BUG/MEDIUM: buffers: Fix how input/output data are injected into buffers
The function buffer_contig_space is buggy and could lead to pernicious bugs
(never hitted until now, AFAIK). This function should return the number of bytes
that can be written into the buffer at once (without wrapping).
First, this function is used to inject input data (bi_putblk) and to inject
output data (bo_putblk and bo_inject). But there is no context. So it cannot
decide where contiguous space should placed. For input data, it should be after
bi_end(buf) (ie, buf->p + buf->i modulo wrapping calculation). For output data,
it should be after bo_end(buf) (ie, buf->p) and input data are assumed to not
exist (else there is no space at all).
Then, considering we need to inject input data, this function does not always
returns the right value. And when we need to inject output data, we must be sure
to have no input data at all (buf->i == 0), else the result can also be wrong
(but this is the caller responsibility, so everything should be fine here).
The buffer can be in 3 different states:
1) no wrapping
<---- o ----><----- i ----->
+------------+------------+-------------+------------+
| |oooooooooooo|iiiiiiiiiiiii|xxxxxxxxxxxx|
+------------+------------+-------------+------------+
^ <contig_space>
p ^ ^
l r
2) input wrapping
...---> <---- o ----><-------- i -------...
+-----+------------+------------+--------------------+
|iiiii|xxxxxxxxxxxx|oooooooooooo|iiiiiiiiiiiiiiiiiiii|
+-----+------------+------------+--------------------+
<contig_space> ^
^ ^ p
l r
3) output wrapping
...------ o ------><----- i -----> <----...
+------------------+-------------+------------+------+
|oooooooooooooooooo|iiiiiiiiiiiii|xxxxxxxxxxxx|oooooo|
+------------------+-------------+------------+------+
^ <contig_space>
p ^ ^
l r
buffer_contig_space returns (l - r). The cases 1 and 3 are correctly
handled. But for the second case, r is wrong. It points on the buffer's end
(buf->data + buf->size). It should be bo_end(buf) (ie, buf->p - buf->o).
To fix the bug, the function has been splitted. Now, bi_contig_space and
bo_contig_space should be used to know the contiguous space available to insert,
respectively, input data and output data. For bo_contig_space, input data are
assumed to not exist. And the right version is used, depending what we want to
do.
In addition, to clarify the buffer's API, buffer_realign does not return value
anymore. So it has the same API than buffer_slow_realign.
This patch can be backported in 1.7, 1.6 and 1.5.
2 files changed