And do so by type, rather than by value. While here, introduce a C++20 concept
for this Backend-offered proxy and adjust the template parameters appropriately.
This will be useful for the process sandbox code, which needs to mediate stores
to the pagemap, but can provide a read-only view.
This is the set of changes required for snmalloc2 to be usable by the
process sandboxing code and incorporates some API changes that reduce
the amount of code required to embed snmalloc. Highlights:
- Merge the config and back-end classes.
- Everything in config is now global (all methods are static)
- The GlobalState class is gone (all global state is managed by global
methods on the config class)
- LocalState is now a member of the config class, all methods are
instance methods.
- Not every configuration needs to use the lazy initialisation hooks.
They now need to be provided only if they are used. If the
configuration does not provide an `ensure_init` method, it is not
called. If it does not provide an `is_initialised` method then the
global initialisation state is not checked.
- There is now an `snmalloc::Options` class that default initialises
itself to the default behaviour. Every configuration must provide a
`constexpr` instance of this class. Each flag can be separately
overridden and new flags can be added without breaking any existing
API consumers.
The config classes are moved into the backend directory.
* Make address space manager use pagemap for next pointers
The address space manager uses the pagemap entry to form linked lists of
unused address space above MIN_CHUNK_SIZE. It continues to use
references in the block below that threshold.
In the CHECK_CLIENT mode this makes it hard to corrupt the ASM as only
meta-data uses allocations below MIN_CHUNK_SIZE from the ASM. This
allocations will be protected with guard pages by the backend.
* address_space_core: use FreeChunk struct
Purely stylistic, NFCI. This hides some somewhat gnarly reinterpret_cast-s in
favor of more, but hopefully less gnarly, casts elsewhere.
* Apply suggestions from code review
Co-authored-by: Nathaniel Wesley Filardo <nfilardo@microsoft.com>
The meta-data in the CHECK_CLIENT mode is allocated from its own areas,
where most of the pages have been disabled, and the location within this
range is randomised.
This protects from trivial OOB read/write hitting the meta-data.
More complex controlled offsets are also mitigated due to the sparse
level of pages being turned on (i.e. not PROT_NONE).
The Pal was providing policy for overallocating a block of memory to
achieve alignment make that part of the backend.
The backend should be responsible for layout policy.
This PR exposes a pagemap interface to specify ranges that are being
used. The overall invariant is that any memory in the address space
manager has the pagemap committed. This means that individual operations
do not need to commit entries.
This is important for Windows that does not support lazy commit. It is
also important if we want to PROT_NONE most of the pagemap to reduce the
risk of memory safety issues getting access to the pagemap.
There are minor changes to test to pull memory directly from the Pal.
There are also bug fixes in the pagemap tests.
The pagemap allocates it self directly either from
* the original fixed address range it is supplied, and returns the
remaining space after the pagemap is removed; or
* directly allocated from the PAL without using the address space
manager.
This change in layering is required for the next commit, which imposes
the invariant that the pagemap has been committed for all spaced managed
by the address space manager.
# Pagemap
The Pagemap now stores all the meta-data for the object allocation. The meta-data in the pagemap is effectively a triple of the sizeclass, the remote allocator, and a pointer to a 64 byte block of meta-data for this chunk of memory. By storing the pointer to a block, it allows the pagemap to handle multiple slab sizes without branching on the fast path. There is one entry in the pagemap per 16KiB of address space, but by using the same entry in the pagemap for 4 adjacent entries, then we can treat a 64KiB range can be treated as a single slab of allocations.
This change also means there is almost no capability amplification required by the implementation on CHERI for finding meta-data. The only amplification is required, when we change the way a chunk is used to a size of object allocation.
# Backend
There is a second major aspect of the refactor that there is now a narrow API that abstracts the Pagemap, PAL and address space management. This should better enable the compartmentalisation and makes it easier to produce alternative backends for various research directions. This is a template parameter that can be used to specialised by the front-end in different ways.
# Thread local state
The thread local state has been refactored into two components, one (called 'localalloc') that is stored directly in the TLS and is constant initialised, and one that is allocated in the address space (called 'coreallloc') which is lazily created and pooled.
# Difference
This removes Superslabs/Medium slabs as there meta-data is now part of the pagemap.
