[PATCH libcrypto v3 2/3] compiler: introduce at_least parameter decoration pseudo keyword

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Clang and recent gcc support warning if they are able to prove that the
user is passing to a function an array that is too short in size. For
example:

    void blah(unsigned char herp[at_least 7]);
    static void schma(void)
    {
        unsigned char good[] = { 1, 2, 3, 4, 5, 6, 7 };
        unsigned char bad[] = { 1, 2, 3, 4, 5, 6 };
        blah(good);
        blah(bad);
    }

The notation here, `static 7`, which this commit makes explicit by
allowing us to write it as `at_least 7`, means that it's incorrect to
pass anything less than 7 elements. This is section 6.7.5.3 of C99:

    If the keyword static also appears within the [ and ] of the array
    type derivation, then for each call to the function, the value of
    the corresponding actual argument shall provide access to the first
    element of an array with at least as many elements as specified by
    the size expression.

Here is the output from gcc 15:

    zx2c4@thinkpad /tmp $ gcc -c a.c
    a.c: In function ‘schma’:
    a.c:9:9: warning: ‘blah’ accessing 7 bytes in a region of size 6 [-Wstringop-overflow=]
        9 |         blah(bad);
          |         ^~~~~~~~~
    a.c:9:9: note: referencing argument 1 of type ‘unsigned char[7]’
    a.c:2:6: note: in a call to function ‘blah’
        2 | void blah(unsigned char herp[at_least 7]);
          |      ^~~~

And from clang 21:

    zx2c4@thinkpad /tmp $ clang -c a.c
    a.c:9:2: warning: array argument is too small; contains 6 elements, callee requires at least 7
          [-Warray-bounds]
        9 |         blah(bad);
          |         ^    ~~~
    a.c:2:25: note: callee declares array parameter as static here
        2 | void blah(unsigned char herp[at_least 7]);
          |                         ^   ~~~~~~~~~~
    1 warning generated.

So these are covered by, variously, -Wstringop-overflow and
-Warray-bounds.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Changes v2->v3:
- Added Ard's ack.
- Conditionalize macro on __CHECKER__.

 include/linux/compiler.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 5b45ea7dff3e..a9356b158d33 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -379,6 +379,21 @@ static inline void *offset_to_ptr(const int *off)
  */
 #define prevent_tail_call_optimization()	mb()
 
+/*
+ * This designates the minimum number of elements a passed array parameter must
+ * have. For example:
+ *
+ *     void some_function(u8 param[at_least 7]);
+ * 
+ * If a caller passes an array with fewer than 7 elements, the compiler will
+ * emit a warning.
+ */
+#ifndef __CHECKER__
+#define at_least static
+#else
+#define at_least
+#endif
+
 #include <asm/rwonce.h>
 
 #endif /* __LINUX_COMPILER_H */
-- 
2.52.0