BUG/MEDIUM: ebtree: use a byte-per-byte memcmp() to compare memory blocks
As reported in issue #689, there is a subtle bug in the ebtree code used
to compared memory blocks. It stems from the platform-dependent memcmp()
implementation. Original implementations used to perform a byte-per-byte
comparison and to stop at the first non-matching byte, as in this old
example:
https://www.retro11.de/ouxr/211bsd/usr/src/lib/libc/compat-sys5/memcmp.c.html
The ebtree code has been relying on this to detect the first non-matching
byte when comparing keys. This is made so that a zero-terminated string
can fail to match against a longer string.
Over time, especially with large busses and SIMD instruction sets,
multi-byte comparisons have appeared, making the processor fetch bytes
past the first different byte, which could possibly be a trailing zero.
This means that it's possible to read past the allocated area for a
string if it was allocated by strdup().
This is not correct and definitely confuses address sanitizers. In real
life the problem doesn't have visible consequences. Indeed, multi-byte
comparisons are implemented so that aligned words are loaded (e.g. 512
bits at once to process a cache line at a time). So there is no way such
a multi-byte access will cross a page boundary and end up reading from
an unallocated zone. This is why it was never noticed before.
This patch addresses this by implementing a one-byte-at-a-time memcmp()
variant for ebtree, called eb_memcmp(). It's optimized for both small and
long strings and guarantees to stop after the first non-matching byte. It
only needs 5 instructions in the loop and was measured to be 3.2 times
faster than the glibc's AVX2-optimized memcmp() on short strings (1 to
257 bytes), since that latter one comes with a significant setup cost.
The break-even seems to be at 512 bytes where both version perform
equally, which is way longer than what's used in general here.
This fix should be backported to stable versions and reintegrated into
the ebtree code.
4 files changed