linux-kernel - Re: [RFC][CFT] selftest for permission checks in mount propagation changes

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Message-ID: <20250814063745.GP222315@ZenIV>
Date: Thu, 14 Aug 2025 07:37:45 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: linux-fsdevel@...r.kernel.org
Cc: Tycho Andersen <tycho@...ho.pizza>, Andrei Vagin <avagin@...gle.com>,
	Andrei Vagin <avagin@...il.com>,
	Christian Brauner <brauner@...nel.org>,
	Pavel Tikhomirov <snorcht@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, criu@...ts.linux.dev,
	Linux API <linux-api@...r.kernel.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [RFC][CFT] selftest for permission checks in mount propagation
 changes

> void do_unshare(void)
> {
> 	FILE *f;
> 	uid_t uid = geteuid();
> 	gid_t gid = getegid();
> 	unshare(CLONE_NEWNS|CLONE_NEWUSER);
> 	f = fopen("/proc/self/uid_map", "w");
> 	fprintf(f, "0 %d 1", uid);
> 	fclose(f);
> 	f = fopen("/proc/self/setgroups", "w");
> 	fprintf(f, "deny");
> 	fclose(f);
> 	f = fopen("/proc/self/gid_map", "w");
> 	fprintf(f, "0 %d 1", gid);
> 	fclose(f);
> 	mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL);
> }

This obviously needs error checking - in this form it won't do
anything good without userns enabled (coredump on the first
fprintf() in there, since there won't be /proc/self/uid_map);
should probably just report CLONE_NEWUSER failure, warn about
skipped tests, fall back to unshare(CLONE_NEWNS) and skip
everything in in_child()...