Re: [PATCH] nextfd(2)

[KOSAKI Motohiro]

>>> But it still has the same braindamage: one system call per loop
>>> invocation, and we can do better.  I would much rather see fdwalk() in SUS.
>>
>> Why would we bother to do better?
>>
>> System calls are cheap, and usually you actually do want to do
>> something about the fd, so you actually want to iterate over them.
>>
>> I'd much rather have simple cheap interfaces than anything else. If
>> SuS has a F_NEXT fcntl, let's just do that thing. Much simpler than
>> doing something more complex and then just having to emulate the
>> simple thing in user space anyway.
>>
>> If a standard interface exists, we should just use it.
>>
>
> I went back and looked at the post, and also the discussion on the SUS
> mailing list.
>
> The proposal for FD_NEXT was rejected with some serious vitriol.
>
> fdwalk() was considered just more palatable since there is an existing
> implementation (in Solaris) and since it might be possible to provide a
> way to hide specific fds from fdwalk(), but a much bigger issue raised
> is that *ALL* of these interfaces are inherently broken.  Closing random
> file descriptors is:
>
> a) inherently racy in a multithreaded environment;

I would say two things. 1) I know and I agree we _can_ misuse the interface.
but many already existed interface also can be misused. 2) As I
already explained
this can be used correctly.

So, I have a question. Why do you bother a possibility of misuse? Of
if you didn't point
out misuse, can you please point out a real world use case of multi
threads + fd interation?


> b) unsafe because there might be file descriptors used by libc itself.

I agree this. Even though almost developer don't use libc message catalogue and
we can avoid such issue by using nextfd() + fcntl(O_CLOEXEC).


> Instead, from the resolution text:
>
>> Therefore, the rest of this proposal seeks to document the problem
>> with closing arbitrary file descriptors, and a new bugid will be
>> opened to propose standardizing some recent interfaces and interface
>> extensions first appearing in Linux (new interfaces such as pipe2( ),
>> accept4( ), mkostemps( ), ..., and extensions like fopen(,"we")) to
>> guarantee the atomic creation of file descriptors with the cloexec
>> bit already set, as was already done in the standard with O_CLOEXEC
>> in open( ) and F_DUPFD_CLOEXEC in fcntl( ). See also 0000368 for
>> a related proposal to require CLOEXEC on hidden descriptors.
>
> I say we ask the new glibc people to provide fdwalk() since it already
> has an implementation history (and because it can be implemented without
> new system calls, thereby working on old kernels), but the *big*
> takeaway from this is that if there is way to create a file descriptor
> so that it doesn't have O_CLOEXEC set from the very beginning, *that* is
> what we need to fix.

Yeah, I don't think fdwalk() is problematic. It's an option if I
understand Alexey's mail
correctly. but I disagree almost all developers should fix a design
and rewrite their
applications. In theory, they can avoid glibc or they can rewrite all
of their code or
avoid linux. but there is one problem. unrealistic.
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