[PATCH v2 05/17] tracing: Add __print_untrusted_str()

[Mickaël Salaün]

Steve, Masami, Mathieu, are you ok with this new helper?

On Mon, Apr 06, 2026 at 04:37:03PM +0200, Mickaël Salaün wrote:
> Landlock tracepoints expose filesystem paths and process names
> that may contain spaces, equal signs, or other characters that
> break ftrace field parsing.
> 
> Add a new __print_untrusted_str() helper to safely print strings after
> escaping all special characters, including common separators (space,
> equal sign), quotes, and backslashes.  This transforms a string from an
> untrusted source (e.g. user space) to make it:
> - safe to parse,
> - easy to read (for simple strings),
> - easy to get back the original.
> 
> Cc: Günther Noack <gnoack at google.com>
> Cc: Masami Hiramatsu <mhiramat at kernel.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Cc: Steven Rostedt <rostedt at goodmis.org>
> Cc: Tingmao Wang <m at maowtm.org>
> Signed-off-by: Mickaël Salaün <mic at digikod.net>
> ---
> 
> Changes since v1:
> https://lore.kernel.org/r/20250523165741.693976-4-mic@digikod.net
> - Remove WARN_ON() (pointed out by Steven Rostedt).
> ---
>  include/linux/trace_events.h               |  2 ++
>  include/trace/stages/stage3_trace_output.h |  4 +++
>  include/trace/stages/stage7_class_define.h |  1 +
>  kernel/trace/trace_output.c                | 41 ++++++++++++++++++++++
>  4 files changed, 48 insertions(+)
> 
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index 37eb2f0f3dd8..7f4325d327ee 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -57,6 +57,8 @@ trace_print_hex_dump_seq(struct trace_seq *p, const char *prefix_str,
>  			 int prefix_type, int rowsize, int groupsize,
>  			 const void *buf, size_t len, bool ascii);
>  
> +const char *trace_print_untrusted_str_seq(struct trace_seq *s, const char *str);
> +
>  int trace_raw_output_prep(struct trace_iterator *iter,
>  			  struct trace_event *event);
>  extern __printf(2, 3)
> diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
> index fce85ea2df1c..62e98babb969 100644
> --- a/include/trace/stages/stage3_trace_output.h
> +++ b/include/trace/stages/stage3_trace_output.h
> @@ -133,6 +133,10 @@
>  	trace_print_hex_dump_seq(p, prefix_str, prefix_type,		\
>  				 rowsize, groupsize, buf, len, ascii)
>  
> +#undef __print_untrusted_str
> +#define __print_untrusted_str(str)							\
> +		trace_print_untrusted_str_seq(p, __get_str(str))
> +
>  #undef __print_ns_to_secs
>  #define __print_ns_to_secs(value)			\
>  	({						\
> diff --git a/include/trace/stages/stage7_class_define.h b/include/trace/stages/stage7_class_define.h
> index fcd564a590f4..1164aacd550f 100644
> --- a/include/trace/stages/stage7_class_define.h
> +++ b/include/trace/stages/stage7_class_define.h
> @@ -24,6 +24,7 @@
>  #undef __print_array
>  #undef __print_dynamic_array
>  #undef __print_hex_dump
> +#undef __print_untrusted_str
>  #undef __get_buf
>  
>  /*
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 1996d7aba038..9d14c7cc654d 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -16,6 +16,7 @@
>  #include <linux/btf.h>
>  #include <linux/bpf.h>
>  #include <linux/hashtable.h>
> +#include <linux/string_helpers.h>
>  
>  #include "trace_output.h"
>  #include "trace_btf.h"
> @@ -321,6 +322,46 @@ trace_print_hex_dump_seq(struct trace_seq *p, const char *prefix_str,
>  }
>  EXPORT_SYMBOL(trace_print_hex_dump_seq);
>  
> +/**
> + * trace_print_untrusted_str_seq - print a string after escaping characters
> + * @s: trace seq struct to write to
> + * @src: The string to print
> + *
> + * Prints a string to a trace seq after escaping all special characters,
> + * including common separators (space, equal sign), quotes, and backslashes.
> + * This transforms a string from an untrusted source (e.g. user space) to make
> + * it:
> + * - safe to parse,
> + * - easy to read (for simple strings),
> + * - easy to get back the original.
> + */
> +const char *trace_print_untrusted_str_seq(struct trace_seq *s,
> +					   const char *src)
> +{
> +	int escaped_size;
> +	char *buf;
> +	size_t buf_size = seq_buf_get_buf(&s->seq, &buf);
> +	const char *ret = trace_seq_buffer_ptr(s);
> +
> +	/* Buffer exhaustion is normal when the trace buffer is full. */
> +	if (!src || buf_size == 0)
> +		return NULL;
> +
> +	escaped_size = string_escape_mem(src, strlen(src), buf, buf_size,
> +		ESCAPE_SPACE | ESCAPE_SPECIAL | ESCAPE_NAP | ESCAPE_APPEND |
> +		ESCAPE_OCTAL, " ='\"\\");
> +	if (unlikely(escaped_size >= buf_size)) {
> +		/* We need some room for the final '\0'. */
> +		seq_buf_set_overflow(&s->seq);
> +		s->full = 1;
> +		return NULL;
> +	}
> +	seq_buf_commit(&s->seq, escaped_size);
> +	trace_seq_putc(s, 0);
> +	return ret;
> +}
> +EXPORT_SYMBOL(trace_print_untrusted_str_seq);
> +
>  int trace_raw_output_prep(struct trace_iterator *iter,
>  			  struct trace_event *trace_event)
>  {
> -- 
> 2.53.0
> 
>