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Date: Tue, 20 Jul 2021 13:19:58 -0600
From: Christopher Hodgkins <>
Subject: Failing assertions in get_meta

Hi all,
I'm working on a modified version of musl for some research.
We replaced the typical memory provisioning methods (mmap/brk)
with our own versions which change the backing of the provided memory,
but maintain the same semantics from the perspective of the caller,
except that the program break is no longer adjacent to the rest of the
mapped binary.

We have been encountering various failed assertions in mallocng during
in the get_meta function. Specifically, we see the assertion at line 141 in
meta.h (meta == mem->base)
failing when free()-ing memory previously allocated with malloc(),
and the assertion at line 131 ( !((uintptr_t)p & 15) ) failing for calls to
with an alignment of 16 and a length of 704. Could our changes in the
backing of provisioned memory cause this?

Related question: We are aware of (and have disabled) the "donation"
functionality in the loader.
Are there other out-of-band (i.e. not mmap/sbrk) sources of memory used by
The errors are caused in some way by our changes, but we haven't changed
anything inside the allocator,
except to point the brk() macro to our own implementation rather than the
So I assume there must be either some memory getting used that we don't
know about,
or some error in our implementation.

Thanks for your assistance,

Build info:
Working from commit 1f0c7cb1cc2170bf230623dc0b57d9a9f001af08 on master
(HEAD at time of writing).
Installed at /usr in a Docker container running Alpine Linux 3.14 on x86_64.
Musl was compiled with --enable-debug and -O0. Our test programs are
compiled with normal gcc inside the
container after installing musl.

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