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Commit 07114f0f authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar
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x86_64: Add a comment explaining the TASK_SIZE_MAX guard page 



That guard page is absolutely necessary; explain why for
posterity.

Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 1ad83c85

arch/x86/include/asm/processor.h

+7 −1
Original line number Diff line number Diff line
@@ -893,7 +893,13 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk); 


#else

/*

 * User space process size. 47bits minus one guard page.

 * User space process size. 47bits minus one guard page.  The guard

 * page is necessary on Intel CPUs: if a SYSCALL instruction is at

 * the highest possible canonical userspace address, then that

 * syscall will enter the kernel with a non-canonical return

 * address, and SYSRET will explode dangerously.  We avoid this

 * particular problem by preventing anything from being mapped

 * at the maximum canonical address.

 */

#define TASK_SIZE_MAX	((1UL << 47) - PAGE_SIZE)