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Re: [PATCH v3, 14/16] xfsprogs: metadump: fix duplicate handling once an

To: Alex Elder <aelder@xxxxxxx>
Subject: Re: [PATCH v3, 14/16] xfsprogs: metadump: fix duplicate handling once and for all
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 24 Feb 2011 19:39:45 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <201102182121.p1ILL2Wl029181@xxxxxxxxxxxxxxxxxxxxxx>
References: <201102182121.p1ILL2Wl029181@xxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri, Feb 18, 2011 at 03:21:02PM -0600, Alex Elder wrote:
> This is a case where I think I've solved a problem to death.

:)

> The metadump code now stops rather than spinning forever in the face
> of finding no obfuscated name that hasn't already been seen.
> Instead, it simply gives up and passes the original name back to use
> without obfuscation.
> 
> Unfortunately, as a result it actually creates entries with
> duplicate names in a directory (or inode attribute fork).  And at
> least in the case of directories, xfs_mdrestore(8) will populate the
> directory it restores with duplicate entries.  That even seems to
> work, but xfs_repair(8) does identify this as a problem and fixes it
> (by moving duplicates to "lost+found").
> 
> This might have been OK, given that it was a rare occurence.  But
> it's possible, with short (5-character) names, for the obfuscation
> algorithm to come up with only a single possible alternate name,
> and I felt that was just not acceptable.
> 
> This patch fixes all that by creating a way to generate alternate
> names directly from existing names by carefully flipping pairs of
> bits in the characters making up the name.
> 
> 
> The first change is that a name is only ever obfuscated once.
> If the obfuscated name can't be used, an alternate is computed
> based on that name rather than re-starting the obfuscation
> process.  (Names shorter than 5 characters are still not
> obfuscated.)
> 
> Second, once a name is selected for use (obfuscated or not), it is
> checked for duplicates.  The name table is consulted to see if it
> has already been seen, and if it has, an alternate for that name is
> created (a different name of the same length that has the same hash
> value).  That name is checked in the name table, and if it too is
> already there the process repeats until an unused one is found.
> 
> Third, alternates are generated methodically rather than by
> repeatedly trying to come up with new random names.  A sequence
> number uniquely defines a particular alternate name, given an
> existing name.  (Note that some of those alternates aren't valid
> because they contain at least one unallowed character.)
> 
> Finally, because all names are now maintained in the name table,
> and because of the way alternates are generated, it's actually
> possible for short names to get modified in order to avoid
> duplicates.
> 
> The algorithm for doing all of this is pretty well explained in
> the comments in the code itself, so I'll avoid duplicating any
> more of that here.
> 
> Signed-off-by: Alex Elder <aelder@xxxxxxx>
> 
> This is a new change, not posted with this series previously.
> 
> ---
>  db/metadump.c |  222 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 204 insertions(+), 18 deletions(-)
> 
> Index: b/db/metadump.c
> ===================================================================
> --- a/db/metadump.c
> +++ b/db/metadump.c
> @@ -27,6 +27,10 @@
>  #include "sig.h"
>  #include "xfs_metadump.h"
>  
> +#ifndef ARRAY_SIZE
> +#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> +#endif
> +

Doesn't that belong in a header file somewhere?

