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SubjectRe: Subject: [PATCH 02/16] Squashfs: directory lookup operations
On Tue, 28 Oct 2008, Michael Tokarev wrote:
> Geert Uytterhoeven wrote:
> > On Fri, 17 Oct 2008, Phillip Lougher wrote:
> > > --- /dev/null
> > > +++ b/fs/squashfs/namei.c
> >
> > > +static int get_dir_index_using_name(struct super_block *s,
> > > + long long *next_block, unsigned int *next_offset,
> > > + long long index_start, unsigned int index_offset,
> > > + int i_count, const char *name, int len)
> > > +{
> > > + struct squashfs_sb_info *msblk = s->s_fs_info;
> > > + int i, size, length = 0;
> > > + struct squashfs_dir_index *index;
> > > + char *str;
> > > +
> > > + TRACE("Entered get_dir_index_using_name, i_count %d\n", i_count);
> > > +
> > > + str = kmalloc(sizeof(*index) + (SQUASHFS_NAME_LEN + 1) * 2,
> > > GFP_KERNEL);
> > > + if (str == NULL) {
> > > + ERROR("Failed to allocate squashfs_dir_index\n");
> > > + goto out;
> > > + }
> > > +
> > > + index = (struct squashfs_dir_index *) (str + SQUASHFS_NAME_LEN + 1);
> >
> > As str has been returned by kmalloc(), and SQUASHFS_NAME_LEN is equal to
> > 256,
> > `str + SQUASHFS_NAME_LEN + 1` is an odd address.
> >
> [..]
> > > + size = le32_to_cpu(index->size) + 1;
> > ^^^^^^^^^^^
> [.]
> > Hence accessing multi-byte fields in struct squashfs_dir_index causes
> > unaligned
> > accesses, which are emulated on some architectures (e.g. on MIPS).
> >
> > Use get_unaligned_le32() for unaligned accesses.
>
> How about aligning it properly in the first place instead?
> Three ways:
>
> 1) reordering index and str here, so that index comes first,
> str next.
>
> 2) using another constant instead of +1
>
> 3) using separate allocations for separate objects.

You're right.

Somehow I was convinced this was part of the on-disk layout, so it could not be
changed. But that's not the case...

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium

Phone: +32 (0)2 700 8453
Fax: +32 (0)2 700 8622
E-mail: Geert.Uytterhoeven@sonycom.com
Internet: http://www.sony-europe.com/

A division of Sony Europe (Belgium) N.V.
VAT BE 0413.825.160 · RPR Brussels
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