kcsan, debugfs: Move debugfs file creation out of early init
diff mbox series

Message ID 20210303093845.2743309-1-elver@google.com
State Accepted
Commit e36299efe7d749976fbdaaf756dee6ef32543c2c
Headers show
Series
  • kcsan, debugfs: Move debugfs file creation out of early init
Related show

Commit Message

Marco Elver March 3, 2021, 9:38 a.m. UTC
Commit 56348560d495 ("debugfs: do not attempt to create a new file
before the filesystem is initalized") forbids creating new debugfs files
until debugfs is fully initialized. This breaks KCSAN's debugfs file
creation, which happened at the end of __init().

There is no reason to create the debugfs file during early
initialization. Therefore, move it into a late_initcall() callback.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: stable <stable@vger.kernel.org>
Fixes: 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
Signed-off-by: Marco Elver <elver@google.com>
---
I've marked this for 'stable', since 56348560d495 is also intended for
stable, and would subsequently break KCSAN in all stable kernels where
KCSAN is available (since 5.8).
---
 kernel/kcsan/core.c    | 2 --
 kernel/kcsan/debugfs.c | 4 +++-
 kernel/kcsan/kcsan.h   | 5 -----
 3 files changed, 3 insertions(+), 8 deletions(-)

Comments

Greg KH March 3, 2021, 9:57 a.m. UTC | #1
On Wed, Mar 03, 2021 at 10:38:45AM +0100, Marco Elver wrote:
> Commit 56348560d495 ("debugfs: do not attempt to create a new file
> before the filesystem is initalized") forbids creating new debugfs files
> until debugfs is fully initialized. This breaks KCSAN's debugfs file
> creation, which happened at the end of __init().

How did it "break" it?  The files shouldn't have actually been created,
right?

> There is no reason to create the debugfs file during early
> initialization. Therefore, move it into a late_initcall() callback.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: stable <stable@vger.kernel.org>
> Fixes: 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> I've marked this for 'stable', since 56348560d495 is also intended for
> stable, and would subsequently break KCSAN in all stable kernels where
> KCSAN is available (since 5.8).

No objection from me, just odd that this actually fixes anything :)

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Marco Elver March 3, 2021, 10:27 a.m. UTC | #2
On Wed, 3 Mar 2021 at 11:24, Greg KH <gregkh@linuxfoundation.org> wrote:
> On Wed, Mar 03, 2021 at 11:18:06AM +0100, Marco Elver wrote:
> > On Wed, 3 Mar 2021 at 10:57, Greg KH <gregkh@linuxfoundation.org> wrote:
> > >
> > > On Wed, Mar 03, 2021 at 10:38:45AM +0100, Marco Elver wrote:
> > > > Commit 56348560d495 ("debugfs: do not attempt to create a new file
> > > > before the filesystem is initalized") forbids creating new debugfs files
> > > > until debugfs is fully initialized. This breaks KCSAN's debugfs file
> > > > creation, which happened at the end of __init().
> > >
> > > How did it "break" it?  The files shouldn't have actually been created,
> > > right?
> >
> > Right, with 56348560d495 the debugfs file isn't created anymore, which
> > is the problem. Before 56348560d495 the file exists (syzbot wants the
> > file to exist.)
> >
> > > > There is no reason to create the debugfs file during early
> > > > initialization. Therefore, move it into a late_initcall() callback.
> > > >
> > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > > Cc: stable <stable@vger.kernel.org>
> > > > Fixes: 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > > > ---
> > > > I've marked this for 'stable', since 56348560d495 is also intended for
> > > > stable, and would subsequently break KCSAN in all stable kernels where
> > > > KCSAN is available (since 5.8).
> > >
> > > No objection from me, just odd that this actually fixes anything :)
> >
> > 56348560d495 causes the file to just not be created if we try to
> > create at the end of __init(). Having it created as late as
> > late_initcall() gets us the file back.
> >
> > When you say "fixes anything", should the file be created even though
> > it's at the end of __init()? Perhaps I misunderstood what 56348560d495
> > changes, but I verified it to be the problem by reverting (upon which
> > the file exists as expected).
>
> All my change did is explicitly not allow you to create a file if
> debugfs had not been initialized.  If you tried to do that before, you
> should have gotten an error from the vfs layer that the file was not
> created, as otherwise how would it have succeeded?
>
> I just moved the check up higher in the "stack" to the debugfs code, and
> not relied on the vfs layer to do a lot of work only to reject things
> later on.
>
> So there "should" not have been any functional change with this patch.
> If there was, then something is really odd as how can the vfs layer
> create a file for a filesystem _before_ that filesystem has been
> registered with the vfs layer?

Ah, I see. I do confirm that the file has been created until
56348560d495, without any errors.

Thanks,
-- Marco

Patch
diff mbox series

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 8c3867640c21..45c821d4e8bd 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -644,8 +644,6 @@  void __init kcsan_init(void)
 
 	BUG_ON(!in_task());
 
-	kcsan_debugfs_init();
-
 	for_each_possible_cpu(cpu)
 		per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
 
diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
index c837ce6c52e6..c1dd02f3be8b 100644
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -266,7 +266,9 @@  static const struct file_operations debugfs_ops =
 	.release = single_release
 };
 
-void __init kcsan_debugfs_init(void)
+static void __init kcsan_debugfs_init(void)
 {
 	debugfs_create_file("kcsan", 0644, NULL, NULL, &debugfs_ops);
 }
+
+late_initcall(kcsan_debugfs_init);
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 594a5dd4842a..9881099d4179 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -31,11 +31,6 @@  extern bool kcsan_enabled;
 void kcsan_save_irqtrace(struct task_struct *task);
 void kcsan_restore_irqtrace(struct task_struct *task);
 
-/*
- * Initialize debugfs file.
- */
-void kcsan_debugfs_init(void);
-
 /*
  * Statistics counters displayed via debugfs; should only be modified in
  * slow-paths.