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

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Date:   Thu, 24 Jun 2021 17:38:35 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Chen Huang <chenhuang5@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        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 2021-06-24 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.