osdep: Make MIN/MAX evaluate arguments only once

I'm not aware of any immediate bugs in qemu where a second runtime
evaluation of the arguments to MIN() or MAX() causes a problem, but
proactively preventing such abuse is easier than falling prey to an
unintended case down the road.  At any rate, here's the conversation
that sparked the current patch:
https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg05718.html

Update the MIN/MAX macros to only evaluate their argument once at
runtime; this uses typeof(1 ? (a) : (b)) to ensure that we are
promoting the temporaries to the same type as the final comparison (we
have to trigger type promotion, as typeof(bitfield) won't compile; and
we can't use typeof((a) + (b)) or even typeof((a) + 0), as some of our
uses of MAX are on void* pointers where such addition is undefined).

However, we are unable to work around gcc refusing to compile ({}) in
a constant context (such as the array length of a static variable),
even when only used in the dead branch of a __builtin_choose_expr(),
so we have to provide a second macro pair MIN_CONST and MAX_CONST for
use when both arguments are known to be compile-time constants and
where the result must also be usable as a constant; this second form
evaluates arguments multiple times but that doesn't matter for
constants.  By using a void expression as the expansion if a
non-constant is presented to this second form, we can enlist the
compiler to ensure the double evaluation is not attempted on
non-constants.

Alas, as both macros now rely on compiler intrinsics, they are no
longer usable in preprocessor #if conditions; those will just have to
be open-coded or the logic rewritten into #define or runtime 'if'
conditions (but where the compiler dead-code-elimination will probably
still apply).

I tested that both gcc 10.1.1 and clang 10.0.0 produce errors for all
forms of macro mis-use.  As the errors can sometimes be cryptic, I'm
demonstrating the gcc output:

Use of MIN when MIN_CONST is needed:

In file included from /home/eblake/qemu/qemu-img.c:25:
/home/eblake/qemu/include/qemu/osdep.h:249:5: error: braced-group within expression allowed only inside a function
  249 |     ({                                                  \
      |     ^
/home/eblake/qemu/qemu-img.c:92:12: note: in expansion of macro ‘MIN’
   92 | char array[MIN(1, 2)] = "";
      |            ^~~

Use of MIN_CONST when MIN is needed:

/home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’:
/home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be
 1225 |             i = MIN_CONST(i, n);
      |               ^

Use of MIN in the preprocessor:

In file included from /home/eblake/qemu/accel/tcg/translate-all.c:20:
/home/eblake/qemu/accel/tcg/translate-all.c: In function ‘page_check_range’:
/home/eblake/qemu/include/qemu/osdep.h:249:6: error: token "{" is not valid in preprocessor expressions
  249 |     ({                                                  \
      |      ^

Fix the resulting callsites that used #if or computed a compile-time
constant min or max to use the new macros.  cpu-defs.h is interesting,
as CPU_TLB_DYN_MAX_BITS is sometimes used as a constant and sometimes
dynamic.

It may be worth improving glib's MIN/MAX definitions to be saner, but
that is a task for another day.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200625162602.700741-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 47 insertions, 10 deletions

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index ff7c17b..0d26a1b 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -236,18 +236,55 @@ extern int daemon(int, int);
 #define SIZE_MAX ((size_t)-1)
 #endif
 
-#ifndef MIN
-#define MIN(a, b) (((a) < (b)) ? (a) : (b))
-#endif
-#ifndef MAX
-#define MAX(a, b) (((a) > (b)) ? (a) : (b))
-#endif
+/*
+ * Two variations of MIN/MAX macros. The first is for runtime use, and
+ * evaluates arguments only once (so it is safe even with side
+ * effects), but will not work in constant contexts (such as array
+ * size declarations) because of the '{}'.  The second is for constant
+ * expression use, where evaluating arguments twice is safe because
+ * the result is going to be constant anyway, but will not work in a
+ * runtime context because of a void expression where a value is
+ * expected.  Thus, both gcc and clang will fail to compile if you use
+ * the wrong macro (even if the error may seem a bit cryptic).
+ *
+ * Note that neither form is usable as an #if condition; if you truly
+ * need to write conditional code that depends on a minimum or maximum
+ * determined by the pre-processor instead of the compiler, you'll
+ * have to open-code it.
+ */
+#undef MIN
+#define MIN(a, b)                                       \
+    ({                                                  \
+        typeof(1 ? (a) : (b)) _a = (a), _b = (b);       \
+        _a < _b ? _a : _b;                              \
+    })
+#define MIN_CONST(a, b)                                         \
+    __builtin_choose_expr(                                      \
+        __builtin_constant_p(a) && __builtin_constant_p(b),     \
+        (a) < (b) ? (a) : (b),                                  \
+        ((void)0))
+#undef MAX
+#define MAX(a, b)                                       \
+    ({                                                  \
+        typeof(1 ? (a) : (b)) _a = (a), _b = (b);       \
+        _a > _b ? _a : _b;                              \
+    })
+#define MAX_CONST(a, b)                                         \
+    __builtin_choose_expr(                                      \
+        __builtin_constant_p(a) && __builtin_constant_p(b),     \
+        (a) > (b) ? (a) : (b),                                  \
+        ((void)0))
 
-/* Minimum function that returns zero only iff both values are zero.
- * Intended for use with unsigned values only. */
+/*
+ * Minimum function that returns zero only if both values are zero.
+ * Intended for use with unsigned values only.
+ */
 #ifndef MIN_NON_ZERO
-#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \
-                                ((b) == 0 ? (a) : (MIN(a, b))))
+#define MIN_NON_ZERO(a, b)                              \
+    ({                                                  \
+        typeof(1 ? (a) : (b)) _a = (a), _b = (b);       \
+        _a == 0 ? _b : (_b == 0 || _b > _a) ? _a : _b;  \
+    })
 #endif
 
 /* Round number down to multiple */