'array_idx' uses a mask to sanitize user controllable array indexes,
i.e. generate a 0 mask if idx >= sz, and a ~0 mask otherwise. While the
default array_idx_mask handles the carry-bit from the (index - size)
result in software. The x86 'array_idx_mask' does the same, but the
carry-bit is handled in the processor CF flag without conditional
instructions in the control flow.
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
arch/x86/include/asm/barrier.h | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 01727dbc294a..30419b674ebd 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -24,6 +24,28 @@
#define wmb() asm volatile("sfence" ::: "memory")
#endif
+/**
+ * array_idx_mask - generate a mask for array_idx() that is ~0UL when
+ * the bounds check succeeds and 0 otherwise
+ *
+ * mask = 0 - (idx < sz);
+ */
+#define array_idx_mask array_idx_mask
+static inline unsigned long array_idx_mask(unsigned long idx, unsigned long sz)
+{
+ unsigned long mask;
+
+#ifdef CONFIG_X86_32
+ asm ("cmpl %1,%2; sbbl %0,%0;"
+#else
+ asm ("cmpq %1,%2; sbbq %0,%0;"
+#endif
+ :"=r" (mask)
+ :"r"(sz),"r" (idx)
+ :"cc");
+ return mask;
+}
+
#ifdef CONFIG_X86_PPRO_FENCE
#define dma_rmb() rmb()
#else
