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[NFC][asan] Cleanup comments 

D132090 is too verbose.
Don't explain obvious.
Add simpler explanation of rounding reasoning.

Differential Revision: https://reviews.llvm.org/D138773
This commit is contained in:
Vitaly Buka 2022-11-27 22:48:58 -08:00
parent 2b026fedca
commit cc27860165
1 changed files with 14 additions and 39 deletions
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53
compiler-rt/lib/asan/asan_poisoning.cpp
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@ -539,65 +539,40 @@ void __sanitizer_annotate_double_ended_contiguous_container(
CHECK_LT(new_beg, old_end);
CHECK_GT(new_end, old_beg);
// There are two situations: we are poisoning or unpoisoning.
// WARNING: at the moment we do not poison prefixes of blocks described by
// one byte in shadow memory, so we have to unpoison prefixes of blocks with
// content. Up to (granularity - 1) bytes not-in-use may not be poisoned.
if (new_beg < old_beg) {
// We are unpoisoning.
// Round down because we can't poison prefixes.
uptr a = RoundDownTo(new_beg, granularity);
// Round down and ignore the [c, old_beg) as its state defined by unchanged
// [old_beg, old_end).
uptr c = RoundDownTo(old_beg, granularity);
// State at the moment is:
// [storage_beg, a] is poisoned and should remain like that.
// [a, c] is poisoned as well (interval may be empty if new_beg
// and old_beg are in the same block). If the container is not
// empty, first element starts somewhere in [c, c+granularity]. Because we
// do not poison prefixes, memory [c, container_end] is not poisoned and
// we won't change it. If container is empty, we have to unpoison memory
// for elements after c, so [c, container_end]
PoisonShadow(a, c - a, 0);
} else if (new_beg > old_beg) {
// We are poisoning as beginning moved further in memory.
// Round down and poison [a, old_beg) because it was unpoisoned only as a
// prefix.
uptr a = RoundDownTo(old_beg, granularity);
// Round down and ignore the [c, new_beg) as its state defined by unchanged
// [new_beg, old_end).
uptr c = RoundDownTo(new_beg, granularity);
// State at the moment is:
// [storage_beg, a] is poisoned and should remain like that.
// [a, c] is not poisoned (interval may be empty if new_beg and
// old_beg are in the same block) [c, container_end] is not
// poisoned. If there are remaining elements in the container:
// We have to poison [a, c], but because we do not poison prefixes, we
// cannot poison memory after c (even that there are not elements of the
// container). Up to granularity-1 unused bytes will not be poisoned.
// Otherwise:
// We have to poison the last byte as well.
PoisonShadow(a, c - a, kAsanContiguousContainerOOBMagic);
}
if (new_end > old_end) {
// We are unpoisoning memory.
// Round down to poison the prefix.
uptr a = RoundDownTo(old_end, granularity);
// Round down and handle remainder below.
uptr c = RoundDownTo(new_end, granularity);
// State at the moment is:
// if container_beg < a : [container_beg, a] is correct and we will not be
// changing it. else [a, container_beg] cannot be poisoned, so we do not
// have to think about it. we have to makr as unpoisoned [a, c]. [c, end]
// is correctly poisoned.
PoisonShadow(a, c - a, 0);
if (!AddrIsAlignedByGranularity(new_end)) // ends in the middle of a block
if (!AddrIsAlignedByGranularity(new_end))
*(u8 *)MemToShadow(c) = static_cast<u8>(new_end - c);
} else if (new_end < old_end) {
// We are poisoning memory.
// State at the moment is:
// [storage_beg, a] is correctly annotated
// if container is empty after the removal, then a < container_beg and we
// will have to poison memory which is adressable only because we are not
// poisoning prefixes.
// Round up and handle remained below.
uptr a2 = RoundUpTo(new_end, granularity);
// Round up to poison entire granule as we had nothing in [old_end, c2).
uptr c2 = RoundUpTo(old_end, granularity);
PoisonShadow(a2, c2 - a2, kAsanContiguousContainerOOBMagic);
if (!AddrIsAlignedByGranularity(new_end)) {
// Starts in the middle of the block
uptr a = RoundDownTo(new_end, granularity);
*(u8 *)MemToShadow(a) = static_cast<u8>(new_end - a);
}