Wietse Venema reported in the thread below that we have a signedness
issue with our hashes implementations: due to the use of const char*
for the input key that's often text, the crc32, sdbm, djb2, and wt6
algorithms return a platform-dependent value for binary input keys
containing bytes with bit 7 set. This means that an ARM or PPC
platform will hash binary inputs differently from an x86 typically.
Worse, some algorithms are well defined in the industry (like CRC32)
and do not provide the expected result on x86, possibly causing
interoperability issues (e.g. a user-agent would fail to compare the
CRC32 of a message body against the one computed by haproxy).
Fortunately, and contrary to the first impression, the CRC32c variant
used in the PROXY protocol processing is not affected. Thus the impact
remains very limited (the vast majority of input keys are text-based,
such as user-agent headers for exmaple).
This patch addresses the issue by fixing all hash functions' prototypes
(even those not affected, for API consistency). A reg test will follow
in another patch.
The vast majority of users do not use these hashes. And among those
using them, very few will pass them on binary inputs. However, for the
rare ones doing it, this fix MAY have an impact during the upgrade. For
example if the package is upgraded on one LB then on another one, and
the CRC32 of a binary input is used as a stick table key (why?) then
these CRCs will not match between both nodes. Similarly, if
"hash-type ... crc32" is used, LB inconsistency may appear during the
transition. For this reason it is preferable to apply the patch on all
nodes using such hashes at the same time. Systems upgraded via their
distros will likely observe the least impact since they're expected to
be upgraded within a short time frame.
And it is important for distros NOT to skip this fix, in order to avoid
distributing an incompatible implementation of a hash. This is the
reason why this patch is tagged as MAJOR, eventhough it's extremely
unlikely that anyone will ever notice a change at all.
This patch must be backported to all supported branches since the
hashes were introduced in 1.5-dev20 (commit 98634f0c). Some parts
may be dropped since implemented later.
Link to Wietse's report:
https://marc.info/?l=postfix-users&m=157879464518535&w=2
This function will be used to perform CRC32 computations. This one wa
loosely inspired from crc32b found here, and focuses on size and speed
at the same time :
http://www.hackersdelight.org/hdcodetxt/crc.c.txt
Much faster table-based versions exist but are pointless for our usage
here, this hash already sustains gigabit speed which is far faster than
what we'd ever need. Better preserve the CPU's cache instead.
When we were generating a hash, it was done using an unsigned long. When the hash was used
to select a backend, it was sent as an unsigned int. This made it difficult to predict which
backend would be selected.
This patch updates get_hash, and the hash methods to use an unsigned int, to remain consistent
throughout the codebase.
This fix should be backported to 1.5 and probably in part to 1.4.
This function was designed for haproxy while testing other functions
in the past. Initially it was not planned to be used given the not
very interesting numbers it showed on real URL data : it is not as
smooth as the other ones. But later tests showed that the other ones
are extremely sensible to the server count and the type of input data,
especially DJB2 which must not be used on numeric input. So in fact
this function is still a generally average performer and it can make
sense to merge it in the end, as it can provide an alternative to
sdbm+avalanche or djb2+avalanche for consistent hashing or when hashing
on numeric data such as a source IP address or a visitor identifier in
a URL parameter.
Summary:
In testing at tumblr, we found that using djb2 hashing instead of the
default sdbm hashing resulted is better workload distribution to our backends.
This commit implements a change, that allows the user to specify the hash
function they want to use. It does not limit itself to consistent hashing
scenarios.
The supported hash functions are sdbm (default), and djb2.
For a discussion of the feature and analysis, see mailing list thread
"Consistent hashing alternative to sdbm" :
http://marc.info/?l=haproxy&m=138213693909219
Note: This change does NOT make changes to new features, for instance,
applying an avalance hashing always being performed before applying
consistent hashing.
