linux-kernel - Re: [PATCH] arm64: uprobes: Optimize cache flushes for xol slot

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Message-ID: <20240920110301.GB15795@redhat.com>
Date: Fri, 20 Sep 2024 13:03:01 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Liao, Chang" <liaochang1@...wei.com>
Cc: mhiramat@...nel.org, peterz@...radead.org, catalin.marinas@....com,
	will@...nel.org, mark.rutland@....com, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: uprobes: Optimize cache flushes for xol slot

On 09/20, Liao, Chang wrote:
>
> 在 2024/9/19 22:18, Oleg Nesterov 写道:
> > On 09/19, Liao Chang wrote:
> >>
> >> --- a/arch/arm64/kernel/probes/uprobes.c
> >> +++ b/arch/arm64/kernel/probes/uprobes.c
> >> @@ -17,12 +17,16 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> >>  	void *xol_page_kaddr = kmap_atomic(page);
> >>  	void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK);
> >>
> >> +	if (!memcmp(dst, src, len))
> >> +		goto done;
> >
> > can't really comment, I know nothing about arm64...
> >
> > but don't we need to change __create_xol_area()
> >
> > 	-	area->page = alloc_page(GFP_HIGHUSER);
> > 	+	area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO);
> >
> > to avoid the false positives?
>
> Indeed, it would be safer.
>
> Could we tolerate these false positives? Even if the page are not reset
> to zero bits, if the existing bits are the same as the instruction being
> copied, it still can execute the correct instruction.

OK, agreed, the task should the same data after page fault.

Oleg.