Re: [PATCH] trace_printk: replace _______STR with __UNIQUE_ID(STR)

[Qian-Yu Lin <tiffany019230@gmail.com>]

On Wed, Apr 29, 2026 at 10:47:28PM +0100, David Laight wrote:
> On Wed, 29 Apr 2026 13:42:26 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Thu, 30 Apr 2026 00:57:07 +0800
> > Qian-Yu Lin <tiffany019230@gmail.com> wrote:
> > 
> > > The macro trace_printk() uses a hardcoded identifier _______STR
> > > within a statement expression, which can lead to variable name
> > > shadowing if a caller happens to use the same name in its scope.  
> > 
> > Has this ever been a problem?
> > 
> > > 
> > > Following the pattern in commit 24ba53017e18 ("rcu: Replace ________p1
> > > and _________p1 with __UNIQUE_ID(rcu)") and commit 589a9785ee3a
> > >  ("min/max: remove sparse warnings when they're nested"), replace the
> > >  hardcoded identifier with __UNIQUE_ID(STR).
> 
> min and max do get nested - so it is important that the 'local'
> variables have different names - otherwise you can get invalid expansions.
> 
> No one is going to have: trace_printk(fmt1, trace_printk(ftm2, ...), ...)
> it just doesn't make sense.
> 
> There is a slight problem the ____________________________STR might
> be used by an entirely different #define.
> Just using _trace_printk_str[] would make this pretty unlikely.
> It would also have the advantage of making the .i file a bit easier
> to read, all the UNIQUE names in min/max output make it really hard
> to see what the output actually means.
> 

Thank you for the suggestion. However, _trace_printk_str still carries
a theoretical shadowing risk if a caller happens to use the same name.

> > > 
> > > Since __UNIQUE_ID() must be expanded once to remain consistent across
> > > declaration and sizeof() within the statement expression, introduce a
> > > nested helper macro ___trace_printk.  
> > 
> > Hmm, so we are replacing one name with underscores with another name
> > with underscores?
> > 
> > > 
> > > Signed-off-by: Qian-Yu Lin <tiffany019230@gmail.com>
> > > ---
> > >  include/linux/trace_printk.h | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/include/linux/trace_printk.h b/include/linux/trace_printk.h
> > > index 2670ec7f4262..060eccb40838 100644
> > > --- a/include/linux/trace_printk.h
> > > +++ b/include/linux/trace_printk.h
> > > @@ -2,6 +2,7 @@
> > >  #ifndef _LINUX_TRACE_PRINTK_H
> > >  #define _LINUX_TRACE_PRINTK_H
> > >  
> > > +#include <linux/compiler.h>  
> > 
> > People are already saying that trace_printk.h slows down the compile.
> > Does this add any overhead to the compile?
> 
> A little - nothing is free.
> 
> 	David
> 

Indeed. I measured compile time of kernel/trace/ring_buffer_benchmark.o
after make clean on an x86_64 machine running Ubuntu 24.04 LTS:

  - Original _______STR:                 49.8s
  - v1 with __UNIQUE_ID (compiler.h):    53.5s
  - compound literal (no extra include): 33.2s

A compound literal eliminates the local variable entirely, removing
the shadowing risk without any extra include or compile overhead:

#define trace_printk(fmt, ...)                          \
do {                                                    \
    if (sizeof((char[])                             \
        {__stringify((__VA_ARGS__))}) > 3)      \
        do_trace_printk(fmt, ##__VA_ARGS__);    \
    else                                            \
        trace_puts(fmt);                        \
} while (0)

Qian-Yu

> > 
> > -- Steve
> > 
> > 
> > >  #include <linux/compiler_attributes.h>
> > >  #include <linux/instruction_pointer.h>
> > >  #include <linux/stddef.h>
> > > @@ -84,15 +85,18 @@ do {									\
> > >   * let gcc optimize the rest.
> > >   */
> > >  
> > > -#define trace_printk(fmt, ...)				\
> > > +#define ___trace_printk(fmt, str, ...)				\
> > >  do {							\
> > > -	char _______STR[] = __stringify((__VA_ARGS__));	\
> > > -	if (sizeof(_______STR) > 3)			\
> > > +	char str[] = __stringify((__VA_ARGS__));	\
> > > +	if (sizeof(str) > 3)			\
> > >  		do_trace_printk(fmt, ##__VA_ARGS__);	\
> > >  	else						\
> > >  		trace_puts(fmt);			\
> > >  } while (0)
> > >  
> > > +#define trace_printk(fmt, ...) \
> > > +	___trace_printk(fmt, __UNIQUE_ID(str), ##__VA_ARGS__)
> > > +
> > >  #define do_trace_printk(fmt, args...)					\
> > >  do {									\
> > >  	static const char *trace_printk_fmt __used			\  
> > 
> > 
>