From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Add shift_overflow() helper to assist driver authors in ensuring that
shift operations don't cause overflows or other odd conditions.
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxxxx>
[kees: tweaked comments and commit log, dropped unneeded assignment]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
include/linux/overflow.h | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 8712ff70995f..69fc366ce865 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -202,6 +202,37 @@
#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
+/** check_shift_overflow() - Calculate a left-shifted value and check overflow
+ *
+ * @a: Value to be shifted
+ * @b: How many bits left to shift
+ * @d: Pointer to where to store the result
+ *
+ * Computes *@d = (@a << @s)
+ *
+ * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
+ * make sense. Example conditions:
+ * - 'a << s' causes bits to be lost when stored in *d.
+ * - 's' is garbage (e.g. negative) or so large that the result of
+ * 'a << s' is guaranteed to be 0.
+ * - 'a' is negative.
+ * - 'a << s' sets the sign bit, if any, in '*d'.
+ *
+ * '*d' will hold the results of the attempted shift, but is not
+ * considered "safe for use" if false is returned.
+ */
+#define check_shift_overflow(a, s, d) ({ \
+ typeof(a) _a = a; \
+ typeof(s) _s = s; \
+ typeof(d) _d = d; \
+ u64 _a_full = _a; \
+ unsigned int _to_shift = \
+ _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \
+ *_d = (_a_full << _to_shift); \
+ (_to_shift != _s || *_d < 0 || _a < 0 || \
+ (*_d >> _to_shift) != _a); \
+})
+
/**
* array_size() - Calculate size of 2-dimensional array.
*
--
2.17.1
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