[PATCH v2] kexec: fix the unexpected kexec_dprintk() macro

[Baoquan He]

On 04/12/24 at 03:28pm, Dave Young wrote:
> On Tue, 9 Apr 2024 at 12:23, Baoquan He <bhe at redhat.com> wrote:
> >
> > Jiri reported that the current kexec_dprintk() always prints out
> > debugging message whenever kexec/kdmmp loading is triggered. That is
> > not wanted. The debugging message is supposed to be printed out when
> > 'kexec -s -d' is specified for kexec/kdump loading.
> >
> > After investigating, the reason is the current kexec_dprintk() takes
> > printk(KERN_INFO) or printk(KERN_DEBUG) depending on whether '-d' is
> > specified. However, distros usually have defaulg log level like below:
> >
> >  [~]# cat /proc/sys/kernel/printk
> >  7       4      1       7
> >
> > So, even though '-d' is not specified, printk(KERN_DEBUG) also always
> > prints out. I thought printk(KERN_DEBUG) is equal to pr_debug(), it's
> > not.
> >
> > Fix it by changing to use pr_info() instead which are expected to work.
> 
> Could you also update the kernel/crash_core.c and
> kernel/crash_reserve.c to include the filename prefix?
> #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

OK, thanks for testing and spotting this. 

When I tested this patch, I only did test kexec reboot functionality.
So the line of printing in kernel/crash_core.c is missed. While there
seems to be no kexec_dprintk() in kernel/crash_reserve.c. Anyway,
adding pr_fmt() is a good idea for debug printing in that file. I will
post a standalone patch to add these.

> 
> Otherwise:
> Reviewed-by: Dave Young <dyoung at redhat.com>

Thanks again.

> 
> >
> > Fixes: cbc2fe9d9cb2 ("kexec_file: add kexec_file flag to control debug printing")
> > Signed-off-by: Baoquan He <bhe at redhat.com>
> > Reported-by: Jiri Slaby <jirislaby at kernel.org>
> > Closes: https://lore.kernel.org/all/4c775fca-5def-4a2d-8437-7130b02722a2@kernel.org
> > ---
> > v1->v2:
> > - Change to use pr_info() only when "kexec -s -d" is specified. With
> >   this change, those debugging message for "kexec -c -d" of kexec_load
> >   will be missed. We'll see if we need add them for kexec_load too, if
> >   someone explicitly requests it.
> >
> >  include/linux/kexec.h | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index 060835bb82d5..f31bd304df45 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -461,10 +461,8 @@ static inline void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages) {
> >
> >  extern bool kexec_file_dbg_print;
> >
> > -#define kexec_dprintk(fmt, ...)                                        \
> > -       printk("%s" fmt,                                        \
> > -              kexec_file_dbg_print ? KERN_INFO : KERN_DEBUG,   \
> > -              ##__VA_ARGS__)
> > +#define kexec_dprintk(fmt, arg...) \
> > +        do { if (kexec_file_dbg_print) pr_info(fmt, ##arg); } while (0)
> >
> >  #else /* !CONFIG_KEXEC_CORE */
> >  struct pt_regs;
> > --
> > 2.41.0
> >
> >
>