Re: [PATCH v2 1/3] overflow.h: Add arithmetic shift helper
From: Randy Dunlap <hidden>
Date: 2018-08-01 01:59:52
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On 07/31/2018 05:00 PM, Kees Cook wrote:
quoted hunk ↗ jump to hunk
From: Jason Gunthorpe <redacted> 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 <redacted> Signed-off-by: Leon Romanovsky <redacted> [kees: tweaked comments and commit log, dropped unneeded assignment] Signed-off-by: Kees Cook <redacted> --- 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)
@b)
+ *
+ * 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.
*-- ~Randy