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| author | Andy Lutomirski <luto@amacapital.net> | 2014-11-05 02:46:21 +0300 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2014-11-10 12:43:13 +0300 |
| commit | 07114f0f1cda8b2ef6e884d0c7b268a32cce7903 (patch) | |
| tree | d5a4cdabfea6290b9b2f77515c4dc4c3b50d301a /arch/x86/include/asm/processor.h | |
| parent | 1ad83c858c7d4ea210429142c99a1548e6715a35 (diff) | |
| download | linux-07114f0f1cda8b2ef6e884d0c7b268a32cce7903.tar.xz | |
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@amacapital.net>
Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/include/asm/processor.h')
| -rw-r--r-- | arch/x86/include/asm/processor.h | 8 |
1 files changed, 7 insertions, 1 deletions
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) |