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[v3,12/19,5.15.y] minmax: fix up min3() and max3() too

Message ID 20251008152946.29285-13-farbere@amazon.com
State Not Applicable
Headers show
Series Backport minmax.h updates from v6.17-rc7 | expand

Commit Message

Farber, Eliav Oct. 8, 2025, 3:29 p.m. UTC 
From: Linus Torvalds <torvalds@linux-foundation.org>

[ Upstream commit 21b136cc63d2a9ddd60d4699552b69c214b32964 ]

David Laight pointed out that we should deal with the min3() and max3()
mess too, which still does excessive expansion.

And our current macros are actually rather broken.

In particular, the macros did this:

  #define min3(x, y, z) min((typeof(x))min(x, y), z)
  #define max3(x, y, z) max((typeof(x))max(x, y), z)

and that not only is a nested expansion of possibly very complex
arguments with all that involves, the typing with that "typeof()" cast
is completely wrong.

For example, imagine what happens in max3() if 'x' happens to be a
'unsigned char', but 'y' and 'z' are 'unsigned long'.  The types are
compatible, and there's no warning - but the result is just random
garbage.

No, I don't think we've ever hit that issue in practice, but since we
now have sane infrastructure for doing this right, let's just use it.
It fixes any excessive expansion, and also avoids these kinds of broken
type issues.

Requested-by: David Laight <David.Laight@aculab.com>
Acked-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Eliav Farber <farbere@amazon.com>
---
 include/linux/minmax.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)
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Patch

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 41da6f85a407..98008dd92153 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -152,13 +152,20 @@ 
 #define umax(x, y)	\
 	__careful_cmp(max, (x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull)
 
+#define __careful_op3(op, x, y, z, ux, uy, uz) ({			\
+	__auto_type ux = (x); __auto_type uy = (y);__auto_type uz = (z);\
+	BUILD_BUG_ON_MSG(!__types_ok3(x,y,z,ux,uy,uz),			\
+		#op"3("#x", "#y", "#z") signedness error");		\
+	__cmp(op, ux, __cmp(op, uy, uz)); })
+
 /**
  * min3 - return minimum of three values
  * @x: first value
  * @y: second value
  * @z: third value
  */
-#define min3(x, y, z) min((typeof(x))min(x, y), z)
+#define min3(x, y, z) \
+	__careful_op3(min, x, y, z, __UNIQUE_ID(x_), __UNIQUE_ID(y_), __UNIQUE_ID(z_))
 
 /**
  * max3 - return maximum of three values
@@ -166,7 +173,8 @@ 
  * @y: second value
  * @z: third value
  */
-#define max3(x, y, z) max((typeof(x))max(x, y), z)
+#define max3(x, y, z) \
+	__careful_op3(max, x, y, z, __UNIQUE_ID(x_), __UNIQUE_ID(y_), __UNIQUE_ID(z_))
 
 /**
  * min_not_zero - return the minimum that is _not_ zero, unless both are zero