linux-kernel - Re: [PATCH 1/4] exec: Change uselib(2) IS

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Message-ID: <20200519132931.3b7yugfv2ajry6y7@wittgenstein>
Date:   Tue, 19 May 2020 15:29:31 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andreas Schwab <schwab@...ux-m68k.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Eric Biggers <ebiggers3@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] exec: Change uselib(2) IS_SREG() failure to EACCES

On Tue, May 19, 2020 at 07:28:46AM -0500, Eric W. Biederman wrote:
> Andreas Schwab <schwab@...ux-m68k.org> writes:
> 
> > On Mai 19 2020, Eric W. Biederman wrote:
> >
> >> I am wondering if there are source trees for libc4 or libc5 around
> >> anywhere that we can look at to see how usage of uselib evolved.
> >
> > libc5 is available from archive.debian.org.
> >
> > http://archive.debian.org/debian-archive/debian/pool/main/libc/libc/libc_5.4.46.orig.tar.gz
> 
> Interesting.
> 
> It appears that the old a.out code to make use of uselib remained in
> the libc5 sources but it was all conditional on the being compiled not
> to use ELF.
> 
> libc5 did provide a wrapper for the uselib system call.
> 
> It appears glibc also provides a wrapper for the uselib system call
> named: uselib@...BC_2.2.5.
> 
> I don't see a glibc header file that provides a declaration for uselib
> though.
> 
> So the question becomes did anyone use those glibc wrappers.

The only software I could find was ski, the ia64 instruction set
emulator, which apparently used to make use of this and when glibc
removed they did:

#define uselib(libname) syscall(__NR_uselib, libname)

but they only define it for the sake of the internal syscall list they
maintain so not actively using it. I just checked, ski is available on
Fedora 31 and Fedora has USELIB disabled.
Codesearch on Debian yields no users that actively use the syscall for
anything.

Christian