linux-kernel - [PATCH v4 0/4] Add ARRAY_END(), and use it to fix off-by-one bugs

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Message-ID: <cover.1762717358.git.alx@kernel.org>
Date: Sun, 9 Nov 2025 20:45:04 +0100
From: Alejandro Colomar <alx@...nel.org>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: Alejandro Colomar <alx@...nel.org>, Kees Cook <kees@...nel.org>, 
	Christopher Bazley <chris.bazley.wg14@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Marco Elver <elver@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Jann Horn <jannh@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "Maciej W. Rozycki" <macro@...am.me.uk>
Subject: [PATCH v4 0/4] Add ARRAY_END(), and use it to fix off-by-one bugs

ARRAY_END() is a macro to calculate a pointer to one past the last
element of an array argument.  This is a very common pointer, which is
used to iterate over all elements of an array:

        for (T *p = a; p < ARRAY_END(a); p++)
                ...

Of course, this pointer should never be dereferenced.  A pointer one
past the last element of an array should not be dereferenced; it's
perfectly fine to hold such a pointer --and a good thing to do--, but
the only thing it should be used for is comparing it with other pointers
derived from the same array.

Due to how special these pointers are, it would be good to use
consistent naming.  It's common to name such a pointer 'end' --in fact,
we have many such cases in the kernel--.  C++ even standardized this
name with std::end().  Let's try naming such pointers 'end', and try
also avoid using 'end' for pointers that are not the result of
ARRAY_END().

It has been incorrectly suggested that these pointers are dangerous, and
that they should never be used, suggesting to use something like

	#define ARRAY_LAST(a)  ((a) + ARRAY_SIZE(a) - 1)

	for (T *p = a; p <= ARRAY_LAST(a); p++)
		...

This is bogus, as it doesn't scale down to arrays of 0 elements.  In the
case of an array of 0 elements, ARRAY_LAST() would underflow the
pointer, which not only it can't be dereferenced, it can't even be held
(it produces Undefined Behavior).  That would be a footgun.  Such arrays
don't exist per the ISO C standard; however, GCC supports them as an
extension (with partial support, though; GCC has a few bugs which need
to be fixed).

This patch set fixes a few places where it was intended to use the array
end (that is, one past the last element), but accidentally a pointer to
the last element was used instead, thus wasting one byte.

It also replaces other places where the array end was correctly
calculated with ARRAY_SIZE(), by using the simpler ARRAY_END().

Also, there was one drivers/ file that already defined this macro.  We
remove that definition, to not conflict with this one.

---

Hi,

v4:

-  Use 'arr' as the macro parameter, for consistency with ARRAY_SIZE().
   [Reported-by: "Maciej W. Rozycki" <macro@...am.me.uk>]

Have a lovely night!
Alex

Alejandro Colomar (4):
  array_size.h: Add ARRAY_END()
  mm: Fix benign off-by-one bugs
  kernel: Fix off-by-one benign bugs
  mm: Use ARRAY_END() instead of open-coding it

 drivers/block/floppy.c     | 2 --
 include/linux/array_size.h | 6 ++++++
 kernel/kcsan/kcsan_test.c  | 4 ++--
 mm/kfence/kfence_test.c    | 4 ++--
 mm/kmemleak.c              | 2 +-
 mm/kmsan/kmsan_test.c      | 2 +-
 mm/memcontrol-v1.c         | 4 ++--
 7 files changed, 14 insertions(+), 10 deletions(-)

Range-diff against v3:
1:  2cb4ddff93b3 ! 1:  9f87d6208a6c array_size.h: Add ARRAY_END()
    @@ Commit message
         Cc: Michal Hocko <mhocko@...e.com>
         Cc: Linus Torvalds <torvalds@...ux-foundation.org>
         Cc: Al Viro <viro@...iv.linux.org.uk>
    +    Cc: "Maciej W. Rozycki" <macro@...am.me.uk>
         Signed-off-by: Alejandro Colomar <alx@...nel.org>
         Message-ID: <37b1088dbd01a21d2f9d460aa510726119b3bcb0.1752193588.git.alx@...nel.org>

    @@ include/linux/array_size.h
      #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

     +/**
    -+ * ARRAY_END - get a pointer to one past the last element in array @a
    -+ * @a: array
    ++ * ARRAY_END - get a pointer to one past the last element in array @arr
    ++ * @arr: array
     + */
    -+#define ARRAY_END(a)  (&(a)[ARRAY_SIZE(a)])
    ++#define ARRAY_END(arr)  (&(arr)[ARRAY_SIZE(arr)])
     +
      #endif  /* _LINUX_ARRAY_SIZE_H */
2:  831155f02bec = 2:  ac55d92551e4 mm: Fix benign off-by-one bugs
3:  d8128f0c8b9f = 3:  ca8dec7f5bc9 kernel: Fix off-by-one benign bugs
4:  9646a1d194a5 = 4:  980c8fe8a6de mm: Use ARRAY_END() instead of open-coding it

base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
-- 
2.51.0