#define task_pt_regs(task) \
({ \
struct pt_regs *__regs__; \
- __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
+ __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
+ TOP_OF_KERNEL_STACK_PADDING); \
__regs__ - 1; \
})
#include <asm/types.h>
/*
+ * TOP_OF_KERNEL_STACK_PADDING is a number of unused bytes that we
+ * reserve at the top of the kernel stack. We do it because of a nasty
+ * 32-bit corner case. On x86_32, the hardware stack frame is
+ * variable-length. Except for vm86 mode, struct pt_regs assumes a
+ * maximum-length frame. If we enter from CPL 0, the top 8 bytes of
+ * pt_regs don't actually exist. Ordinarily this doesn't matter, but it
+ * does in at least one case:
+ *
+ * If we take an NMI early enough in SYSENTER, then we can end up with
+ * pt_regs that extends above sp0. On the way out, in the espfix code,
+ * we can read the saved SS value, but that value will be above sp0.
+ * Without this offset, that can result in a page fault. (We are
+ * careful that, in this case, the value we read doesn't matter.)
+ *
+ * In vm86 mode, the hardware frame is much longer still, but we neither
+ * access the extra members from NMI context, nor do we write such a
+ * frame at sp0 at all.
+ *
+ * x86_64 has a fixed-length stack frame.
+ */
+#ifdef CONFIG_X86_32
+# define TOP_OF_KERNEL_STACK_PADDING 8
+#else
+# define TOP_OF_KERNEL_STACK_PADDING 0
+#endif
+
+/*
* low level task data that entry.S needs immediate access to
* - this struct should fit entirely inside of one cache line
* - this struct shares the supervisor stack pages