linux-kernel - Re: [RESEND 2][PATCH v4] hexdump: fix for non-aligned buffers

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Message-Id: <20150708170341.322f6eb3fcbad5ec3c77f10e@linux-foundation.org>
Date:	Wed, 8 Jul 2015 17:03:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Horacio Mijail Antón Quiles <hmijail@...il.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	David Howells <dhowells@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
	trivial@...nel.org, Joe Perches <joe@...ches.com>
Subject: Re: [RESEND 2][PATCH v4] hexdump: fix for non-aligned buffers

On Thu, 9 Jul 2015 01:44:18 +0200 Horacio Mijail Ant__n Quiles <hmijail@...il.com> wrote:

> An hexdump with a buf not aligned to the groupsize causes
> non-naturally-aligned memory accesses. This was causing a kernel panic on
> the processor BlackFin BF527, when such an unaligned buffer was fed by the
> function ubifs_scanned_corruption in fs/ubifs/scan.c .
> 
> To fix this, if the buffer is not aligned to groupsize in a platform which
> does not define CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, then change the
> groupsize to 1, so alignment is no longer a problem.
> This behavior is coherent with the way the function currently deals with
> inappropriate parameter combinations, which is to fall back to safe
> "defaults", even if that means changing the output format and the implicit
> access patterns that could have been expected.

CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS seems inappropriate for this. 
Having this unset means "can do unaligned accesses, but they're
inefficient".  It doesn't mean "unaligned accesses go oops".

But I can't the appropriate config knob.  There's a
CONFIG_CPU_HAS_NO_UNALIGNED, but that's an m68k-private thing.

> Resent on 8 Jul because of no answers.
> 
> Resent on 29 Jun because of no answers.

During the merge window.  So I have everything sitting there in my
patches-to-process pile.  The backlog is excessive this time (700+
emails) so I'm thinking I'll change things so I'll henceforth be
processing patches-for-the-next-cycle during this-cycle, while keeping
patches-for-next-cycle out of linux-next.

> --- a/lib/hexdump.c
> +++ b/lib/hexdump.c
> @@ -124,6 +124,11 @@ int hex_dump_to_buffer(const void *buf, size_t len, int rowsize, int groupsize,
>  	if ((len % groupsize) != 0)	/* no mixed size output */
>  		groupsize = 1;
>  
> +	/* fall back to 1-byte groups if buf is not aligned to groupsize */

That isn't a great comment.  It tells us what the code does (which is
quite obvious just from reading it) but it doesn't tell us *why* it
does it.  Something like "if buf is not aligned to groupsize then fall
back to 1-byte groups to avoid unaligned memory accesses on
architectures which do not support them"?

But as mentioned above, that's different from "architectures which do
not support them efficently"!  Maybe we need a new config variable.

Or maybe blackfin should be handling the unaligned access trap and
transparently handling it, like sparc?

> +	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
> +	    !IS_ALIGNED((uintptr_t)buf, groupsize))
> +		groupsize = 1;
> +
>
> ...
>
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