On Thu, 19 May 2016, Andrey Ryabinin wrote: > 2016-05-18 22:28 GMT+03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > > On Wed, 18 May 2016, Andrey Ryabinin wrote: > > > >> 2016-05-18 19:09 GMT+03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > >> > On Wed, 18 May 2016, Andrey Ryabinin wrote: > >> > > >> >> 2016-05-18 17:40 GMT+03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>: > >> >> > >> >> > All right, I'm getting very tired of all these bug reports. Besides, > >> >> > Andrey has a point: Unless you're Linus, arguing against the C standard > >> >> > is futile. (Even though the language dialect used in the kernel is not > >> >> > standard C.) > >> >> > > >> >> > Does this patch make UBSAN happy? The runtime overhead is minimal. > >> >> > > >> >> > >> >> It does. However, you could fool ubsan way more easy: > >> >> u32 __iomem *hostpc_reg = ehci->regs->hostpc + > >> >> (wIndex & 0xff) - 1; > > > > This probably should be considered to be a bug in UBSAN. It ought to > > treat pointer addition the same as index addition. > > > > It's more like a missing feature. UBSAN doesn't guarantee that every possible > UB will be detected. > > > >> > >> So it silences UBSAN, but still undefined. > >> I think it's up to you to decide - more code churn or undefined behavior. > > > > Well, I don't want the compiler to eliminate code that's necessary. > > > > On the other hand, it's not clear how much we need to worry about the > > standard. After all, zero-length arrays are a GNU extension to C. > > Since the array objects in question are defined like this: > > > > u32 port_status[0]; /* up to N_PORTS */ > > > > it's hard to guess what the compiler will think about out-of-bounds > > pointer values. > > > > Maybe the best thing to do is eliminate the underflow while leaving the > > calculation unchanged. What does UBSAN think about this? Does it > > dislike -1 as an index value as much as it dislikes -1u? > > Type of doesn't change anything here. Okay, then I'll go back to the more verbose but guaranteed safe patch. Thanks for your comments, Alan Stern -- 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
