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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Szabolcs Nagy' <nsz@port70.net>,
        Christian Brauner <christian.brauner@ubuntu.com>
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Subject: RE: [PATCH v5 1/3] open: add close_range()
Thread-Topic: [PATCH v5 1/3] open: add close_range()
Thread-Index: AQHWO0lrkRMjBAtF4kyptmA+TsanYKjNIo+Q
Date:   Sun, 7 Jun 2020 13:22:48 +0000
Message-ID: <46cd4282193641bf801fb150d482c624@AcuMS.aculab.com>
References: <20200602204219.186620-1-christian.brauner@ubuntu.com>
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 <20200605145549.GC673948@port70.net>
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From: Szabolcs Nagy
> Sent: 05 June 2020 15:56
...
> currently there is no libc interface contract in place that
> says which calls may use libc internal fds e.g. i've seen
> 
>   openlog(...) // opens libc internal syslog fd
>   ...
>   fork()
>   closefrom(...) // close syslog fd
>   open(...) // something that reuses the closed fd
>   syslog(...) // unsafe: uses the wrong fd
>   execve(...)
> 
> syslog uses a libc internal fd that the user trampled on and
> this can go bad in many ways depending on what libc apis are
> used between closefrom (or equivalent) and exec.

It is, of course, traditional that daemons only call
close(0); close(1); close(2);
Took us ages to discover that a misspelt fprintf()
was adding data to the stdout buffer and eventually
flushing 10k of ascii text into an inter-process pipe
that had a 32bit field for 'message extension length'.

FWIW isn't syslog() going to go badly wrong after fork()
anyway?
Unless libc's fork() calls closelog().

	David

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