| Message ID | 20200625162602.700741-1-eblake@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v5] osdep: Make MIN/MAX evaluate arguments only once | expand |
On 6/25/20 6:26 PM, Eric Blake wrote: > 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> > Nice! (btw why extra line after s-o-b?) Reviewed-by: Philippe Mathieu-Daudé <philmd@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(-)
On Thu, Jun 25, 2020 at 6:36 PM Philippe Mathieu-Daudé <philmd@redhat.com> wrote: > On 6/25/20 6:26 PM, Eric Blake wrote: > > 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> > > > > Nice! (btw why extra line after s-o-b?) > > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> Tested-by: Philippe Mathieu-Daudé <philmd@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(-)
On 6/25/20 9:26 AM, Eric Blake wrote: > 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: Richard Henderson <richard.henderson@linaro.org> r~
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 fb4e8a8e29cb..fc403d456b70 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..0d26a1b9bd07 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 */ diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c index c3d37058a17c..2afa46bd2b1f 100644 --- a/accel/tcg/translate-all.c +++ b/accel/tcg/translate-all.c @@ -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; 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;
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(-)