8eacda58b6
commit fd0c42c4dea54335967c5a86f15fc064235a2797 upstream.
and change to pseudorandom numbers, as this is a traffic dithering
operation that doesn't need crypto-grade.
The previous code operated in 4 steps:
1. Generate a random byte 0 <= rand_tq <= 255
2. Multiply it by BATADV_TQ_MAX_VALUE - tq
3. Divide by 255 (= BATADV_TQ_MAX_VALUE)
4. Return BATADV_TQ_MAX_VALUE - rand_tq
This would apperar to scale (BATADV_TQ_MAX_VALUE - tq) by a random
value between 0/255 and 255/255.
But! The intermediate value between steps 3 and 4 is stored in a u8
variable. So it's truncated, and most of the time, is less than 255, after
which the division produces 0. Specifically, if tq is odd, the product is
always even, and can never be 255. If tq is even, there's exactly one
random byte value that will produce a product byte of 255.
Thus, the return value is 255 (511/512 of the time) or 254 (1/512
of the time).
If we assume that the truncation is a bug, and the code is meant to scale
the input, a simpler way of looking at it is that it's returning a random
value between tq and BATADV_TQ_MAX_VALUE, inclusive.
Well, we have an optimized function for doing just that.
Fixes: 3c12de9a5c75 ("batman-adv: network coding - code and transmit packets if possible")
Signed-off-by: George Spelvin <lkml@sdf.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.