Re: [RFC v1] man/man2/close.2: CAVEATS: Document divergence from POSIX.1-2024

[The 8472 <kernel@infinite-source.de>]

On 24/01/2026 22:39, Rich Felker wrote:
> On Sat, Jan 24, 2026 at 08:34:01PM +0100, The 8472 wrote:
>> On 23/01/2026 01:33, Zack Weinberg wrote:
>>
>> [...]
>>
>>> ERRORS
>>>          EBADF  The fd argument was not a valid, open file descriptor.
>>
>> Unfortunately EBADF from FUSE is passed through unfiltered by the kernel
>> on close[0], that makes it more difficult to reliably detect bugs relating
>> to double-closes of file descriptors.
> 
> Wow, that's a nasty bug. Are the kernel folks not amenable to fixing
> it?

Not when I brought it up last time, no[0]

> I wonder if that could even have security implications. I think
> you could detect these fraudulent EBADFs (albeit not under conditions
> where there's a race bug) by performing fcntl/F_GETFD before close and
> knowing the EBADF from close is fake is fcntl didn't EBADF, but that
> seems like an unreasonable cost to work around FUSE behaving badly.
> 
> Rich

That's pretty much the workaround[1] we use, but due to the extra syscall it's
only done in debug builds.

[0] https://lore.kernel.org/linux-fsdevel/1b946a20-5e8a-497e-96ef-f7b1e037edcb@infinite-source.de/
[1] https://github.com/rust-lang/rust/blob/021fc25b7a48f6051bee1e1f06c7a277e4de1cc9/library/std/src/sys/fs/unix.rs#L981-L999