>  #define DEFAULT_MAX_EXT_SIZE 1000
>  
>  /*
> @@ -469,41 +473,221 @@ in_lost_found(
>  }
>  
>  /*
> + * Flip a bit in each of two bytes at the end of the given name.
> + * This is used in generating a series of alternate names to be used
> + * in the event a duplicate is found.
> + *
> + * The bits flipped are selected such that they both affect the same
> + * bit in the name's computed hash value, so flipping them both will
> + * preserve the hash.
> + *
> + * The following diagram aims to show the portion of a computed
> + * hash that a given byte of a name affects.
> + *
> + *         31    28      24    21            14           8 7       3     0
> + *         +-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+
> + * hash:   | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
> + *         +-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+
> + *        last-4 ->|           |<-- last-2 --->|           |<--- last ---->|
> + *               |<-- last-3 --->|           |<-- last-1 --->|     |<- last-4
> + *                       |<-- last-7 --->|           |<-- last-5 --->|
> + *         . . . and so on
> + *
> + * The last byte of the name directly affects the low-order byte of
> + * the hash.  The next-to-last affects bits 7-14, the next one back
> + * affects bits 14-21, and so on.  The effect wraps around when it
> + * goes beyond the top of the hash.
> + *
> + * The last two bytes both affect bit 7, for example, so that's a
> + * pair of bytes (the first one tried, actually) that can be used
> + * for this "flip bit" operation.
> + *
> + * A table defines pairs of bytes--and bits within them--that can be
> + * used this way.  The byte offset is relative to a starting point
> + * within the name, which will be set to affect the bytes at the end
> + * of the name.
> + *
> + * The function is called with a "bitseq" value which indicates
> + * which bit flip is desired, that is, selects one entry in the
> + * bit_to_flip[] table to apply.
> + *
> + * The table is ordered by increasing byte offset, so that the
> + * earliest entries can apply to the shortest strings.  But the
> + * first bit to flip will be the one furthest down in the table
> + * (which makes the earlier bit flips tend to occur as close to the
> + * end of the name as possible).
> + *
> + * The function returns 1 if the operation was successful.  It
> + * returns 0 if the result produced a character that's not valid in
> + * a name (either '/' or a '\0').  Finally, it returns -1 if the bit
> + * sequence number is beyond what is supported for a name of this
> + * length.
> + */
> +static int
> +flip_bit(
> +     size_t          name_len,
> +     uchar_t         *name,
> +     uint32_t        bitseq)
> +{
> +     int     index;
> +     size_t  offset;
> +     uchar_t *p0, *p1;
> +     uchar_t m0, m1;
> +     struct {
> +         int         byte;   /* Offset from start within name */
> +         uchar_t     bit;    /* Bit within that byte */
> +     } bit_to_flip[][2] = {  /* Sorted by second entry's byte */
> +         { { 0, 0 }, { 1, 7 } },     /* Each row defines a pair */
> +         { { 1, 0 }, { 2, 7 } },     /* of bytes and a bit within */
> +         { { 2, 0 }, { 3, 7 } },     /* each byte.  Each bit in */
> +         { { 0, 4 }, { 4, 0 } },     /* a pair affects the same */
> +         { { 0, 5 }, { 4, 1 } },     /* bit in the hash, so flipping */
> +         { { 0, 6 }, { 4, 2 } },     /* both will change the name */
> +         { { 0, 7 }, { 4, 3 } },     /* while preserving the hash. */
> +         { { 3, 0 }, { 4, 7 } },
> +         { { 0, 0 }, { 5, 3 } },     /* The first entry's byte offset */
> +         { { 0, 1 }, { 5, 4 } },     /* must be less than the second. */
> +         { { 0, 2 }, { 5, 5 } },
> +         { { 0, 3 }, { 5, 6 } },     /* The table can be extended to */
> +         { { 0, 4 }, { 5, 7 } },     /* an arbitrary number of entries */
> +         { { 4, 0 }, { 5, 7 } },     /* but there's not much point. */
> +             /* . . . */
> +     };

OK, so there are 14 different bit swaps defined here....

> +     /* Find the first entry *not* usable for name of this length */
> +
> +     for (index = 0; index < ARRAY_SIZE(bit_to_flip); index++)
> +             if (bit_to_flip[index][1].byte >= name_len)
> +                     break;

but only 2 for name_len == 2 and 3 for name_len == 3....

> +     /*
> +      * Back up to the last usable entry.  If that number is
> +      * smaller than the bit sequence number, inform the caller
> +      * that nothing this large (or larger) will work.
> +      */
> +     if (bitseq > --index)
> +             return -1;

Which means that we only get one/two bit swaps allowed for them,
and hence very few alternates. i.e the number of alternates are
determined by the name length and the number of bit swaps defined
for that name length in the above table....

