Commit-ID: 07114f0f1cda8b2ef6e884d0c7b268a32cce7903 Gitweb: http://git.kernel.org/tip/07114f0f1cda8b2ef6e884d0c7b268a32cce7903 Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx> AuthorDate: Tue, 4 Nov 2014 15:46:21 -0800 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Mon, 10 Nov 2014 10:43:13 +0100 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@xxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@xxxxxxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- 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 eb71ec7..82d93ea 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) -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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