linux-kernel - Re: [PATCH 2/3] arm64: use PAGE_KERNEL_ROX directly in alloc_insn

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Date:   Tue, 23 Jun 2020 10:07:58 +0100
From:   Will Deacon <will@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dexuan Cui <decui@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jessica Yu <jeyu@...nel.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] arm64: use PAGE_KERNEL_ROX directly in
 alloc_insn_page

On Tue, Jun 23, 2020 at 11:05:05AM +0200, Christoph Hellwig wrote:
> On Sat, Jun 20, 2020 at 07:16:16PM -0700, Andrew Morton wrote:
> > On Thu, 18 Jun 2020 08:43:06 +0200 Christoph Hellwig <hch@....de> wrote:
> > 
> > > Use PAGE_KERNEL_ROX directly instead of allocating RWX and setting the
> > > page read-only just after the allocation.
> > > 
> > > --- a/arch/arm64/kernel/probes/kprobes.c
> > > +++ b/arch/arm64/kernel/probes/kprobes.c
> > > @@ -120,15 +120,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
> > >  
> > >  void *alloc_insn_page(void)
> > >  {
> > > -	void *page;
> > > -
> > > -	page = vmalloc_exec(PAGE_SIZE);
> > > -	if (page) {
> > > -		set_memory_ro((unsigned long)page, 1);
> > > -		set_vm_flush_reset_perms(page);
> > > -	}
> > > -
> > > -	return page;
> > > +	return __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
> > > +			GFP_KERNEL, PAGE_KERNEL_ROX, VM_FLUSH_RESET_PERMS,
> > > +			NUMA_NO_NODE, __func__);
> > >  }
> > >  
> > >  /* arm kprobe: install breakpoint in text */
> > 
> > But why.  I think this is just a cleanup, doesn't address any runtime issue?
> 
> It doesn't "fix" an issue - it just simplifies and speeds up the code.

Ok, but I don't understand the PLT comment from Peter in
20200618092754.GF576905@...ez.programming.kicks-ass.net:

  | I think this has the exact same range issue as the x86 user. But it
  | might be less fatal if their PLT magic can cover the full range.

Peter, please could you elaborate on your concern? I feel like I'm missing
some context.

Cheers,

Will