Re: [BUG] arm64: an infinite loop in generic_perform_write()

From: Robin Murphy <robin.murphy@arm.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>,
	Christoph Hellwig <hch@infradead.org>,
	Chen Huang <chenhuang5@huawei.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Randy Dunlap <rdunlap@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linux-mm <linux-mm@kvack.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write()
Date: Thu, 24 Jun 2021 17:38:35 +0100	[thread overview]
Message-ID: <7896a3c7-2e14-d0f4-dbb9-286b6f7181b5@arm.com> (raw)
In-Reply-To: <YNSyZaZtPTmTa5P8@zeniv-ca.linux.org.uk>

On 2021-06-24 17:27, Al Viro wrote:
> On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote:
> 
>> FWIW I think the only way to make the kernel behaviour any more robust here
>> would be to make the whole uaccess API more expressive, such that rather
>> than simply saying "I only got this far" it could actually differentiate
>> between stopping due to a fault which may be recoverable and worth retrying,
>> and one which definitely isn't.
> 
> ... and propagate that "more expressive" information through what, 3 or 4
> levels in the call chain?
> 
>  From include/linux/uaccess.h:
> 
>   * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting
>   * at to must become equal to the bytes fetched from the corresponding area
>   * starting at from.  All data past to + size - N must be left unmodified.
>   *
>   * If copying succeeds, the return value must be 0.  If some data cannot be
>   * fetched, it is permitted to copy less than had been fetched; the only
>   * hard requirement is that not storing anything at all (i.e. returning size)
>   * should happen only when nothing could be copied.  In other words, you don't
>   * have to squeeze as much as possible - it is allowed, but not necessary.
> 
> arm64 instances violate the aforementioned hard requirement.  Please, fix
> it there; it's not hard.  All you need is an exception handler in .Ltiny15
> that would fall back to (short) byte-by-byte copy if the faulting address
> happened to be unaligned.  Or just do one-byte copy, not that it had been
> considerably cheaper than a loop.  Will be cheaper than propagating that extra
> information up the call chain, let alone paying for extra ->write_begin()
> and ->write_end() for single byte in generic_perform_write().

And what do we do if we then continue to fault with an external abort 
because whatever it is that warranted being mapped as Device-type memory 
in the first place doesn't support byte accesses?

Robin.