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

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

[kernel test robot]

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@intel.com/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@intel.com/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@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/

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

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

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

[David Laight]

On Wed, 21 Jan 2026 07:50:16 +0800
kernel test robot <lkp@intel.com> 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@intel.com/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@intel.com/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@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/
> 
> 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  
> 

When CONFIG_DEBUG_PREEMPT or CONFIG_TRACE_PREEMP_TOGGLE is set
the preempt_count_[en|dis]able() calls inside [put|get]_cpu_var()
become real functions.

Maybe __preempt_count_[inc|dec]() can be called (with this_cpu_ptr()).

	David

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

[David Laight]

On Wed, 21 Jan 2026 10:20:17 +0000
David Laight <david.laight.linux@gmail.com> wrote:

> On Wed, 21 Jan 2026 07:50:16 +0800
> kernel test robot <lkp@intel.com> 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@intel.com/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@intel.com/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@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/
> > 
> > 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    
> >   
> 
> When CONFIG_DEBUG_PREEMPT or CONFIG_TRACE_PREEMP_TOGGLE is set
> the preempt_count_[en|dis]able() calls inside [put|get]_cpu_var()
> become real functions.
> 
> Maybe __preempt_count_[inc|dec]() can be called (with this_cpu_ptr()).

Or the code could just use the per-cpu data without disabling preemption.
Usually that isn't a good idea at all, but it can't matter in this case.
Might give a noticeable performance gain, disabling preemption is
non-trivial and/or an atomic operation on some architectures.

If anyone is worried about preemption causing the output be repeated, that
would be (mostly) mitigated by checking that s[1234] haven't changed prior
to writing the new values.
I think a 'not locked at all' compare of two of the four values will
stop everything except two threads doing system calls at the same time
getting the same output from the prng.

The whole thing is very unlikely and there will be much easier ways
to break the prng.
Provided s[1234] are only written with valid values (ie ones which aren't
effectively zero) it will continue generating numbers.

	David


> 
> 	David
> 

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

[Ryan Roberts]

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@intel.com/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@intel.com/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@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/
> 
> 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.

Thanks,
Ryan

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

[Mark Rutland]

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@intel.com/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@intel.com/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@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/
> > 
> > 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().

Mark.

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@intel.com/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@intel.com/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@intel.com>
>>> | Closes: https://lore.kernel.org/oe-kbuild-all/202601210752.6Nsv9et9-lkp@intel.com/
>>>
>>> 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.