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[RFC] Detecting malloc errors.
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: libc-alpha at sourceware dot org
- Date: Fri, 13 Dec 2013 15:44:30 +0100
- Subject: [RFC] Detecting malloc errors.
- Authentication-results: sourceware.org; auth=none
As I looked through various malloc sanity checks and with bit of effort
we could use much stronger checks with similar slowdown. A main problem
here is that a data layout needs to be changed. Then these default checks
would replace what mcheck does.
For a 64bit systems we do not need most of time 64bits to describe a
chunk size so we can get with 32bit sizes that allow us start with 64bit
sentinel. If a more is needed then we will use a different sentinel
value followed by pointer to actual chunk header.
We need to move flags to prev_size if we want to detect off-by-one
writes. These will be done by writing a rotated sentinel after user
data, which allows us to also use next chunk's sentinel (unless its
unlikely case of a custom sentinel.)
For a 32bit systems we need to adjust sizes accordingly.
A orthogonal improvement is detecting double frees by having a flag that
memory is in use and different flag that memory is freed (and chunks in
fastbins have both bits set.)
uint32_t prev_size; /* Size of previous chunk (if free). */
uint32_t size; /* Size in bytes as requested. */
struct malloc_chunk* fd; /* double links -- used only if free. */
struct malloc_chunk* bk;
/* Only used for large blocks: pointer to next larger size. */
struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */
struct malloc_chunk* bk_nextsize