[RFC PATCH v1 3/5] tracing: Add __print_untrusted_str()

[Steven Rostedt]

On Fri, 23 May 2025 18:57:39 +0200
Mickaël Salaün <mic at digikod.net> wrote:

> 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.

Hmm, so this can be an issue if this is printed out via seq_file()?

I'm curious to what exactly can be "unsafe" about a string being printed
via "%s"?

I'm not against this change, I just want to understand more about what the
issue is.

> 
> 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>
> ---
>  include/linux/trace_events.h               |  3 ++
>  include/trace/stages/stage3_trace_output.h |  4 +++
>  include/trace/stages/stage7_class_define.h |  1 +
>  kernel/trace/trace_output.c                | 40 ++++++++++++++++++++++
>  4 files changed, 48 insertions(+)
> 
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index fa9cf4292dff..78f543bb7558 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -54,6 +54,9 @@ 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);
> +
> +
>  struct trace_iterator;
>  struct trace_event;
>  
> diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
> index 1e7b0bef95f5..36947ca2abcb 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..bc10b69b755d 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_string
>  #undef __get_buf
>  
>  /*
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index b9ab06c99543..17d576941147 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"
> @@ -297,6 +298,45 @@ 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);
> +
> +	if (!src || WARN_ON(buf_size == 0))

		    WARN_ON_ONCE() please.

-- Steve

> +		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)
>  {