Trait wiggle::GuestMemory
source · pub unsafe trait GuestMemory: Send + Sync {
// Required methods
fn base(&self) -> &[UnsafeCell<u8>];
fn has_outstanding_borrows(&self) -> bool;
fn is_mut_borrowed(&self, r: Region) -> bool;
fn is_shared_borrowed(&self, r: Region) -> bool;
fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
fn mut_unborrow(&self, h: BorrowHandle);
fn shared_unborrow(&self, h: BorrowHandle);
// Provided methods
fn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
where Self: Sized,
T: ?Sized + Pointee { ... }
fn is_shared_memory(&self) -> bool { ... }
}Expand description
A trait which abstracts how to get at the region of host memory that contains guest memory.
All GuestPtr types will contain a handle to this trait, signifying where
the pointer is actually pointing into. This type will need to be implemented
for the host’s memory storage object.
§Safety
Safety around this type is tricky, and the trait is unsafe since there are
a few contracts you need to uphold to implement this type correctly and have
everything else in this crate work out safely.
The most important method of this trait is the base method. This returns,
in host memory, a pointer and a length. The pointer should point to valid
memory for the guest to read/write for the length contiguous bytes
afterwards.
The region returned by base must not only be valid, however, but it must
be valid for “a period of time before the guest is reentered”. This isn’t
exactly well defined but the general idea is that GuestMemory is allowed
to change under our feet to accommodate instructions like memory.grow or
other guest modifications. Memory, however, cannot be changed if the guest
is not reentered or if no explicitly action is taken to modify the guest
memory.
This provides the guarantee that host pointers based on the return value of
base have a dynamic period for which they are valid. This time duration
must be “somehow nonzero in length” to allow users of GuestMemory and
GuestPtr to safely read and write interior data.
This type also provides methods for run-time borrow checking of references
into the memory. The safety of this mechanism depends on there being exactly
one associated tracking of borrows for a given WebAssembly memory. There
must be no other reads or writes of WebAssembly the memory by either Rust or
WebAssembly code while there are any outstanding borrows, as given by
GuestMemory::has_outstanding_borrows().
§Using References
The GuestPtr::as_slice or GuestPtr::as_str will return smart
pointers GuestSlice and GuestStr. These types, which implement
std::ops::Deref and std::ops::DerefMut, provide mutable references
into the memory region given by a GuestMemory.
These smart pointers are dynamically borrow-checked by the borrow checker
methods on this trait. While a GuestSlice or a GuestStr are live, the
GuestMemory::has_outstanding_borrows() method will always return true.
If you need to re-enter the guest or otherwise read or write to the contents
of a WebAssembly memory, all GuestSlices and GuestStrs for the memory
must be dropped, at which point GuestMemory::has_outstanding_borrows()
will return false.
Required Methods§
sourcefn base(&self) -> &[UnsafeCell<u8>]
fn base(&self) -> &[UnsafeCell<u8>]
Returns the base allocation of this guest memory, located in host memory.
A pointer/length pair are returned to signify where the guest memory lives in the host, and how many contiguous bytes the memory is valid for after the returned pointer.
Note that there are safety guarantees about this method that
implementations must uphold, and for more details see the
GuestMemory documentation.
sourcefn has_outstanding_borrows(&self) -> bool
fn has_outstanding_borrows(&self) -> bool
Indicates whether any outstanding borrows are known to the
GuestMemory. This function must be false in order for it to be
safe to recursively call into a WebAssembly module, or to manipulate
the WebAssembly memory by any other means.
sourcefn is_mut_borrowed(&self, r: Region) -> bool
fn is_mut_borrowed(&self, r: Region) -> bool
Check if a region of linear memory is exclusively borrowed. This is called during any
GuestPtr::read or GuestPtr::write operation to ensure that wiggle is not reading or
writing a region of memory which Rust believes it has exclusive access to.
Check if a region of linear memory has any shared borrows.
sourcefn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>
fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>
Exclusively borrow a region of linear memory. This is used when constructing a
GuestSliceMut or GuestStrMut. Those types will give Rust &mut access
to the region of linear memory, therefore, the GuestMemory impl must
guarantee that at most one BorrowHandle is issued to a given region,
GuestMemory::has_outstanding_borrows is true for the duration of the
borrow, and that GuestMemory::is_mut_borrowed of any overlapping region
is false for the duration of the borrow.
Shared borrow a region of linear memory. This is used when constructing a
GuestSlice or GuestStr. Those types will give Rust & (shared reference) access
to the region of linear memory.
sourcefn mut_unborrow(&self, h: BorrowHandle)
fn mut_unborrow(&self, h: BorrowHandle)
Unborrow a previously borrowed mutable region. As long as GuestSliceMut and
GuestStrMut are implemented correctly, a mut BorrowHandle should only be
unborrowed once.
Unborrow a previously borrowed shared region. As long as GuestSlice and
GuestStr are implemented correctly, a shared BorrowHandle should only be
unborrowed once.
Provided Methods§
sourcefn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
fn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
Convenience method for creating a GuestPtr at a particular offset.
Note that T can be almost any type, and typically offset is a u32.
The exception is slices and strings, in which case offset is a (u32, u32) of (offset, length).
Check if the underlying memory is shared across multiple threads; e.g., with a WebAssembly shared memory.