Re: [PATCH] LoongArch: Increase default mmap randomization bits

[Huacai Chen]

On Sun, Apr 12, 2026 at 11:49 AM Bingwu Zhang <xtex@xxxxxxxx> wrote:
>
> On Sunday, March 29, 2026 11:48:02 AM China Standard Time Huacai Chen wrote:
> > Hi, Bingwu,
> >
> > On Sun, Mar 29, 2026 at 6:59 AM Bingwu Zhang <xtex@xxxxxxxx> wrote:
> > > From: Bingwu Zhang <xtex@xxxxxxxxxxxxx>
> > >
> > > Increase default mmap randomization bits from 12 to 18 on 64-bit
> > > platforms for better strength.
> > >
> > > The original default, 12, means that ASLR offset has only (1 << 12) =
> > > 4096 possibilities. On average, it can be brute-forced in 2048 attempts.
> > > If a service is configured to restart automatically or can be started
> > > easily (e.g. execve a suid program), then trying for 4k times can be
> > > done in one day even when each attempt takes 20s.
> > > Increasing it to 18 makes brute-force much more difficult and leaves
> > > more time for operators to find out attacks.
> > >
> > > On 64-bit platforms, virtual address space is cheap, so the
> > > randomization bits can be increased safely without disturbing userland
> > > much and security comes first instead of availability.
> >
> > Don't change ARCH_MMAP_RND_BITS_MIN because it may compact
> > performance, and you cannot blindly increase ARCH_MMAP_RND_BITS_MAX
> > because it should no more than "VA_BITS - PAGE_SHIFT - 3" (see
> > arch/riscv/Kconfig and arch/csky/Kconfig). For LoongArch, VA_BITS can
> > be 39 (half of VA40), PAGE_SHIFT can be 16 (64KB page), so
> > ARCH_MMAP_RND_BITS_MAX can only increase up to 20 (39 - 16 - 3 =20).
>
> Sorry for the late reply.
>
> It was my mistake that I didn't notice the upper bound of MAX. Thank you for
> pointing out!
>
> However, I didn't get where increasing MIN could compact performance.
> Theoretically, the only difference is that the gap between heaps and stacks
> are larger, from [128, 128+64]M to [128, 128+4096]M. This shouldn't affect
> performance much. MIPS once used MIN=16, RISC-V 64 and PowerPC and ARM 64 are
> using 18, and x86-64 is using 28, but I hadn't heard anyone complaining about
> the performance.
>
> I ran some benchmarks last weekend using the LLVM build benchmark of AOSC OS
> https://github.com/AOSC-Dev/buildbot-benchmark/blob/master/buildbot-benchmark.bash
> and here are the results
>
> Processor       Rand bits       Time/secs
> Average Variance        Range
> 3C6000/S        18      612.927 614.394 614.513 613.343 614.76
> 613.9874        0.515637840000006       1.83299999999997
> 3C6000/S        12      614.322 613.93  614.825 613.859
> 615.875 614.5622        0.548874160000007       2.01599999999996
> 3A5000  18      3547.165        3548.764        3552.192        3545.697
> 3547.567        3548.277        4.79228759999993        6.49499999999989
> 3A5000  12      3549.029        3551.894        3549.191        3567.869
> 3587.409        3561.0784       222.052853440003        38.3800000000001
>
> I didn't see any statistically meaningful performance defect.
OK, then we can increase ARCH_MMAP_RND_BITS_MIN, but the range 18~20
seems too small and then useless, can we consider using 15~20?


Huacai

>
> >
> > These are done in:
> > https://github.com/chenhuacai/linux/commit/ff35475dcfbd4c6e9c275aaef3c92da71
> > 0894058
> >
> > Huacai
> >
> > > Fixes: fa96b57c1490 ("LoongArch: Add build infrastructure")
> > > Signed-off-by: Bingwu Zhang <xtex@xxxxxxxxxxxxx>
> > > ---
> > >
> > >  arch/loongarch/Kconfig | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> > > index 92068ff38685..b47c4e4ecbb7 100644
> > > --- a/arch/loongarch/Kconfig
> > > +++ b/arch/loongarch/Kconfig
> > > @@ -740,9 +740,11 @@ config MMU
> > >
> > >         default y
> > >
> > >  config ARCH_MMAP_RND_BITS_MIN
> > >
> > > +       default 18 if 64BIT
> > >
> > >         default 12
> > >
> > >  config ARCH_MMAP_RND_BITS_MAX
> > >
> > > +       default 32 if 64BIT
> > >
> > >         default 18
> > >
> > >  config ARCH_SUPPORTS_UPROBES
> > >
> > > base-commit: be762d8b6dd7efacb61937d20f8475db8f207655
> > > --
> > > 2.52.0
>
> Best regards,
>
> --
> Bingwu Zhang @ Sun, 12 Apr 2026 03:28:14 +0000