Issue with 'Unknown Error: -512'

[Robert Watson]

On Mon, 18 Jul 2011, Andriy Gapon wrote:

>> In recent branches (confirmed with 224119) builds compiled with clang 
>> happen to throw 'Unknown error: -512' in a lot of places, making the system 
>> unusable. (Untested on gcc compiled systems). Originally I thought the 
>> problem was with specific programs, then I narrowed it down to file I/O, 
>> and now I've narrowed it down to open() with O_TRUNC. Without O_TRUNC there 
>> seems to be no issues whatsoever. With O_TRUNC on open() it fails with that 
>> 'Unknown error: -512' every other time you run the program. Common issues, 
>> portsnap is affected, making it impossible to fetch/extract ports. As well 
>> as redirecting output in shells eg `echo 'hi' > test` fails every other 
>> try. You have the same issue with text editors like `edit` where it fails 
>> every other save. There are no issues with `echo 'hi' >> test` as there is 
>> no O_TRUNC, it only seems to be an O_TRUNC error.
>>
>> Any tips? Otherwise I'll be looking into this today myself.
>
> Just a hint that you could try using DTrace syscall and fbt providers to see 
> where in kernel (if in kernel) that -512 return value originates.

Jon Anderson spotted that here during some Capsicum work -- initially we were 
concerned it was a local patch, but it sounds like it might be less local.  I 
think he saw it on calls to open(2) as well, and I couldn't help but wonder 
(given its recent arrival) if it was an outcome of the change to break falloc 
into two parts, leading to some or another problematic handling of file 
descriptor numbers.  I.e., it's not so much that -512 is being returned, as a 
number that's a bad file descriptor.  (Although now having seen 512 twice on 
two different machines, that particular explanation seems less credible). 
Perhaps this is indeed unrelated to Capsicum, and triggered by a clang bug or 
something else.

I've CC'd Jon, maybe he has gained further insight since we chatted.

Robert