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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> |
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| bus.c | ||
| ccid-card-emulated.c | ||
| ccid-card-passthru.c | ||
| ccid.h | ||
| chipidea.c | ||
| combined-packet.c | ||
| core.c | ||
| desc-msos.c | ||
| desc.c | ||
| desc.h | ||
| dev-audio.c | ||
| dev-hid.c | ||
| dev-hub.c | ||
| dev-mtp.c | ||
| dev-network.c | ||
| dev-serial.c | ||
| dev-smartcard-reader.c | ||
| dev-storage.c | ||
| dev-uas.c | ||
| dev-wacom.c | ||
| hcd-dwc2.c | ||
| hcd-dwc2.h | ||
| hcd-ehci-pci.c | ||
| hcd-ehci-sysbus.c | ||
| hcd-ehci.c | ||
| hcd-ehci.h | ||
| hcd-musb.c | ||
| hcd-ohci-pci.c | ||
| hcd-ohci.c | ||
| hcd-ohci.h | ||
| hcd-uhci.c | ||
| hcd-xhci-nec.c | ||
| hcd-xhci.c | ||
| hcd-xhci.h | ||
| host-libusb.c | ||
| host-stub.c | ||
| host.h | ||
| imx-usb-phy.c | ||
| Kconfig | ||
| libhw.c | ||
| Makefile.objs | ||
| quirks-ftdi-ids.h | ||
| quirks-pl2303-ids.h | ||
| quirks.c | ||
| quirks.h | ||
| redirect.c | ||
| trace-events | ||
| tusb6010.c | ||
| xen-usb.c | ||