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Re: [PATCH v4] osdep: Make MIN/MAX evaluate arguments only once
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From: |
Paolo Bonzini |
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Subject: |
Re: [PATCH v4] osdep: Make MIN/MAX evaluate arguments only once |
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Date: |
Tue, 23 Jun 2020 11:31:04 +0200 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
On 04/06/20 23:52, Eric Blake wrote:
> 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 (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.
>
> 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
> ---
> 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 | 51 +++++++++++++++++++++++++++++++++------
> accel/tcg/translate-all.c | 6 ++---
> migration/qemu-file.c | 2 +-
> 7 files changed, 59 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..b18584f32ff6 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -236,18 +236,53 @@ 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), \
> + __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;
>
Queued, thnaks.
Paolo