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

[Rafael J. Wysocki]

On Mon, Aug 7, 2023 at 10:34 AM Vlastimil Babka <vbabka at suse.cz> wrote:
>
> 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 at suse.cz>
> Reviewed-by: Christoph Hellwig <hch at lst.de>
> Acked-by: Greg Kroah-Hartman <gregkh at 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--;
> --

Applied as 6.5-rc material, thanks!