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osdep: Make MIN/MAX evaluate arguments only once

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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>
This commit is contained in:
Eric Blake 2020-06-25 11:26:02 -05:00 committed by Paolo Bonzini
parent 47f0d11d21
commit f9919116b8
7 changed files with 63 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
accel/tcg/translate-all.c
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@ -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;

2
hw/usb/hcd-xhci.h
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@ -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;

4
include/block/block.h
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@ -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)
/*

8
include/exec/cpu-all.h
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@ -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

7
include/exec/cpu-defs.h
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@ -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 {

57
include/qemu/osdep.h
View File

@ -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 */

2
migration/qemu-file.c
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@ -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;