have the exact same issue.
> > >
> > > For example, if CPU X is in the prologue fetches the old AUIPC and the new
> > > JALR (because it races with CPU Y modifying those), CPU X will branch to the
> > > wrong address. The race window is much smaller in the absence of preemption,
> > > but it's still there (and will be exacerbated in virtual machines since the
> > > hypervisor can preempt a vCPU at any time).
> > >
> > > Note that the above is even assuming that instruction fetches are atomic, which
> > > I'm not sure is the case; for example arm64 has special CMODX / "Concurrent
> > > MODification and eXecutuion of instructions" rules which mean only certain
> > > instructions can be patched atomically.
> > >
> > > Either I'm missing something that provides mutual exclusion between the
> > > patching and execution of the ftrace_prologue, or this patch is not sufficient.
> > This patch is sufficient because riscv isn't the same as arm64. It
> > uses default arch_ftrace_update_code, which uses stop_machine.
> > See kernel/trace/ftrace.c:
> > void __weak arch_ftrace_update_code(int command)
> > {
> >         ftrace_run_stop_machine(command);
> > }
>
> Ah; sorry, I had misunderstood here, since the commit message spoke in terms of
> removing that.
>
> As long as stop_machine() is used I agree this is safe; sorry for the noise.
Okay.

Hi Palmer,

Please take Andy's fixup patch. We would continue to find a way for PREEMPTION.

>
> > ps:
> >  Yes, it's not good, and it's expensive.
>
> We can't have everything! :)
>
> Thanks,
> Mark.



-- 
Best Regards
 Guo Ren