mirror of https://github.com/xemu-project/xemu.git
f9919116b8
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> |
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| .. | ||
| atomic.h | ||
| atomic128.h | ||
| base64.h | ||
| bcd.h | ||
| bitmap.h | ||
| bitops.h | ||
| bswap.h | ||
| buffer.h | ||
| co-shared-resource.h | ||
| compiler.h | ||
| config-file.h | ||
| coroutine.h | ||
| coroutine_int.h | ||
| cpuid.h | ||
| crc32c.h | ||
| ctype.h | ||
| cutils.h | ||
| dbus.h | ||
| drm.h | ||
| envlist.h | ||
| error-report.h | ||
| event_notifier.h | ||
| fifo8.h | ||
| fifo32.h | ||
| filemonitor.h | ||
| futex.h | ||
| guest-random.h | ||
| hbitmap.h | ||
| help_option.h | ||
| host-utils.h | ||
| id.h | ||
| int128.h | ||
| iov.h | ||
| iova-tree.h | ||
| jhash.h | ||
| job.h | ||
| lockable.h | ||
| log-for-trace.h | ||
| log.h | ||
| main-loop.h | ||
| memfd.h | ||
| mmap-alloc.h | ||
| module.h | ||
| notify.h | ||
| nvdimm-utils.h | ||
| option.h | ||
| option_int.h | ||
| osdep.h | ||
| path.h | ||
| plugin-memory.h | ||
| plugin.h | ||
| pmem.h | ||
| processor.h | ||
| progress_meter.h | ||
| qdist.h | ||
| qemu-plugin.h | ||
| qemu-print.h | ||
| qht.h | ||
| qsp.h | ||
| queue.h | ||
| range.h | ||
| ratelimit.h | ||
| rcu.h | ||
| rcu_queue.h | ||
| readline.h | ||
| selfmap.h | ||
| seqlock.h | ||
| sockets.h | ||
| stats64.h | ||
| sys_membarrier.h | ||
| systemd.h | ||
| thread-posix.h | ||
| thread-win32.h | ||
| thread.h | ||
| throttle-options.h | ||
| throttle.h | ||
| timed-average.h | ||
| timer.h | ||
| tsan.h | ||
| typedefs.h | ||
| unicode.h | ||
| units.h | ||
| uri.h | ||
| uuid.h | ||
| vfio-helpers.h | ||
| win_dump_defs.h | ||
| xattr.h | ||
| xxhash.h | ||