On Tue, May 28, 2024 at 01:36:11PM +0200, Alexander Potapenko wrote: > On Fri, May 24, 2024 at 7:30 PM Kent Overstreet > <[email protected]> wrote: > > > > On Fri, May 24, 2024 at 12:04:11PM -0400, Mathieu Desnoyers wrote: > > > On 2024-05-24 11:35, Mathieu Desnoyers wrote: > > > > [ Adding clang/llvm and KMSAN maintainers/reviewers in CC. ] > > > > > > > > On 2024-05-24 11:28, Kent Overstreet wrote: > > > > > On Thu, May 23, 2024 at 01:53:42PM -0400, Mathieu Desnoyers wrote: > > > > > > Hi Kent, > > > > > > > > > > > > Looking around in the bcachefs code for possible causes of this > > > > > > KMSAN > > > > > > bug report: > > > > > > > > > > > > https://lore.kernel.org/lkml/[email protected]/ > > > > > > > > > > > > I notice the following pattern in the bcachefs structures: > > > > > > zero-length > > > > > > arrays members are inserted in structures (not always at the end), > > > > > > seemingly to achieve a result similar to what could be done with a > > > > > > union: > > > > > > > > > > > > fs/bcachefs/bcachefs_format.h: > > > > > > > > > > > > struct bkey_packed { > > > > > > __u64 _data[0]; > > > > > > > > > > > > /* Size of combined key and value, in u64s */ > > > > > > __u8 u64s; > > > > > > [...] > > > > > > }; > > > > > > > > > > > > likewise: > > > > > > > > > > > > struct bkey_i { > > > > > > __u64 _data[0]; > > > > > > > > > > > > struct bkey k; > > > > > > struct bch_val v; > > > > > > }; > > I took a glance at the LLVM IR for fs/bcachefs/bset.c, and it defines > struct bkey_packed and bkey_i as: > > %struct.bkey_packed = type { [0 x i64], i8, i8, i8, [0 x i8], [37 x i8] } > %struct.bkey_i = type { [0 x i64], %struct.bkey, %struct.bch_val } > > , which more or less looks as expected, so I don't think it could be > causing problems with KMSAN right now. > Moreover, there are cases in e.g. include/linux/skbuff.h where > zero-length arrays are used for the same purpose, and KMSAN handles > them just fine. > > Yet I want to point out that even GCC discourages the use of > zero-length arrays in the middle of a struct: > https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html, so Clang is not > unique here. > > Regarding the original KMSAN bug, as noted in > https://lore.kernel.org/all/[email protected]/T/, > we might be missing the event of copying data from the disk to > bcachefs structs. > I'd appreciate help from someone knowledgeable about how disk I/O is > implemented in the kernel.
If that was missing I'd expect everything to be breaking. What's the helper that marks memory as initialized?
