Re: Use of zero-length arrays in bcachefs structures inner fields

[Mathieu Desnoyers]

[ 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/000000000000fd5e7006191f78dc@xxxxxxxxxx/
> >
> > 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;
> > };
> >
> > (and there are many more examples of this pattern in bcachefs)
> >
> > AFAIK, the C11 standard states that array declarator constant expression
> >
> > Effectively, we can verify that this code triggers an undefined behavior
> > with:
> >
> > #include <stdio.h>
> >
> > struct z {
> >          int x[0];
> >          int y;
> >          int z;
> > } __attribute__((packed));
> >
> > int main(void)
> > {
> >          struct z a;
> >
> >          a.y = 1;
> >          printf("%d\n", a.x[0]);
> > }
> > delimited by [ ] shall have a value greater than zero.
>
> Yet another example of the C people going absolutely nutty with
> everything being undefined. Look, this isn't ok, we need to get work
> done, and I've already wasted entirely too much time on ZLA vs. flex
> array member nonsense.
>
> There's a bunch of legit uses for zero length arrays, and your example,
> where we're not even _assigning_ to x, is just batshit. Someone needs to
> get his head examined.
>
> > So I wonder if the issue reported by KMSAN could be caused by this
> > pattern ?
>
> Possibly; the KMSAN errors I've been looking at do look suspicious. But
> it sounds like we need a real fix that involves defining proper
> semantics, not compiler folks giving up and saying 'aiee!'.
>
> IOW, clang/KMSAN are broken if they simply choke on a zero length array
> being present.

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com