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

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Message-ID: <aXDHSIn3hIvESjXg@J2N7QTR9R3>
Date: Wed, 21 Jan 2026 12:32:08 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: kernel test robot <lkp@...el.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Jeremy Linton <jeremy.linton@....com>,
	David Laight <david.laight.linux@...il.com>, llvm@...ts.linux.dev,
	oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
	linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH v4 3/3] randomize_kstack: Unify random source across
 arches

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@...el.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.