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Date:	Wed, 11 Jul 2012 22:22:45 +0100
From:	Gordan Bobic <gordan@...ich.net>
To:	Theodore Ts'o <tytso@....edu>
CC:	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: e2fsprogs alignment issues

On 11/07/2012 21:05, Theodore Ts'o wrote:
> On Wed, Jul 11, 2012 at 10:04:51AM -0500, Eric Sandeen wrote:
>> On 7/11/12 8:20 AM, Gordan Bobic wrote:
>>> I would like to bring the following bug report to people's
>>> attention:
>>>
>>> https://bugzilla.redhat.com/show_bug.cgi?id=680090
>>>
>>> The issue is that the code used in e2fsprogs WRT allocating unaligned
>>> buffers and then casting them into structs is non-portable and
>>> dangerous - eyewateringly dangerous in something like e2fsprogs where
>>> it can lead to thorough trashing of the file system.
>
> I think you're being a bit over-dramatic here; can you point out a
> place where this is actually going to lead to "trashing of the file
> system"?

I haven't gone through the code with a fine tooth comb, but the fact 
that this sort of unportable operations happens implies that it wasn't 
written with safety and portability considerations in mind.

> I will observe that being able to derefrence unaligned pointers is
> something that Linus has said any architecture which does *not*
> provide fixups is probably doomed to fail.

That sounds like a resigned recognition that a lot of developers will 
use unsafe practices anyway, not a model for how things should be done.

> There's an awful lot of
> userspace code out there which assumes this works.  Doing a quick
> grep, I'm seeing code in the blkid library (which has now been moved
> into util-linux) which does this.  I'm also seeing one case of it in
> the tdb library (which we imported from Samba).

I make a point with running with fixup+warn on all my ARMv5 devices, and 
I file bugs against packages that cause these to crop up. It is 
relatively rare, though, from what I have been able to observe.

> That being said, doing fixups is slow since it requires a kernel
> fault, and I have no problem with trying to avoid this practice.

Splendid, we agree, then. :)

>>> Can something be done to improve the portability of the userspace
>>> code?
>
> Yes, someone can send me patches.  :-)

Sure, I can sed __attribute__((aligned(4))) on every char[] definition, 
as long as no art rather than engineering oriented individuals aren;t 
going to complain that it "looks ugly". :)

> Ranting and raving about how file systems are being trashed will be
> met with the observation that no one has actually reported this.  And
> this won't be the only place where the Linux kernel code makes
> assumptions which are not guaranteed by the C standard, but the
> official position that Linus has taken is any architecture (or C
> compiler) is insane, and it's up to the architecture to provide
> workarounds.

Extending that view to it's logical conclusion, let's drop support for 
ARM < v7 completely, then? I just don't think cutting corners at the 
expense of correctness should be deemed acceptable.

>> Many of these are the result of things like:
>>
>>          char buf[4096] = "";
>>          struct fiemap *fiemap = (struct fiemap *)buf;
>>
>>
>> Ted, is this construct just an attempt to avoid malloc/free for
>> simplicity of the code?
>
> Probably; it's the way Kalpak Shaw wrote the code when he submitted
> it.  And while I do have higher standards of portability that the
> Linux kernel, this is not one of the things that I test for.

That's OK - somebody will find it. :)

> Please do note that malloc() is guaranteed to return a block of memory
> which satisifies the worse case alignment requirements of the
> architecture, so there are plenty of places where we cast a buffer
> pointer to an int * or a struct * which are completely safe.  It's
> only when someone is allocating space on the stack where this is
> potentially problematic.

Indeed, and only when allocating it using arrays of type that is smaller 
than word, which most of the time means char. Everything else will be 
word aligned anyway.

>>   I think Gordan suggested (if I understand it
>> right) that doing an array of ints might also solve the problem, since
>> ints should be on natural alignment.  Or maybe in some cases malloc/free
>> would be more obvious, if handling errors isn't too tricky.
>
> In the specific case which Gordon has pointed out, the obvious thing
> to do is to just to set errno to ENOMEM, and return -1. since we
> already reflect an error code up to the caller if the FIEMAP ioctl()
> fails.
>
> If someone sends me the patch, I will happily apply it.

As opposed to aligning the char arrays explicitly?

>> (IIRC "make gcc-wall" will also emit warnings for casts which change
>> natural alignment, among other things)
>
> I'd have to check to be sure, but I don't think so, since it would
> have way too many false positives.  We *do* have code where we take
> char *'s and and then cast them to some other pointer type, and then
> dereference them.  And we do currently assume that it is safe to do
> this for on-disk data structures which are 4 byte aligned, in the
> directory entry code, for example.
>
> I will *not* accept a patch which uses memcpy to copy each field in
> the on-disk superblock, or directory entry, into an int, just in case
> there is some insane architecture which requires that 4 byte integers
> be 32-byte aligned, or something else insane like that.
>
> What we are currently doing may not be 100% portable, but we're not
> going to penalize sane archiectures like x86 just because there might
> be some future insane architecture that requires 32-byte aligned ints!
> All the ranting in the world about how this could hypothetically cause
> file system corruption on said insane architecture will just cause me
> to laugh at you.

By that definition, the majority of architectures supported by Linux are 
insane. Word alignment requirements apply to ARM, and IIRC to Itanium 
and SPARC (not sure about the most recent SPARCs) too. If you think that 
Linux should become x86-only, that's fair enough, I'm sure the ARM 
kernel maintainers are getting sufficiently frustrated with the current 
situation that they might think this is a positive development. But 
meanwhile, back in the real world, we do need to deal with the issue.

> But that being said, changing places where we allocate a char []
> buffer on the stack and assuming that it is 4-byte aligned does seem
> reasonable.  But I see this as an optimization issue more than I do a
> performance issue....

I'm not sure exactly what you mean here. But either way, what's wrong 
with any of the following:

1) Checking sizeof(int) and definining the buffer in suitably sized 
array of int instead of char

or

2) making sure that arrays of char are explicitly aligned to 4 bytes (or 
even 8 bytes, just to be on the safe side for 64-bit architectures - 
that should be safe for quite a foreseeable future


Gordan
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