On Mon, Jul 04, 2022 at 09:07:43PM +0100, Matthew Wilcox wrote: > On Fri, Jul 01, 2022 at 04:23:09PM +0200, Alexander Potapenko wrote: > > Functions implementing the a_ops->write_end() interface accept the > > `void *fsdata` parameter that is supposed to be initialized by the > > corresponding a_ops->write_begin() (which accepts `void **fsdata`). > > > > However not all a_ops->write_begin() implementations initialize `fsdata` > > unconditionally, so it may get passed uninitialized to a_ops->write_end(), > > resulting in undefined behavior. > > ... wait, passing an uninitialised variable to a function *which doesn't > actually use it* is now UB? What genius came up with that rule? What > purpose does it serve? "The value we are passing might be utter bollocks, but that way it's obfuscated enough to confuse anyone, compiler included". Defensive progamming, don'cha know? I would suggest a different way to obfuscate it, though - pass const void ** and leave it for the callee to decide whether they want to dereferences it. It is still 100% dependent upon the ->write_end() being correctly matched with ->write_begin(), with zero assistance from the compiler, but it does look, er, safer. Or something. Of course, a clean way to handle that would be to have ->write_begin() return a partial application of foo_write_end to whatever it wants for fsdata, to be evaluated where we would currently call ->write_end(). _That_ could be usefully typechecked, but... we don't have usable partial application.
