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Date: Fri, 13 Oct 2023 00:40:32 +0100
From: Gabriel Ravier <gabravier@...il.com>
To: musl@...ts.openwall.com, Rich Felker <dalias@...c.org>,
Markus Wichmann <nullplan@....net>
Subject: Re: aio_close needed in dup2?
On 10/12/23 18:43, Rich Felker wrote:
> On Thu, Oct 12, 2023 at 06:43:20PM +0200, Markus Wichmann wrote:
>> Hi all,
>>
>> I noticed something today: In close(), we call __aio_close() to both
>> prevent AIO from using invalid file descriptors and implement the
>> requirement that outstanding AIO be cancelled. But in dup2() and dup3(),
>> that doesn't happen.
>>
>> POSIX only says that dup2() closes newfd if it already is a valid file
>> descriptor. While not explicitly stated, I can't really find a sensible
>> interpretation of that requirement that is different from "as if by way
>> of close()". POSIX has no concept of closing file descriptors in any
>> other way. And dup3() is an extension function, but I think most
>> programmers will understand it to be an extension of dup2(), so the same
>> argument applies there.
>>
>> So, do we need to call __aio_close() in dup2() and dup3()?
> I'm not sure. Unlike close, which invalidates the fd and makes any
> subsequent use by aio a use-after-close bug (extremely dangerous),
> dup2/dup3 does not invalidate the fd. They change what it refers to.
> While I'm not sure this is sanctioned by POSIX, it would be reasonable
> to want to *atomically replace* an fd that aio is pending on, such
> that the operations happen either on the previously-referenced open
> file description or the new one (but never zero or both).
I can confirm that current POSIX wants dup2 to be atomic:
> All of the following functions shall be atomic with respect to each
other in the effects specified in
POSIX.1-2008 when they operate on regular files or symbolic links:
> [list that includes many functions operating on file descriptors,
including dup2]
- POSIX-1.2008, 2.9.7 *Thread Interactions with Regular File Operations*
The latest draft has similar wording which also includes dup3, in
particular:
> Except where specified otherwise, all of the following functions
shall be atomic with respect to each other in the effects specified in
POSIX.1-202x when they operate on file descriptors that are open, or
being opened, to files in the file hierarchy:
> [list that includes many functions operating on file descriptors,
including dup2 and dup3]
> If two threads each call one of these functions, each call shall
either see all of the specified effects of the other call, or none of
them. The requirement on the close() function shall also apply whenever
a file descriptor is successfully closed, however caused (for example,
as a consequence of calling close(), calling dup2(), or of process
termination).
- POSIX.1-202x
with the specification of `close` including:
> When there is an outstanding cancelable asynchronous I/O operation
against fildes when close() is called, that I/O operation may be
canceled. An I/O operation that is not canceled completes as if the
close() operation had not yet occurred. All operations that are not
canceled shall complete as if the close() blocked until the operations
completed. The close() operation itself need not block awaiting such I/O
completion. Whether any I/O operation is canceled, and which I/O
operation may be canceled upon close(), is implementation-defined.
- POSIX.1-202x
and nothing in the specification of dup3 or dup2 indicating that they
are not atomic.
>
> On the other hand, I don't see a safe way to implement __aio_close
> semantics for dup2/dup3, because you can't know in advance whether
> it's going to succeed, and once it succeeds, you can no longer do the
> work __aio_close would have done. I think you'd have to juggle around
> temporary fds to make it work in any reasonable way.
>
> Most likely, it's just supposed to be UB to replace an fd with pending
> aio operations on it using dup2/dup3. (BTW, dup3 is not just an
> extension, it's POSIX-future, so all this applies to it too.)
>
> Rich
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