From: Eric Blake <eblake@redhat.com>
To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
"open list:Block layer core" <qemu-block@nongnu.org>,
crosthwaite.peter@gmail.com, quintela@redhat.com,
f4bug@amsat.org, dgilbert@redhat.com, kraxel@redhat.com,
dirty.ice.hu@gmail.com, pbonzini@redhat.com,
Max Reitz <mreitz@redhat.com>,
rth@twiddle.net
Subject: [PATCH v3] osdep: Make MIN/MAX evaluate arguments only once
Date: Tue, 2 Jun 2020 20:36:03 -0500 [thread overview]
Message-ID: <20200603013603.2400199-1-eblake@redhat.com> (raw)
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
next reply other threads:[~2020-06-03 1:37 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-03 1:36 Eric Blake [this message]
2020-06-03 2:07 ` [PATCH v3] osdep: Make MIN/MAX evaluate arguments only once Richard Henderson
2020-06-03 2:29 ` Eric Blake
2020-06-03 15:33 ` Eric Blake
2020-06-03 16:32 ` Richard Henderson
2020-06-03 19:26 ` Eric Blake
2020-06-04 18:39 ` Paolo Bonzini
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