Re: [PATCH v4 3/3] randomize_kstack: Unify random source across arches

[Ryan Roberts]

On 21/01/2026 12:32, Mark Rutland wrote:
> On Wed, Jan 21, 2026 at 10:52:21AM +0000, Ryan Roberts wrote:
>> On 20/01/2026 23:50, kernel test robot wrote:
>>> Hi Ryan,
>>>
>>> kernel test robot noticed the following build warnings:
>>>
>>> [auto build test WARNING on akpm-mm/mm-everything]
>>> [also build test WARNING on linus/master v6.19-rc6 next-20260119]
>>> [cannot apply to tip/sched/core kees/for-next/hardening kees/for-next/execve]
>>> [If your patch is applied to the wrong git tree, kindly drop us a note.
>>> And when submitting patch, we suggest to use '--base' as documented in
>>> https://git-scm.com/docs/git-format-patch#_base_tree_information]
>>>
>>> url:    https://github.com/intel-lab-lkp/linux/commits/Ryan-Roberts/randomize_kstack-Maintain-kstack_offset-per-task/20260119-210329
>>> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
>>> patch link:    https://lore.kernel.org/r/20260119130122.1283821-4-ryan.roberts%40arm.com
>>> patch subject: [PATCH v4 3/3] randomize_kstack: Unify random source across arches
>>> config: x86_64-allmodconfig (https://download.01.org/0day-ci/archive/20260121/202601210752.6Nsv9et9-lkp@xxxxxxxxx/config)
>>> compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
>>> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260121/202601210752.6Nsv9et9-lkp@xxxxxxxxx/reproduce)
>>>
>>> If you fix the issue in a separate patch/commit (i.e. not just a new version of
>>> the same patch/commit), kindly add following tags
>>> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
>>> | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@xxxxxxxxx/
>>>
>>> All warnings (new ones prefixed by >>):
>>>
>>>>> vmlinux.o: warning: objtool: do_syscall_64+0x2c: call to preempt_count_add() leaves .noinstr.text section
>>>>> vmlinux.o: warning: objtool: __do_fast_syscall_32+0x3d: call to preempt_count_add() leaves .noinstr.text section
>>
>> Hmm, clearly Dave was correct not to rush this through... yuck. I'll take a
>> look, but I guess there is no rush if this won't go into -next until shortly
>> after -rc1.
> 
> Sorry, I should have checked the entry sequencing more thoroughly when I
> reviewed this,.
> 
> From a quick look, I suspect the right thing to do is to pull the call
> to add_random_kstack_offset() a bit later in a few cases; after the
> entry logic has run, and after instrumentation_begin() (if the arch code
> uses that), such that it doesn't matter if this gets instrumented.
> 
> Considering the callers of add_random_kstack_offset(), if we did that:
> 
> * arm64 is fine as-is.
> 
> * loongarch is fine as-is.
> 
> * powerpc's system_call_exception() would need this moved after the
>   user_exit_irqoff(). Given that function is notrace rather than
>   noinstr, it looks like there are bigger extant issues here.
> 
> * riscv is fine as-is.
> 
> * s390's __do_syscall() would need this moved after
>   enter_from_user_mode().
> 
> * On x86:
>   - do_int80_emulation() is fine as-is.
>   - int80_emulation() is fine as-is.
>   - do_int80_syscall_32() would need this moved after
>     instrumentation_begin().
>   - __do_fast_syscall_32() would need this moved after
>     instrumentation_begin().
>   - do_syscall_64() would need this moved after instrumentation_begin().

Thanks for the detailed suggestions, Mark. I've taken this approach, and
assuming perf testing doesn't throw up any issue, I'm going to revert back to
using the out-of-line version of prandom_u32_state() and will drop patch 2.

Thanks,
Ryan


> 
> Mark.