linux-kernel - vdso_join_timens() question

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Date:   Thu, 11 Jun 2020 13:02:21 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrei Vagin <avagin@...il.com>, Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org
Subject: vdso_join_timens() question

Hey,

I'm about to finish a patch to add CLONE_NEWTIME support to setns().
Since setns() now allows to attach to a multiple namespaces at the same
time I've also reworked it to be atomic (already upstream). Either all
namespaces are switched or no namespace is switched. All namespaces
basically now have a commit mode after which installation should ideally
not fail anymore. That could work for CLONE_NEWTIME too, I think. The
only blocker to this is vdso_join_timens() which can fail due to
mmap_write_lock_killable().

Is it possible to change this to mmap_write_lock()? So sm like:

diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index ea7c1f0b79df..5c5b4cc61fce 100644
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -144,8 +144,7 @@ int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
        struct mm_struct *mm = task->mm;
        struct vm_area_struct *vma;

-       if (mmap_write_lock_killable(mm))
-               return -EINTR;
+       mmap_write_lock(mm);

        for (vma = mm->mmap; vma; vma = vma->vm_next) {
                unsigned long size = vma->vm_end - vma->vm_start;

vdso_join_timens() is called in two places. Once during fork() and once
during timens_install(). I would only need the mmap_write_lock() change
for the latter. So alternatively we could have:

__vdso_join_timens_unlocked()

and then have/expose:

vdso_join_timens_fork()
{
        if (mmap_write_lock_killable(mm))
                return -EINTR;
	__vdso_join_timens_unlocked()
	mmap_write_unlock(mm);
}

and 

vdso_join_timens_install()
{
        mmap_write_lock(mm);
	__vdso_join_timens_unlocked()
	mmap_write_unlock(mm);
}

Thanks!
Christian