Messages in this thread | | | Date | Mon, 30 Jan 2023 10:54:40 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH -next V7 1/7] riscv: ftrace: Fixup panic by disabling preemption |
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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@arm.com> wrote: > > > > Hi Guo, > > > > On Thu, Jan 12, 2023 at 04:05:57AM -0500, guoren@kernel.org wrote: > > > From: Andy Chiu <andy.chiu@sifive.com> > > > > > > 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.
> ps: > Yes, it's not good, and it's expensive.
We can't have everything! :)
Thanks, Mark.
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