linux-kernel - RE: [PATCH v5 1/3] open: add close

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Message-ID: <46cd4282193641bf801fb150d482c624@AcuMS.aculab.com>
Date:   Sun, 7 Jun 2020 13:22:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Szabolcs Nagy' <nsz@...t70.net>,
        Christian Brauner <christian.brauner@...ntu.com>
CC:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kyle Evans <self@...e-evans.net>,
        Victor Stinner <victor.stinner@...il.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "ldv@...linux.org" <ldv@...linux.org>
Subject: RE: [PATCH v5 1/3] open: add close_range()

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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