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Message-Id: <23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net> Date: Tue, 4 Nov 2014 15:46:21 -0800 From: Andy Lutomirski <luto@...capital.net> To: x86@...nel.org Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net> Subject: [PATCH] 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: Andy Lutomirski <luto@...capital.net> --- arch/x86/include/asm/processor.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index eb71ec794732..82d93ea13c0c 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -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) -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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