Commit be9afb4b authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

x86/iopl: Fixup misleading comment 



The comment for the sys_iopl() implementation is outdated and actively
misleading in some parts. Fix it up.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarAndy Lutomirski <luto@kernel.org>
parent 0907a09c

arch/x86/kernel/ioport.c

+27 −8
Original line number Diff line number Diff line
@@ -41,7 +41,7 @@ void io_bitmap_exit(void) 
}



/*

 * this changes the io permissions bitmap in the current task.

 * This changes the io permissions bitmap in the current task.

 */

long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)

{
@@ -136,14 +136,24 @@ SYSCALL_DEFINE3(ioperm, unsigned long, from, unsigned long, num, int, turn_on) 
}



/*

 * sys_iopl has to be used when you want to access the IO ports

 * beyond the 0x3ff range: to get the full 65536 ports bitmapped

 * you'd need 8kB of bitmaps/process, which is a bit excessive.

 * The sys_iopl functionality depends on the level argument, which if

 * granted for the task is used by the CPU to check I/O instruction and

 * CLI/STI against the current priviledge level (CPL). If CPL is less than

 * or equal the tasks IOPL level the instructions take effect. If not a #GP

 * is raised. The default IOPL is 0, i.e. no permissions.

 *

 * Here we just change the flags value on the stack: we allow

 * only the super-user to do it. This depends on the stack-layout

 * on system-call entry - see also fork() and the signal handling

 * code.

 * Setting IOPL to level 0-2 is disabling the userspace access. Only level

 * 3 enables it. If set it allows the user space thread:

 *

 * - Unrestricted access to all 65535 I/O ports

 * - The usage of CLI/STI instructions

 *

 * The advantage over ioperm is that the context switch does not require to

 * update the I/O bitmap which is especially true when a large number of

 * ports is accessed. But the allowance of CLI/STI in userspace is

 * considered a major problem.

 *

 * IOPL is strictly per thread and inherited on fork.

 */

SYSCALL_DEFINE1(iopl, unsigned int, level)

{
@@ -164,9 +174,18 @@ SYSCALL_DEFINE1(iopl, unsigned int, level) 
		    security_locked_down(LOCKDOWN_IOPORT))

			return -EPERM;

	}

	/*

	 * Change the flags value on the return stack, which has been set

	 * up on system-call entry. See also the fork and signal handling

	 * code how this is handled.

	 */

	regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) |

		(level << X86_EFLAGS_IOPL_BIT);

	/* Store the new level in the thread struct */

	t->iopl = level << X86_EFLAGS_IOPL_BIT;

	/*

	 * X86_32 switches immediately and XEN handles it via emulation.

	 */

	set_iopl_mask(t->iopl);



	return 0;