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Message-Id: <1639925208.rmnuvpezrd.none@localhost>
Date: Sun, 19 Dec 2021 09:54:32 -0500
From: "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>
To: musl@...ts.openwall.com, Markus Wichmann <nullplan@....net>
Subject: Re: Feasibility of FD_CLOEXEC on all streams
Excerpts from Rich Felker's message of December 18, 2021 12:26 pm:
> On Sat, Dec 18, 2021 at 12:14:15PM -0500, Rich Felker wrote:
>> On Sat, Dec 18, 2021 at 05:33:20PM +0100, Markus Wichmann wrote:
>> > Hi all,
>> >
>> > I was recently reading the source code of popen(), and noticed that it
>> > has to iterate over all open files to close all the open pipe FDs the
>> > child might inherit. And that made me wonder:
>> >
>> > 1. Does POSIX allow for all FILE streams to have FD_CLOEXEC applied by
>> > default?
>>
>> No. Accessing fileno(f) is permissible subject to following the rules
>> for active handle:
>>
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05_01
>>
>> and that entails being able to use them according to the rules for how
>> fds are inherited across exec.
>
> Also, the POSIX spec for fopen is rather explicit:
>
> "[CX] The file descriptor associated with the opened stream shall
> be allocated and opened as if by a call to open() with the
> following flags: ..."
>
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
>
> Rich
>
Playing devil's advocate here, can't the implementation unset FD_CLOEXEC
when fileno is called? This doesn't fix the latter issue, but if that's
the only problem then I would argue that it can be sufficiently covered
by the as-if rule. It also wouldn't fix the popen loop, but would still
add some hardening for poorly written programs.
Cheers,
Alex.
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