fuzz: split write operand using binary approach

fuzz: split write operand using binary approach

Currently, we split the write commands' data from the middle. If it does not
work, try to move the pivot left by one byte and retry until there is no
space.

But, this method has two flaws:

1. It may fail to trim all unnecessary bytes on the right side.

For example, there is an IO write command:

  write addr uuxxxxuu

u is the unnecessary byte for the crash. Unlike ram write commands, in most
case, a split IO write won't trigger the same crash, So if we split from the
middle, we will get:

  write addr uu (will be removed in next round)
  write addr xxxxuu

For xxxxuu, since split it from the middle and retry to the leftmost byte
won't get the same crash, we will be stopped from removing the last two
bytes.

2. The algorithm complexity is O(n) since we move the pivot byte by byte.

To solve the first issue, we can try a symmetrical position on the right if
we fail on the left. As for the second issue, instead moving by one byte, we
can approach the boundary exponentially, achieving O(log(n)).

Give an example:

                   xxxxuu len=6
                        +
                        |
                        +
                 xxx,xuu 6/2=3 fail
                        +
         +--------------+-------------+
         |                            |
         +                            +
  xx,xxuu 6/2^2=1 fail         xxxxu,u 6-1=5 success
                                 +   +
         +------------------+----+   |
         |                  |        +-------------+ u removed
         +                  +
   xx,xxu 5/2=2 fail  xxxx,u 6-2=4 success
                           +
                           |
                           +-----------+ u removed

In some rare cases, this algorithm will fail to trim all unnecessary bytes:

  xxxxxxxxxuxxxxxx
  xxxxxxxx-xuxxxxxx Fail
  xxxx-xxxxxuxxxxxx Fail
  xxxxxxxxxuxx-xxxx Fail
  ...

I think the trade-off is worth it.

Signed-off-by: Qiuhao Li <Qiuhao.Li@outlook.com>
Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <SYCPR01MB3502D26F1BEB680CBBC169E5FCAB0@SYCPR01MB3502.ausprd01.prod.outlook.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>