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Date: Sun, 19 Dec 2021 09:54:32 -0500
From: "Alex Xu (Hello71)" <>
To:, Markus Wichmann <>
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:
>> 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: ..."
> 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.


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