Re: [PATCH v4 44/45] mm: fs: initialize fsdata passed to write_begin/write_end interface

[Al Viro]

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.