Use pointers, not addresses, in more places #188
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davidchisnall
approved these changes
May 16, 2020
mjp41
approved these changes
May 19, 2020
src/mem/alloc.h
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| while (size > 64) | ||
| { | ||
| // This is a large alloc redirect. | ||
| ss = ss - (1ULL << (size - 64)); | ||
| ss = pointer_offset_signed(ss, -(1 << (size - 64))); |
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I'm always worried about lines like this, and that the shift will occur not at 64bits, because it decides 1 is 32bit, and then this doesn't behave as expected.
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Annoyingly, when I use 1LL, my host compiler barfs:
/cheri/source/mainline/snmalloc/src/mem/alloc.h:431:40: error: implicit conversion changes signedness: 'long long' to 'ptrdiff_t' (aka 'long') [-Werror,-Wsign-conversion]
which just seems wrong to me? Anyway, I've worked around it by doing static_cast<ptrdiff_t>(1) which should make everyone happy on all platforms, yes?
Just always work with pointers using the functions defined in ds/address.h. This more obviously preserves provenance through the chain of reasoning. Note that there is still risk of malloc() being used as an amplification oracle on CHERI, but there's no additional risk from this change. Rename the external_address into external_pointer.
Since we anticipate address_t not carrying provenance on CHERI, but rather being vaddr_t there, it doesn't make sense to offer conversion back to a provenance-carrying pointer. Thankfully, there is not much to be done here: the uses were few and could be replaced with the vocabulary of other pointer operations in ds/address.h
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This should produce equivalent code everywhere and more obviously preserves provenance where desired. Ultimately, eliminate
pointer_castentirely since it won't make sense to cast an address back to a pointer on CHERI platforms.