Re: [PATCH v12 33/40] dyndbg: Ignore additional arguments from pr_fmt

[Petr Mladek]

On Tue 2026-10-27 13:22:53, David Laight wrote:
> On Tue, 13 Oct 2026 13:33:10 +0200
> Petr Mladek <pmladek@xxxxxxxx> wrote:
> 
> > On Mon 2026-10-12 20:37:04, Jim Cromie via B4 Relay wrote:
> > > From: Philipp Hahn <phahn-oss@xxxxxx>
> > > 
> > > pr_fmt can be used to add a common prefix to any output from a module:
> > > 	#define pr_fmt(fmt) KBUILD_MODNAME ".%s " fmt, __func__
> > > 
> > > But adding additional arguments breaks dynamic debug:  
> > > > error: macro "DEFINE_DYNAMIC_DEBUG_METADATA_CLS" passed 4 arguments, but takes just 3
> > > > |         pr_debug_ratelimited("%s", "Hello world!");
> > > > |                                                  ^
> > > > note: macro "DEFINE_DYNAMIC_DEBUG_METADATA_CLS" defined here
> > > > | #define DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, cls, fmt)       \
> > > > |
> > > > error: ‘DEFINE_DYNAMIC_DEBUG_METADATA_CLS’ undeclared (first use in this function)
> > > > |         DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, _DPRINTK_CLASS_DFLT, fmt)
> > > > |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
> > 
> > I wonder what this error about undeclared macro means.
> > 
> > > > note: in expansion of macro ‘DEFINE_DYNAMIC_DEBUG_METADATA’
> > > > |         DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, pr_fmt(fmt));         \
> > > > |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > note: in expansion of macro ‘pr_debug_ratelimited’
> > > > |         pr_debug_ratelimited("%s", "Hello world!");
> > > > |         ^~~~~~~~~~~~~~~~~~~~  
> > > 
> > > Add an additional ', ...' to DEFINE_DYNAMIC_DEBUG_METADATA_CLS to slurp
> > > any additional argument, which `pr_fmt` might add.
> > > 
> > > --- a/include/linux/dynamic_debug.h
> > > +++ b/include/linux/dynamic_debug.h
> > > @@ -351,7 +351,7 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
> > >  		dump_stack();					\
> > >  }
> > >  
> > > -#define DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, cls, fmt)	\
> > > +#define DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, cls, fmt, ...)	\
> > >  	static struct _ddebug  __aligned(8)			\
> > >  	__section("__dyndbg_descs") name = {			\
> > >  		.modname = KBUILD_MODNAME,			\
> > > diff --git a/include/linux/printk.h b/include/linux/printk.h
> > > index f594c1266bfd..5d701952e739 100644
> > > --- a/include/linux/printk.h
> > > +++ b/include/linux/printk.h
> > > @@ -742,7 +742,7 @@ do {									\
> > >  	static DEFINE_RATELIMIT_STATE(_rs,				\
> > >  				      DEFAULT_RATELIMIT_INTERVAL,	\
> > >  				      DEFAULT_RATELIMIT_BURST);		\
> > > -	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, pr_fmt(fmt));		\
> > > +	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\  
> > 
> > IMHO, this changes the existing behavior. The prefix added by
> > pr_fmt() will no longer be stored in DEFINE_DYNAMIC_DEBUG_METADATA().
> > Is this intentional, please?
> > 
> > Anyway, if we do not longer pass pr_fmt(fmt) then we should not
> > need to add "..." into DEFINE_DYNAMIC_DEBUG_METADATA_CLS().
> > 
> > By other words, we newly pass just the format "fmt" without the parametes.
> > The 4th parameter was added by pr_fmt().
> > 
> > 
> > IMHO, the right solution is to keep pr_fmt(fmt) here and extend both
> > BEFINE_DYNAMIC_DEBUG_METADATA() and
> > DEFINE_DYNAMIC_DEBUG_METADATA_CLS()
> > to accept var args. I would expect something like:
> > 
> > #define DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, cls, fmt, ...)	\
> > ...
> > 
> > #define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt, ...)		\
> > 	DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, _DPRINTK_CLASS_DFLT, fmt, ##__VA_ARGS__)
> > 
> > 
> > or maybe, we could "eat" the extra parameters in
> > DEFINE_DYNAMIC_DEBUG_METADATA. It would mean to keep
> > DEFINE_DYNAMIC_DEBUG_METADATA_CLS() as is and just add the "..." into
> > DEFINE_DYNAMIC_DEBUG_METADATA(), aka
> > 
> > #define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt, ...)		\
> > 	DEFINE_DYNAMIC_DEBUG_METADATA_CLS(name, _DPRINTK_CLASS_DFLT, fmt)
> > 
> > 
> > Or do I miss something, please?
> 
> Don't think that will work.
> Won't you end up with ".%s" fmt as the string without the extra argument.
> So you need to keep the expansion of pr_fmt() together until is is finally
> substituted into a printf-like call.
> It is possible to enclose it in () so it is passed as one parameter and
> remove them on the final expansion.

My understanding was that the format is stored in the metadata as is.
I mean that the string is not formatted in this code path. So that
the parameter values are or can be ignored. We just need to make
the compiler happy.

But I might miss something.

Best Regards,
Petr