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x86_64: Add a comment explaining the TASK_SIZE_MAX guard page

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

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2014-11-04 15:46:21 -08:00 committed by Ingo Molnar
parent 1ad83c858c
commit 07114f0f1c
1 changed files with 7 additions and 1 deletions
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8
arch/x86/include/asm/processor.h
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@ -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)