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fs: stop sharing fs_struct between init_task and pid 1

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Spawn kernel_init (PID 1) via kernel_clone() directly instead of
user_mode_thread(), without CLONE_FS. This gives PID 1 its own private
copy of init_task's fs_struct rather than sharing it.

This is a prerequisite for isolating kthreads in nullfs: when
init_task's fs is later pointed at nullfs, PID 1 must not share it
or init_userspace_fs() would modify init_task's fs as well, defeating
the isolation.

At this stage PID 1 still gets rootfs (a private copy rather than a
shared reference), so there is no functional change.

Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-19-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-06-01 15:56:52 +02:00
parent 09eca26e7e
commit 14adbd5341
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 6 additions and 1 deletions
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7
init/main.c
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@ -671,6 +671,11 @@ static __initdata DECLARE_COMPLETION(kthreadd_done);
static noinline void __ref __noreturn rest_init(void)
{
struct kernel_clone_args init_args = {
.flags = (CLONE_VM | CLONE_UNTRACED),
.fn = kernel_init,
.fn_arg = NULL,
};
struct task_struct *tsk;
int pid;
@ -680,7 +685,7 @@ static noinline void __ref __noreturn rest_init(void)
* the init task will end up wanting to create kthreads, which, if
* we schedule it before we create kthreadd, will OOPS.
*/
pid = user_mode_thread(kernel_init, NULL, CLONE_FS);
pid = kernel_clone(&init_args);
/*
* Pin init on the boot CPU. Task migration is not properly working
* until sched_init_smp() has been run. It will set the allowed