On 23.10.20 15:09, David Hildenbrand wrote: > On 23.10.20 14:46, David Laight wrote: >> From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> >>> Sent: 22 October 2020 14:51 >> >> I've rammed the code into godbolt. >> >> https://godbolt.org/z/9v5PPW >> >> Definitely a clang bug. >> >> Search for [wx]24 in the clang output. >> nr_segs comes in as w2 and the initial bound checks are done on w2. >> w24 is loaded from w2 - I don't believe this changes the high bits. >> There are no references to w24, just x24. >> So the kmalloc_array() is passed 'huge' and will fail. >> The iov_iter_init also gets the 64bit value. >> >> Note that the gcc code has a sign-extend copy of w2. > > Do we have a result from using "unsigned long" in the base function and > explicitly masking of the high bits? That should definitely work. > > Now, I am not a compiler expert, but as I already cited, at least on > x86-64 clang expects that the high bits were cleared by the caller - in > contrast to gcc. I suspect it's the same on arm64, but again, I am no > compiler expert. > > If what I said and cites for x86-64 is correct, if the function expects > an "unsigned int", it will happily use 64bit operations without further > checks where valid when assuming high bits are zero. That's why even > converting everything to "unsigned int" as proposed by me won't work on > clang - it assumes high bits are zero (as indicated by Nick). > > As I am neither a compiler experts (did I mention that already? ;) ) nor > an arm64 experts, I can't tell if this is a compiler BUG or not. > I just checked against upstream code generated by clang 10 and it properly discards the upper 32bit via a mov w23 w2. So at least clang 10 indeed properly assumes we could have garbage and masks it off. Maybe the issue is somewhere else, unrelated to nr_pages ... or clang 11 behaves differently. -- Thanks, David / dhildenb
