2016-05-18 13:19 GMT+03:00 Oliver Neukum <oneukum@xxxxxxxx>: > On Wed, 2016-05-18 at 12:16 +0300, Andrey Ryabinin wrote: >> 2016-05-18 11:18 GMT+03:00 Oliver Neukum <oneukum@xxxxxxxx>: >> > On Wed, 2016-05-18 at 10:40 +0300, Andrey Ryabinin wrote: >> >> 2016-05-18 1:16 GMT+03:00 Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>: >> >> > On Tue, May 17, 2016 at 05:52:40PM -0400, Valdis Kletnieks wrote: >> >> >> So, not content in the amount of breakage I generate already, I >> >> >> compiled with UBSAN enabled... >> >> >> >> >> >> The immediately relevant part: >> >> >> >> >> >> [ 2.418576] ================================================================================ >> >> >> [ 2.418579] UBSAN: Undefined behaviour in drivers/usb/host/ehci-hub.c:877:47 >> >> >> [ 2.418582] index -1 is out of range for type 'u32 [1]' >> >> > >> >> > <snip> >> >> > >> >> > It's a known bug in ubsan, >> >> >> >> It's not a bug. int *p = &a[-1] is undefined behavior. It doesn't >> >> matter whether that pointer dereferenced or not. >> > >> > That is a bold statement. Pointer arithmetic is defined. How can >> > the computation of an address be undefined behavior while it is >> > not used? >> >> It's defined only if pointer points to array element or one-past-end >> element. Everything else is undefined. >> >> $ 6.5.6.8 >> "If both the pointer operand and the result point to elements of >> the same array object, >> or one past the last element of the array object, the evaluation >> shall not produce an overflow; >> otherwise, the behavior is undefined." > > But we do not care whether the calculation overflows. We don't use it > at all in those cases. > This doesn't make it defined. Also that pointer is unused only if gcc doesn't optimize away '!wIndex' check. If it does, we may actually use it. -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
