FROMLIST: rust: uaccess: add userspace pointers
A pointer to an area in userspace memory, which can be either read-only or read-write. All methods on this struct are safe: invalid pointers return `EFAULT`. Concurrent access, *including data races to/from userspace memory*, is permitted, because fundamentally another userspace thread/process could always be modifying memory at the same time (in the same way that userspace Rust's `std::io` permits data races with the contents of files on disk). In the presence of a race, the exact byte values read/written are unspecified but the operation is well-defined. Kernelspace code should validate its copy of data after completing a read, and not expect that multiple reads of the same address will return the same value. These APIs are designed to make it difficult to accidentally write TOCTOU bugs. Every time you read from a memory location, the pointer is advanced by the length so that you cannot use that reader to read the same memory location twice. Preventing double-fetches avoids TOCTOU bugs. This is accomplished by taking `self` by value to prevent obtaining multiple readers on a given `UserSlicePtr`, and the readers only permitting forward reads. If double-fetching a memory location is necessary for some reason, then that is done by creating multiple readers to the same memory location. Constructing a `UserSlicePtr` performs no checks on the provided address and length, it can safely be constructed inside a kernel thread with no current userspace process. Reads and writes wrap the kernel APIs `copy_from_user` and `copy_to_user`, which check the memory map of the current process and enforce that the address range is within the user range (no additional calls to `access_ok` are needed). This code is based on something that was originally written by Wedson on the old rust branch. It was modified by Alice by removing the `IoBufferReader` and `IoBufferWriter` traits, introducing the `MAX_USER_OP_LEN` constant, and various changes to the comments and documentation. Signed-off-by:Wedson Almeida Filho <wedsonaf@gmail.com> Co-developed-by:
Alice Ryhl <aliceryhl@google.com> Signed-off-by:
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