> +
> +     /*
> +      * We will be switching bits at the end of name, with a
> +      * preference for affecting the last bytes first.  Compute
> +      * where in the name we'll start applying the changes.
> +      */
> +     offset = name_len - (bit_to_flip[index][1].byte + 1);
> +     index -= bitseq;        /* Use later table entries first */
> +
> +     p0 = name + offset + bit_to_flip[index][0].byte;
> +     p1 = name + offset + bit_to_flip[index][1].byte;
> +     m0 = 1 << bit_to_flip[index][0].bit;
> +     m1 = 1 << bit_to_flip[index][1].bit;
> +
> +     /* Only change the bytes if it produces valid characters */
> +
> +     if (is_invalid_char(*p0 ^ m0) || is_invalid_char(*p1 ^ m1))
> +             return 0;
> +
> +     *p0 ^= m0;
> +     *p1 ^= m1;

This looks sane.

> +
> +     return 1;
> +}
> +
> +/*
> + * This function generates a well-defined sequence of "alternate"
> + * names for a given name.  An alternate is a name having the same
> + * length and same hash value as the original name.
> + *
> + * Each bit in the binary representation of the sequence number is
> + * used to select one possible "bit flip" operation to perform on
> + * the name.  So for example:
> + *    seq = 0:       selects no bits to flip
> + *    seq = 1:       selects the 0th bit to flip
> + *    seq = 2:       selects the 1st bit to flip
> + *    seq = 3:       selects the 0th and 1st bit to flip
> + *    ... and so on.
> + *
> + * The flip_bit() function takes care of the details of the bit
> + * flipping within the name.   The "1st bit" in this context is a
> + * bit sequence number; i.e. it doesn't necessarily mean bit 0x02
> + * will be changed.
> + *
> + * If a valid name is produced by this process for the given
> + * sequence number, this function returns 1.  If the result is not
> + * valid, it returns 0.  Returns -1 if the sequence number is beyond
> + * the the maximum for names of the given length.
> + *
> + */
> +static int
> +find_alternate(
> +     size_t          name_len,
> +     uchar_t         *name,
> +     uint32_t        seq)
> +{
> +     uint32_t        bitseq = 0;
> +     uint32_t        bits = seq;
> +
> +     if (!seq)
> +             return 1;       /* alternate 0 is the original name */
> +     if (name_len < 2)       /* Must have 2 bytes to flip */
> +             return -1;
> +
> +     for (bitseq = 0; bits; bitseq++) {
> +             uint32_t        mask = 1 << bitseq;
> +             int             fb;
> +
> +             if (!(bits & mask))
> +                     continue;
> +
> +             fb = flip_bit(name_len, name, bitseq);
> +             if (fb < 1)
> +                     return fb ? -1 : 0;
> +             bits ^= mask;

Ok, so this means we get up to ARRAY_SIZE(bit_to_flip) alternate
names if the name is long enough as the sequence number increases.
If we want more alternates, we need to expand the bit_to_flip()
array above.

However: this is damn hard to understand and I've had to calculate
this long hand to work out what perturbations it actually results
in. As such, it's taken me about 4 hours to understand the mechanics
of the patch well enough to write the above three line comment -
there's no way I'm going to remember how this works in a months
time, let alone when I look at it in a couple of years.

So, is this really any better/more reliable than just using random
characters to generate the alternate? Random characters result in a
far simpler and easier to verify algorithm, so if there's no
practical advantage to this algorithm over random characters (i.e.
how often do random character alternates actually fail?), then
I'd say it's just too complex to bother with. If the simple
solution is good enough, then lets just use the simple solution.

> -/*
> - * Given a name and its hash value, massage the name in such a way
> - * that the result is another name of equal length which shares the
> - * same hash value.
> - */
>  static void
>  obfuscate_name(

The comment should be kept.

>       xfs_dahash_t    hash,
> @@ -593,6 +777,8 @@ generate_obfuscated_name(
>       if (*name == '/')
>               name++;         /* Skip over leading slash (if any). */
>  
> +     /* Obfuscate the name (if possible) */
> +
>       hash = libxfs_da_hashname(name, namelen);
>       obfuscate_name(hash, namelen, name);

And that belongs in a previous patch, I think.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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