[PATCH for 6.5 regression] PM: hibernate: fix resume_store() return value when hibernation not available

[Vlastimil Babka <vbabka@suse.cz>]

On a laptop with hibernation set up but not actively used, and with
secure boot and lockdown enabled kernel, 6.5-rc1 gets stuck on boot with
the following repeated messages:

  A start job is running for Resume from hibernation using device /dev/system/swap (24s / no limit)
  lockdown_is_locked_down: 25311154 callbacks suppressed
  Lockdown: systemd-hiberna: hibernation is restricted; see man kernel_lockdown.7
  ...

Checking the resume code leads to commit cc89c63e2fe3 ("PM: hibernate:
move finding the resume device out of software_resume") which
inadvertently changed the return value from resume_store() to 0 when
!hibernation_available(). This apparently translates to userspace
write() returning 0 as in number of bytes written, and userspace looping
indefinitely in the attempt to write the intended value.

Fix this by returning the full number of bytes that were to be written,
as that's what was done before the commit.

Fixes: cc89c63e2fe3 ("PM: hibernate: move finding the resume device out of software_resume")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Resend with review/ack tags added and not buried in the thread.
Link: https://lore.kernel.org/all/2cfa5f55-1d68-8a4f-d049-13f42e0d1484@suse.cz/

 kernel/power/hibernate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
index e1b4bfa938dd..2b4a946a6ff5 100644
--- a/kernel/power/hibernate.c
+++ b/kernel/power/hibernate.c
@@ -1166,7 +1166,7 @@ static ssize_t resume_store(struct kobject *kobj, struct kobj_attribute *attr,
 	int error;
 
 	if (!hibernation_available())
-		return 0;
+		return n;
 
 	if (len && buf[len-1] == '\n')
 		len--;
-- 
2.41.0