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document -Zmiri-symbolic-alignment-check

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Ralf Jung 2020-08-16 17:12:32 +02:00
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README.md
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@ -189,6 +189,17 @@ Miri adds its own set of `-Z` flags:
entropy. The default seed is 0. **NOTE**: This entropy is not good enough
for cryptographic use! Do not generate secret keys in Miri or perform other
kinds of cryptographic operations that rely on proper random numbers.
* `-Zmiri-symbolic-alignment-check` makes the alignment check more strict. By
default, alignment is checked by casting the pointer to an integer, and making
sure that is a multiple of the alignment. This can lead to cases where a
program passes the alignment check by pure chance, because things "happened to
be" sufficiently aligned. To avoid such cases, the symbolic alignment check
only takes into account the requested alignment of the relevant allocation,
and the offset into that allocation. This avoids such false negatives, but it
also incurs some false positives when the code does manual integer arithmetic
to ensure alignment. (The standard library `align_to` method works fine in
both modes; under symbolic alignment it only fills the middle slice when the
allocation guarantees sufficient alignment.)
* `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
being allocated or freed. This helps in debugging memory leaks and
use after free bugs.