@@ -214,7 +214,7 @@ struct XHCIState {
uint32_t dcbaap_high;
uint32_t config;
- USBPort uports[MAX(MAXPORTS_2, MAXPORTS_3)];
+ USBPort uports[MAX_CONST(MAXPORTS_2, MAXPORTS_3)];
XHCIPort ports[MAXPORTS];
XHCISlot slots[MAXSLOTS];
uint32_t numports;
@@ -133,8 +133,8 @@ typedef struct HDGeometry {
#define BDRV_SECTOR_BITS 9
#define BDRV_SECTOR_SIZE (1ULL << BDRV_SECTOR_BITS)
-#define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \
- INT_MAX >> BDRV_SECTOR_BITS)
+#define BDRV_REQUEST_MAX_SECTORS MIN_CONST(SIZE_MAX >> BDRV_SECTOR_BITS, \
+ INT_MAX >> BDRV_SECTOR_BITS)
#define BDRV_REQUEST_MAX_BYTES (BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS)
/*
@@ -176,11 +176,9 @@ extern unsigned long reserved_va;
* avoid setting bits at the top of guest addresses that might need
* to be used for tags.
*/
-#if MIN(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32
-# define GUEST_ADDR_MAX_ UINT32_MAX
-#else
-# define GUEST_ADDR_MAX_ (~0ul)
-#endif
+#define GUEST_ADDR_MAX_ \
+ ((MIN_CONST(TARGET_VIRT_ADDR_SPACE_BITS, TARGET_ABI_BITS) <= 32) ? \
+ UINT32_MAX : ~0ul)
#define GUEST_ADDR_MAX (reserved_va ? reserved_va - 1 : GUEST_ADDR_MAX_)
#else
@@ -102,8 +102,13 @@ typedef uint64_t target_ulong;
* Skylake's Level-2 STLB has 16 1G entries.
* Also, make sure we do not size the TLB past the guest's address space.
*/
-# define CPU_TLB_DYN_MAX_BITS \
+# ifdef TARGET_PAGE_BITS_VARY
+# define CPU_TLB_DYN_MAX_BITS \
MIN(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS)
+# else
+# define CPU_TLB_DYN_MAX_BITS \
+ MIN_CONST(22, TARGET_VIRT_ADDR_SPACE_BITS - TARGET_PAGE_BITS)
+# endif
# endif
typedef struct CPUTLBEntry {
@@ -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 */
@@ -2582,9 +2582,9 @@ int page_check_range(target_ulong start, target_ulong len, int flags)
/* This function should never be called with addresses outside the
guest address space. If this assert fires, it probably indicates
a missing call to h2g_valid. */
-#if TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS
- assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS));
-#endif
+ if (TARGET_ABI_BITS > L1_MAP_ADDR_SPACE_BITS) {
+ assert(start < ((target_ulong)1 << L1_MAP_ADDR_SPACE_BITS));
+ }
if (len == 0) {
return 0;
@@ -31,7 +31,7 @@
#include "qapi/error.h"
#define IO_BUF_SIZE 32768
-#define MAX_IOV_SIZE MIN(IOV_MAX, 64)
+#define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64)
struct QEMUFile {
const QEMUFileOps *ops;
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> --- v2 was: https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00727.html v3: avoid __auto_type [Richard], document other approaches that fail [Dave], rebase to master v4: use ((void)0) instead of __builtin_unreachable [Dave], update comments v5: fix typo, comment formatting, one more __builtin_unreachable --- hw/usb/hcd-xhci.h | 2 +- include/block/block.h | 4 +-- include/exec/cpu-all.h | 8 +++--- include/exec/cpu-defs.h | 7 ++++- include/qemu/osdep.h | 57 ++++++++++++++++++++++++++++++++------- accel/tcg/translate-all.c | 6 ++--- migration/qemu-file.c | 2 +- 7 files changed, 63 insertions(+), 23 deletions(-)