form_userptr - Linux

Overview

form_userptr provides a way to notify the kernel about a userspace pointer that it can use whenever it needs to refer to the object associated with the pointer. This is useful for passing a pointer to a shared memory region or a character device to the kernel.

Syntax

form_userptr <pointer> <size> <flags>

Where:

  • <pointer> is the userspace pointer to be notified to the kernel.
  • <size> is the size of the object associated with the pointer.
  • <flags> are flags that control the behavior of the command.

Options/Flags

  • -c, –cache: Forces the kernel to cache the pointer. This can improve performance if the pointer is frequently accessed.
  • -u, –uncached: Forces the kernel to not cache the pointer. This can improve security but can also decrease performance.
  • -t, –temp: Makes the kernel automatically release the pointer when the process exits. This is the default behavior.
  • -p, –permanent: Makes the kernel hold onto the pointer until it is explicitly released using the form_unuserptr command.

Examples

To notify the kernel about a pointer to a shared memory region:

form_userptr --cache 0x12345678 4096

To notify the kernel about a pointer to a character device:

form_userptr --uncached 0x87654321 1024

Common Issues

One common issue is that the kernel may not be able to access the object associated with the pointer if the pointer is invalid. This can happen if the pointer is pointing to a memory region that has been freed or if the pointer is pointing to a character device that has been closed.

Another common issue is that the kernel may not be able to release the pointer if the process exits unexpectedly. This can happen if the process crashes or if the user kills the process.

Integration

form_userptr can be combined with other Linux commands and tools to perform advanced tasks. For example, it can be used with the mmap() system call to create a shared memory region and then notify the kernel about the pointer to the shared memory region.

Related Commands

  • form_unuserptr: Releases a pointer that was previously notified to the kernel using the form_userptr command.
  • mmap: Creates a shared memory region.
  • open: Opens a character device.