Merge branch 'master' into ipt-udp-matchers

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+This package defines primitives for sentry access to application memory.
+
+Major types:
+
+-   The `IO` interface represents a virtual address space and provides I/O
+    methods on that address space. `IO` is the lowest-level primitive. The
+    primary implementation of the `IO` interface is `mm.MemoryManager`.
+
+-   `IOSequence` represents a collection of individually-contiguous address
+    ranges in a `IO` that is operated on sequentially, analogous to Linux's
+    `struct iov_iter`.
+
+Major usage patterns:
+
+-   Access to a task's virtual memory, subject to the application's memory
+    protections and while running on that task's goroutine, from a context that
+    is at or above the level of the `kernel` package (e.g. most syscall
+    implementations in `syscalls/linux`); use the `kernel.Task.Copy*` wrappers
+    defined in `kernel/task_usermem.go`.
+
+-   Access to a task's virtual memory, from a context that is at or above the
+    level of the `kernel` package, but where any of the above constraints does
+    not hold (e.g. `PTRACE_POKEDATA`, which ignores application memory
+    protections); obtain the task's `mm.MemoryManager` by calling
+    `kernel.Task.MemoryManager`, and call its `IO` methods directly.
+
+-   Access to a task's virtual memory, from a context that is below the level of
+    the `kernel` package (e.g. filesystem I/O); clients must pass I/O arguments
+    from higher layers, usually in the form of an `IOSequence`. The
+    `kernel.Task.SingleIOSequence` and `kernel.Task.IovecsIOSequence` functions
+    in `kernel/task_usermem.go` are convenience functions for doing so.