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

[Bingwu Zhang]

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.

> 
> 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

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