[−][src]Trait wiggle::GuestMemory

pub unsafe trait GuestMemory {
    fn base(&self) -> (*mut u8, u32);
    fn borrow_checker(&self) -> &BorrowChecker;

    fn validate_size_align(
        &self, 
        offset: u32, 
        align: usize, 
        len: u32
    ) -> Result<*mut u8, GuestError> { ... }
    fn ptr<'a, T: ?Sized>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
    where
        Self: Sized,
        T: Pointee,
    { ... }
}

A trait which abstracts how to get at the region of host memory taht 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 accomodate 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.

Using References

See the safety guarantees of BorrowChecker, which asserts that exactly one BorrowChecker may be constructed for each WebAssembly memory.

The [GuestMemory::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 BorrowChecker given by GuestMemory::borrow_checker(). While a GuestSlice or a GuestStr are live, the BorrowChecker::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 BorrowChecker::has_outstanding_borrows() will return false.

Required methods

`fn base(&self) -> (*mut u8, u32)`

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.

`fn borrow_checker(&self) -> &BorrowChecker`

Gives a reference to the BorrowChecker used to keep track of each outstanding borrow of the memory region. BorrowChecker::new safety rules require that exactly one checker exist for each memory region.

Loading content...

Provided methods

`fn validate_size_align( &self, offset: u32, align: usize, len: u32 ) -> Result<*mut u8, GuestError>`

Validates a guest-relative pointer given various attributes, and returns the corresponding host pointer.

offset - this is the guest-relative pointer, an offset from the base.
align - this is the desired alignment of the guest pointer, and if successful the host pointer will be guaranteed to have this alignment.
len - this is the number of bytes, after offset, that the returned pointer must be valid for.

This function will guarantee that the returned pointer is in-bounds of base, at this time, for len bytes and has alignment align. If any guarantees are not upheld then an error will be returned.

Note that the returned pointer is an unsafe pointer. This is not safe to use in general because guest memory can be relocated. Additionally the guest may be modifying/reading memory as well. Consult the GuestMemory documentation for safety information about using this returned pointer.

`fn ptr<'a, T: ?Sized>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T> where Self: Sized, T: Pointee,`

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).

Loading content...