On Tue, May 28, 2024 at 02:19:51PM +0200, Jason A. Donenfeld wrote: > +/** > + * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom(). > + * > + * @num: On input, a pointer to a suggested hint of how many states to > + * allocate, and on return the number of states actually allocated. > + * > + * @size_per_each: On input, must be zero. On return, the size of each state allocated, > + * so that the caller can split up the returned allocation into > + * individual states. > + * > + * @addr: Reserved, must be zero. > + * > + * @flags: Reserved, must be zero. > + * > + * The getrandom() vDSO function in userspace requires an opaque state, which > + * this function allocates by mapping a certain number of special pages into > + * the calling process. It takes a hint as to the number of opaque states > + * desired, and provides the caller with the number of opaque states actually > + * allocated, the size of each one in bytes, and the address of the first > + * state, which may be split up into @num states of @size_per_each bytes each, > + * by adding @size_per_each to the returned first state @num times, while > + * ensuring that no single state straddles a page boundary. > + * > + * Returns the address of the first state in the allocation on success, or a > + * negative error value on failure. > + * > + * The returned address of the first state may be passed to munmap(2) with a > + * length of `(size_t)num * (size_t)size_per_each`, in order to deallocate the > + * memory, after which it is invalid to pass it to vDSO getrandom(). Wouldn't a munmap with '(size_t)num * (size_t)size_per_each' be potentially too short, due to how the allocation is sized such that states don't cross page boundaries? - Eric
