Commit 0c668477 authored by Jason Gunthorpe's avatar Jason Gunthorpe

overflow.h: Add arithmetic shift helper

Add shift_overflow() helper to assist driver authors in ensuring that
shift operations don't cause overflows or other odd conditions.
Signed-off-by: default avatarJason Gunthorpe <jgg@mellanox.com>
Signed-off-by: default avatarLeon Romanovsky <leonro@mellanox.com>
[kees: tweaked comments and commit log, dropped unneeded assignment]
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent 58796e67
...@@ -202,6 +202,37 @@ ...@@ -202,6 +202,37 @@
#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
/** check_shl_overflow() - Calculate a left-shifted value and check overflow
*
* @a: Value to be shifted
* @s: 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_shl_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. * array_size() - Calculate size of 2-dimensional array.
* *
......
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