The same change was already performed for the cli. The stats applet and the
prometheus exporter are also concerned. Both use the stats API and rely on
pool functions to get total pool usage in bytes. pool_total_allocated() and
pool_total_used() must return 64 bits unsigned integer to avoid any wrapping
around 4G.
This may be backported to all versions.
Till now it was only possible to change the thread local hot cache size
at build time using CONFIG_HAP_POOL_CACHE_SIZE. But along benchmarks it
was sometimes noticed a huge contention in the lower level memory
allocators indicating that larger caches could be beneficial, especially
on machines with large L2 CPUs.
Given that the checks against this value was no longer on a hot path
anymore, there was no reason for continuing to force it to be tuned at
build time. So this patch allows to set it by tune.memory-hot-size.
It's worth noting that during the boot phase the value remains zero so
that it's possible to know if the value was set or not, which opens the
possibility that we try to automatically adjust it based on the per-cpu
L2 cache size or the use of certain protocols (none of this is done yet).
Since the massive pools cleanup that happened in 2.6, the pools
architecture was made quite more hierarchical and many alternate code
blocks could be moved to runtime flags set by -dM. One of them had not
been converted by then, DEBUG_UAF. It's not much more difficult actually,
since it only acts on a pair of functions indirection on the slow path
(OS-level allocator) and a default setting for the cache activation.
This patch adds the "uaf" setting to the options permitted in -dM so
that it now becomes possible to set or unset UAF at boot time without
recompiling. This is particularly convenient, because every 3 months on
average, developers ask a user to recompile haproxy with DEBUG_UAF to
understand a bug. Now it will not be needed anymore, instead the user
will only have to disable pools and enable uaf using -dMuaf. Note that
-dMuaf only disables previously enabled pools, but it remains possible
to re-enable caching by specifying the cache after, like -dMuaf,cache.
A few tests with this mode show that it can be an interesting combination
which catches significantly less UAF but will do so with much less
overhead, so it might be compatible with some high-traffic deployments.
The change is very small and isolated. It could be helpful to backport
this at least to 2.7 once confirmed not to cause build issues on exotic
systems, and even to 2.6 a bit later as this has proven to be useful
over time, and could be even more if it did not require a rebuild. If
a backport is desired, the following patches are needed as well:
CLEANUP: pools: move the write before free to the uaf-only function
CLEANUP: pool: only include pool-os from pool.c not pool.h
REORG: pool: move all the OS specific code to pool-os.h
CLEANUP: pools: get rid of CONFIG_HAP_POOLS
DEBUG: pool: show a few examples in -dMhelp
The poisonning performed on pool_free() used to help a little bit with
use-after-free detection, but usually did more harm than good in that
it was never possible to perform post-mortem analysis on released
objects once poisonning was enabled on allocation. Now that there is
a dedicated DEBUG_POOL_INTEGRITY, let's get rid of this annoyance
which is not even documented in the management manual.
This new option, when set, will cause the callers of pool_alloc() and
pool_free() to be recorded into an extra area in the pool that is expected
to be helpful for later inspection (e.g. in core dumps). For example it
may help figure that an object was released to a pool with some sub-fields
not yet released or that a use-after-free happened after releasing it,
with an immediate indication about the exact line of code that released
it (possibly an error path).
This only works with the per-thread cache, and even objects refilled from
the shared pool directly into the thread-local cache will have a NULL
there. That's not an issue since these objects have not yet been freed.
It's worth noting that pool_alloc_nocache() continues not to set any
caller pointer (e.g. when the cache is empty) because that would require
a possibly undesirable API change.
The extra cost is minimal (one pointer per object) and this completes
well with DEBUG_POOL_INTEGRITY.
When enabled, objects picked from the cache are checked for corruption
by comparing their contents against a pattern that was placed when they
were inserted into the cache. Objects are also allocated in the reverse
order, from the oldest one to the most recent, so as to maximize the
ability to detect such a corruption. The goal is to detect writes after
free (or possibly hardware memory corruptions). Contrary to DEBUG_UAF
this cannot detect reads after free, but may possibly detect later
corruptions and will not consume extra memory. The CPU usage will
increase a bit due to the cost of filling/checking the area and for the
preference for cold cache instead of hot cache, though not as much as
with DEBUG_UAF. This option is meant to be usable in production.
The purpose here is to explain how memory pools work, what their
architecture is depending on the build options (4 possible combinations),
and how the various build options affect their behavior.
Two pool-specific macros that were previously documented in initcalls
were moved to pools.txt.
