[PATCH 5.15.y 2/8] minmax: sanity check constant bounds when clamping

SeongJae Park
2024-07-16T18:33:41Z

From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit 5efcecd9a3b18078d3398b359a84c83f549e22cf upstream.

The clamp family of functions only makes sense if hi>=lo. If hi and lo
are compile-time constants, then raise a build error. Doing so has
already caught buggy code. This also introduces the infrastructure to
improve the clamping function in subsequent commits.

[akpm@linux-foundation.org: coding-style cleanups]
[akpm@linux-foundation.org: s@&&\@&& \@]
Link: https://lkml.kernel.org/r/20220926133435.1333846-1-Jason@zx2c4.com
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 5efcecd9a3b18078d3398b359a84c83f549e22cf)
Signed-off-by: SeongJae Park <sj@kernel.org>
---
include/linux/minmax.h | 26 ++++++++++++++++++++++++--
1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 1aea34b8f19b..8b092c66c5aa 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -37,6 +37,28 @@
__cmp(x, y, op), \
__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))

+#define __clamp(val, lo, hi) \
+ __cmp(__cmp(val, lo, >), hi, <)
+
+#define __clamp_once(val, lo, hi, unique_val, unique_lo, unique_hi) ({ \
+ typeof(val) unique_val = (val); \
+ typeof(lo) unique_lo = (lo); \
+ typeof(hi) unique_hi = (hi); \
+ __clamp(unique_val, unique_lo, unique_hi); })
+
+#define __clamp_input_check(lo, hi) \
+ (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
+ __is_constexpr((lo) > (hi)), (lo) > (hi), false)))
+
+#define __careful_clamp(val, lo, hi) ({ \
+ __clamp_input_check(lo, hi) + \
+ __builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) && \
+ __typecheck(hi, lo) && __is_constexpr(val) && \
+ __is_constexpr(lo) && __is_constexpr(hi), \
+ __clamp(val, lo, hi), \
+ __clamp_once(val, lo, hi, __UNIQUE_ID(__val), \
+ __UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })
+
/**
* min - return minimum of two values of the same or compatible types
* @x: first value
@@ -103,7 +125,7 @@
* This macro does strict typechecking of @lo/@hi to make sure they are of the
* same type as @val. See the unnecessary pointer comparisons.
*/
-#define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
+#define clamp(val, lo, hi) __careful_clamp(val, lo, hi)

/*
* ..and if you can't take the strict
@@ -138,7 +160,7 @@
* This macro does no typechecking and uses temporary variables of type
* @type to make all the comparisons.
*/
-#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
+#define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))

/**
* clamp_val - return a value clamped to a given range using val's type
--
2.39.2