Re: [PATCH -next V3 1/2] riscv: jump_label: Fixup unaligned arch_static_branch function

[Guo Ren]

On Mon, Jan 30, 2023 at 7:57 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> guoren@xxxxxxxxxx writes:
>
> > From: Andy Chiu <andy.chiu@xxxxxxxxxx>
> >
> > Runtime code patching must be done at a naturally aligned address, or we
> > may execute on a partial instruction.
> >
> > We have encountered problems traced back to static jump functions during
> > the test. We switched the tracer randomly for every 1~5 seconds on a
> > dual-core QEMU setup and found the kernel sucking at a static branch
> > where it jumps to itself.
> >
> > The reason is that the static branch was 2-byte but not 4-byte aligned.
> > Then, the kernel would patch the instruction, either J or NOP, with two
> > half-word stores if the machine does not have efficient unaligned
> > accesses. Thus, moments exist where half of the NOP mixes with the other
> > half of the J when transitioning the branch. In our particular case, on
> > a little-endian machine, the upper half of the NOP was mixed with the
> > lower part of the J when enabling the branch, resulting in a jump that
> > jumped to itself. Conversely, it would result in a HINT instruction when
> > disabling the branch, but it might not be observable.
> >
> > ARM64 does not have this problem since all instructions must be 4-byte
> > aligned.
>
> Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxx>
>
> Nice catch! And I guess this is an issue for kprobes as well, no?
> I.e. in general replacing 32b insns with an ebreak. This is only valid
> for natural aligned 32b insns?
>
> @Guo I don't see the point of doing a series for this, and asking the
> maintainers to "pick this patch to stable, and the other for
> next". Isn't that just more work for the maintainers/reviewers?
If these two patches are separated, they are all fixup that issue and
competed with each other. Making my patch an optimization patch must
depend on it. That's why I put them in one series.

>
>
> Björn



-- 
Best Regards
 Guo Ren