RE: [PATCH v2 2/3] tracing: Add "(fault)" name injection to kernel probes

[David Laight <David.Laight@ACULAB.COM>]

From: Steven Rostedt
> Sent: 12 October 2022 11:41
> 
> Have the specific functions for kernel probes that read strings to inject
> the "(fault)" name directly. trace_probes.c does this too (for uprobes)
> but as the code to read strings are going to be used by synthetic events
> (and perhaps other utilities), it simplifies the code by making sure those
> other uses do not need to implement the "(fault)" name injection as well.
> 
> Cc: stable@vger.kernel.org
> Fixes: bd82631d7ccdc ("tracing: Add support for dynamic strings to synthetic events")
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/trace_probe_kernel.h | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/trace/trace_probe_kernel.h b/kernel/trace/trace_probe_kernel.h
> index 1d43df29a1f8..77dbd9ff9782 100644
> --- a/kernel/trace/trace_probe_kernel.h
> +++ b/kernel/trace/trace_probe_kernel.h
> @@ -2,6 +2,8 @@
>  #ifndef __TRACE_PROBE_KERNEL_H_
>  #define __TRACE_PROBE_KERNEL_H_
> 
> +#define FAULT_STRING "(fault)"
> +
>  /*
>   * This depends on trace_probe.h, but can not include it due to
>   * the way trace_probe_tmpl.h is used by trace_kprobe.c and trace_eprobe.c.
> @@ -13,8 +15,16 @@ static nokprobe_inline int
>  kern_fetch_store_strlen_user(unsigned long addr)
>  {
>  	const void __user *uaddr =  (__force const void __user *)addr;
> +	int ret;
> 
> -	return strnlen_user_nofault(uaddr, MAX_STRING_SIZE);
> +	ret = strnlen_user_nofault(uaddr, MAX_STRING_SIZE);
> +	/*
> +	 * strnlen_user_nofault returns zero on fault, insert the
> +	 * FAULT_STRING when that occurs.
> +	 */
> +	if (ret <= 0)
> +		return strlen(FAULT_STRING) + 1;
> +	return ret;
>  }

Isn't that going to do the wrong thing if the user
string is valid memory but just zero length??

	David

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