linux-kernel - Re: [BUG] arm64: an infinite loop in generic_perform

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Message-ID: <92fa298d-9d88-0ca4-40d9-13690dcd42f9@huawei.com>
Date:   Wed, 23 Jun 2021 11:24:54 +0800
From:   Xiaoming Ni <nixiaoming@...wei.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Chen Huang <chenhuang5@...wei.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mm <linux-mm@...ck.org>,
        "open list" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write()

On 2021/6/23 10:50, Al Viro wrote:
> On Wed, Jun 23, 2021 at 10:39:31AM +0800, Chen Huang wrote:
> 
>> Then when kernel handles the alignment_fault, it will not panic. As the
>> arm64 memory model spec said, when the address is not a multiple of the
>> element size, the access is unaligned. Unaligned accesses are allowed to
>> addresses marked as Normal, but not to Device regions. An unaligned access
>> to a Device region will trigger an exception (alignment fault).
>> 	
>> do_alignment_fault
>>      do_bad_area
>> 	__do_kernel_fault
>>             fixup_exception
>>
>> But that fixup cann't handle the unaligned copy, so the
>> copy_page_from_iter_atomic returns 0 and traps in loop.
> 
> Looks like you need to fix your raw_copy_from_user(), then...
> .
> 

Exit loop when iov_iter_copy_from_user_atomic() returns 0.
This should solve the problem, too, and it's easier.

Thanks.
Xiaoming Ni