Re: [PATCH] [v2] tracing: move __printf() attribute on __ftrace_vbprintk()

[David Laight]

On Tue,  3 Feb 2026 17:45:29 +0100
Arnd Bergmann <arnd@xxxxxxxxxx> wrote:

> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> The sunrpc change to use trace_printk() for debugging caused
> a new warning for every instance of dprintk() in some configurations,
> when -Wformat-security is enabled:
> 
> fs/nfs/getroot.c: In function 'nfs_get_root':
> fs/nfs/getroot.c:90:17: error: format not a string literal and no format arguments [-Werror=format-security]
>    90 |                 nfs_errorf(fc, "NFS: Couldn't getattr on root");
> 
> I've been slowly chipping away at those warnings over time with the
> intention of enabling them by default in the future. While I could not
> figure out why this only happens for this one instance, I see that the
> __trace_bprintk() function is always called with a local variable as
> the format string, rather than a literal.
> 
> Move the __printf(2,3) annotation on this function from the declaration
> to the caller. As this is can only be validated for literals, the
         ^ definition ?

	David

> attribute on the declaration causes the warnings every time, but
> removing it entirely introduces a new warning on the __ftrace_vbprintk()
> definition.
> 
> The format strings still get checked because the underlying literal keeps
> getting passed into __trace_printk() in the "else" branch, which is not
> taken but still evaluated for compile-time warnings.
> 
> Fixes: ec7d8e68ef0e ("sunrpc: add a Kconfig option to redirect dfprintk() output to trace buffer")
> Acked-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> v2: included fix for regression reported by kernel test robot <lkp@xxxxxxxxx>
> ---
>  include/linux/trace_printk.h | 1 -
>  kernel/trace/trace_printk.c  | 1 +
>  2 files changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/trace_printk.h b/include/linux/trace_printk.h
> index bb5874097f24..2670ec7f4262 100644
> --- a/include/linux/trace_printk.h
> +++ b/include/linux/trace_printk.h
> @@ -107,7 +107,6 @@ do {									\
>  		__trace_printk(_THIS_IP_, fmt, ##args);			\
>  } while (0)
>  
> -extern __printf(2, 3)
>  int __trace_bprintk(unsigned long ip, const char *fmt, ...);
>  
>  extern __printf(2, 3)
> diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
> index 29f6e95439b6..48c085fcae7a 100644
> --- a/kernel/trace/trace_printk.c
> +++ b/kernel/trace/trace_printk.c
> @@ -197,6 +197,7 @@ struct notifier_block module_trace_bprintk_format_nb = {
>  	.notifier_call = module_trace_bprintk_format_notify,
>  };
>  
> +__printf(2, 3)
>  int __trace_bprintk(unsigned long ip, const char *fmt, ...)
>  {
>  	int ret;