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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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/000000000000fd5e7006191f78dc@google.com/
>>>
>>> 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.

Notice how a.y is first set to 1, then a.x[0] is loaded, expecting to
alias with a.y.

This is the same aliasing pattern found in bcachefs, for instance here:

bcachefs_format.h:

struct jset {
[...]
         __u8                    encrypted_start[0];

         __le16                  _read_clock; /* no longer used */
         __le16                  _write_clock;

         /* Sequence number of oldest dirty journal entry */
         __le64                  last_seq;


         struct jset_entry       start[0];
         __u64                   _data[];
} __packed __aligned(8);

where struct jset last_seq field is set by jset_validate():

		jset->last_seq = jset->seq;

and where journal_read_bucket() uses the encrypted_start member as input:

                 ret = bch2_encrypt(c, JSET_CSUM_TYPE(j), journal_nonce(j),
                              j->encrypted_start,
                              vstruct_end(j) - (void *) j->encrypted_start);

Regards,

Mathieu


>>
>>> 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