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Message-ID: <20191128215642.GN16318@brightrain.aerifal.cx>
Date: Thu, 28 Nov 2019 16:56:42 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: New malloc - first preview
Work on the new malloc is well underway, and I have a draft version
now public at:
https://github.com/richfelker/mallocng-draft
Some highlights:
- Smallest size-class now has 12 of 16 bytes usable for storage,
compared to 8 of 16 (32-bit archs) or 16 of 32 (64-bit archs) with
the old malloc, plus 1.5 bytes (32-bit) or 2.5 bytes (64-bit) of
out-of-band metadata. This overhead (5.5 or 6.5 bytes per
allocation) is uniform across all sizes.
- Small-size allocation granularity/alignment is now 16-byte on both
32-bit and 64-bit archs. No more wasting 64 bytes for a 36-byte
structure.
- Metadata defining heap state is all out-of-band and protected below
by guard pages, where it's not easily reachable through typical
attack vectors.
- Freed memory is quickly returnable to the kernel.
- Common path malloc and free (asymptotically should be ~95% of calls
under high use) do not require exclusive locks -- readlock+atomic
for malloc, pure-atomic for free.
- Size classes are not straight geometric progression but instead 1/7,
1/6, and 1/5 of powers of two in between the whole power-of-two
steps, optimizing fit in whole power-of-two numbers of pages.
- Code size is very close to the same as the old allocator, and may
end up being smaller.
This is still early in development, and it won't be merged in musl
until the next release cycle, after the new year. There may be plenty
of bugs or suboptimal behavior. And I still need to write up the
design.
For anyone wanting to play around with the draft, it can be linked or
used by LD_PRELOAD. A dump_heap function is provided which can show
allocator state. In the status masks, a=available and f=freed; these
are distinct states to allow some control over interval to reuse of
freed memory (which gives increased safety against UAF/DF). The "_"
status is in-use.
Rich
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