[PATCH v3] osdep: Make MIN/MAX evaluate arguments only once

[Eric Blake]

I'm not aware of any immediate bugs in qemu where a second runtime
evalution 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, 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 in
constant contexts.

Fix the resulting callsites that compute 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.  Furthermore, use of either macro in #if now fails.

gcc makes it easy to know which macro to use, thanks to the following
compiler errors (even if they are cryptic):

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 |     ({                                                  \
      |      ^

Signed-off-by: Eric Blake <eblake@redhat.com>

---

I was visiting an old branch, and thought this one is still worth salvaging.

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
---
 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      | 43 +++++++++++++++++++++++++++++++--------
 accel/tcg/translate-all.c |  6 +++---
 migration/qemu-file.c     |  2 +-
 7 files changed, 51 insertions(+), 21 deletions(-)

diff --git a/hw/usb/hcd-xhci.h b/hw/usb/hcd-xhci.h
index 2fad4df2a704..946af51fc25d 100644
--- a/hw/usb/hcd-xhci.h
+++ b/hw/usb/hcd-xhci.h
@@ -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;
diff --git a/include/block/block.h b/include/block/block.h
index 25e299605e19..e8fc8149967f 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -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)

 /*
diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index d14374bdd499..291454fac12e 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -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
diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index 8c44abefa22a..918563233797 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -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 {
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index ff7c17b85735..1f002f035279 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -236,18 +236,45 @@ 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).  The second is for compile-time use, where
+ * evaluating arguments twice is safe because the result is going to
+ * be constant anyway.
+ */
+#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),                                  \
+        __builtin_unreachable())
+#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),                                  \
+        __builtin_unreachable())

 /* Minimum function that returns zero only iff 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 */
diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index 42ce1dfcff77..d77add79b218 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -2565,9 +2565,9 @@ 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;
diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index 1c3a358a140d..be21518c5708 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -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;
-- 
2.27.0