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Message-ID: <CAJgzZoo4Xd67UVnTGNtBTD8y-tVMGkjKYb0vRgNFczu4e9T9ZA@mail.gmail.com>
Date: Thu, 27 Jun 2024 17:31:03 -0400
From: enh <enh@...gle.com>
To: libc-coord@...ts.openwall.com, Zijun Zhao <zijunzhao@...gle.com>
Subject: aligned_realloc()
zijunzhao is working on adding _FORTIFY_SOURCE=3 to bionic, and
hitting the same malloc_usable_size() issues that i think glibc did.
interestingly, there seem to be two main use cases: (a) keeping
statistics about native heap allocations [where there's really no
workaround, and we just need to build that file with
_FORTIFY_SOURCE=2, because the functions for reporting heap usage will
include _all_ allocations, not just the ones they're trying to track]
and (b) implementing an "aligned_realloc()".
C23 doesn't say anything about alignment when describing realloc(). it
might be nice to assume that "The realloc function deallocates the old
object pointed to by ptr and returns a pointer to a new
object that has the size specified by size" actually means "The
realloc function deallocates the old object pointed to by ptr and
returns a pointer to a new object that has the size specified by size,
and the same alignment that the old object had", since that's
presumably what anyone making the call means, but i don't even know
what our allocator does.
i see MS has an _aligned_realloc(), allegedly because their
malloc()/free() allocator couldn't support user-requested alignment,
so they added a whole parallel set of functions [with a leading
underscore] instead of C11's aligned_realloc().
has anyone noticed this before?
there are some pretty unfortunate limitations in all the attempts to
implement this outside of libc that i've seen. (although i haven't
seen anyone go this route yet, i suspect an mmap()/mremap()
implementation would be the least worst ... except it would be
Linux-only.)
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