e2fsprogs alignment issues

e2fsprogs alignment issues

[Gordan Bobic]

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.

Can something be done to improve the portability of the userspace code?

The issue has been discussed on the Fedora-ARM mailing list:
http://lists.fedoraproject.org/pipermail/arm/2012-July/003637.html
but the argument there was that the issue needs to be fixed upstream, 
hence why I am posting it here.

This is an issue on all ARMs up to ARMv5, possibly ARMv6, and likely 
other CPU architectures that don't have transparent alignment fixup in 
hardware (Itanium, SPARC, probably others).

Apart from being dangerous, using unaligned arrays also affects 
performance because it leads to data needlessly straddling cache lines 
in the CPU. But that doesn't really matter in this case.

Many thanks.

Gordan

Re: e2fsprogs alignment issues

[Eric Sandeen]

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.
> 
> Can something be done to improve the portability of the userspace
> code?
> 
> The issue has been discussed on the Fedora-ARM mailing list: 
> http://lists.fedoraproject.org/pipermail/arm/2012-July/003637.html 
> but the argument there was that the issue needs to be fixed upstream,
> hence why I am posting it here.
> 
> This is an issue on all ARMs up to ARMv5, possibly ARMv6, and likely
> other CPU architectures that don't have transparent alignment fixup
> in hardware (Itanium, SPARC, probably others).
> 
> Apart from being dangerous, using unaligned arrays also affects
> performance because it leads to data needlessly straddling cache
> lines in the CPU. But that doesn't really matter in this case.

For what it's worth, I've never seen reports from anywhere that indicate
that it didn't get fixed up.  But I agree that if we can eliminate the need
for the fixups it'd be best.

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?  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.

Gordan, until I get handy access to a real arm box, if you could do a
"make check" in the e2fsprogs tree, it might flush out a few more
of these alignment warnings, and adding them all to the bug for tracking
purposes might be helpful.

(IIRC "make gcc-wall" will also emit warnings for casts which change
natural alignment, among other things)

Thanks,

-Eric


> Many thanks.

Re: e2fsprogs alignment issues

[Theodore Ts'o]

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 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.  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).

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

> > Can something be done to improve the portability of the userspace
> > code?

Yes, someone can send me patches.  :-)

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.

> 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.

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.

>  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.

> (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.

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....

Regards,

						- Ted

Re: e2fsprogs alignment issues

[Gordan Bobic]

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

Re: e2fsprogs alignment issues

[Gordan Bobic]

On 11/07/2012 16:04, Eric Sandeen wrote:

> Gordan, until I get handy access to a real arm box, if you could do a
> "make check" in the e2fsprogs tree, it might flush out a few more
> of these alignment warnings, and adding them all to the bug for tracking
> purposes might be helpful.

Sure, will do.

Re: e2fsprogs alignment issues

[Eric Sandeen]

On 7/11/12 3:05 PM, Theodore Ts'o wrote:
> On Wed, Jul 11, 2012 at 10:04:51AM -0500, Eric Sandeen wrote:

...

>>  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.

Sure, I was planning on it :)

>> (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.

Well, let's just see where we're at, first, and see what it'll take,
case by case.

-Eric

Re: e2fsprogs alignment issues

[Theodore Ts'o]

On Wed, Jul 11, 2012 at 10:22:45PM +0100, Gordan Bobic wrote:
> >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.

The reality is there is a lot of userspace code which does things a
certain way.  Ignoring that reality will very often doom you to
irrelvance.  (For example, Plan9 and Mach assumed that the existing
huge amount of Posix-compatible userspace code wasn't important, and
people would implement new applications to their new OS interfaces
from scratch.)

> >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". :)

Um, no.  NAK.

#1, it's not going to be necessary in all cases, and #2, we try very
hard in e2fsprogs to make sure we are not dependent on GCC extensions.
If you must use a GCC extension, it must be under an #ifdef __GNUC__
and there must be a reasonable alternative if it's compiled using (for
example) the Solaris icc....

Weren't you the one ranting on and on about how portability was so
important?

> >>  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?

The right thing in the case of the filefrag example which you gave is
to allocate the structure dynamically using malloc().

In other cases there may be other solutions which may be a better way
to do things.  Please do *not* bother to do a global sed style patch.
I will reject that summarily.  Thought and care are required, not a
blunt instrument.

> >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.

You're not reading what I wrote.  There are plenty of places where we
read in an on-disk structure into a malloc'ed buffer, where all of the
fields of that structure which have 4-byte alignment for 32-bit
fields, and 2-byte alignment for 16-bit fields.  We dereference those
fields directly, without doing a manual copy operation.  Strictly
speaking, this may not be portable, since there may be architectures
that do something insane with greater than 8 byte alignment
requirements for 4-byte types.

We (both e2fsprogs and the Linux kernel) also assume that the bit
pattern field for NULL is 0x00.  Strictly speaking the C standard does
not require this; it only requires that if you cast an integer value
of 0, the result must be the NULL pointer.  For example, on the
Honeywell GE-645, a NULL pointer was (segment -2, offset 0).

Architectures which have odd alignments or wierd bit patterns for NULL
pointers are what I am calling "insane".  And I don't care about
complete generic portability to weird architectures.  So please don't
try to make arguments about portability as a platonic ideal that we
must adhere to at all costs.  If we did that, we would have to
manually copy each field from the on-disk structure to an in-memory
structure, and that would be insanely slow.  Can you imagine doing
that in the TCP stack?!?

I am willing to support the ARM architecture, but we need to do this
in a careful way; not via some quick and dirty sed script.  But please
note that other architectures, including the Alpha, have always
supported doing unaligned pointer fixups (even if it is slow), because
if they didn't vast amounts of userspace *would* break.

Regards,

						- Ted

Re: e2fsprogs alignment issues

[Dave Chinner]

On Wed, Jul 11, 2012 at 04:05:57PM -0400, Theodore Ts'o wrote:
> 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.

This is a problem we've had to deal with in the XFS userspace for
years because of the way extent records are packed into inodes.
Hence we have "get/set_unaligned_be32" and "get/set_unaligned_be64"
functions to read and write unaligned variables directly out of
buffers without having platform specific alignment issues. It's
exactly the same generic code as the kernel uses for this purpose.
See libxfs/xfs.h....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com