shell: Add synchronization for prompt-string access in shell

Resolved a data race in shell.c by copying the user-provided
prompt-string into a private buffer within the shell, ensuring
proper synchronization with the shell-thread.

Fixes: #64972

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>

include/zephyr/shell/shell.h

@@ -26,6 +26,10 @@
 extern "C" {
 #endif
 
+#ifndef CONFIG_SHELL_PROMPT_BUFF_SIZE
+#define CONFIG_SHELL_PROMPT_BUFF_SIZE 0
+#endif
+
 #ifndef CONFIG_SHELL_CMD_BUFF_SIZE
 #define CONFIG_SHELL_CMD_BUFF_SIZE 0
 #endif
@@ -779,7 +783,11 @@ enum shell_signal {
  * @brief Shell instance context.
  */
 struct shell_ctx {
-	const char *prompt; /*!< shell current prompt. */
+#if defined(CONFIG_SHELL_PROMPT_CHANGE) && CONFIG_SHELL_PROMPT_CHANGE
+	char prompt[CONFIG_SHELL_PROMPT_BUFF_SIZE]; /*!< shell current prompt. */
+#else
+	const char *prompt;
+#endif
 
 	enum shell_state state; /*!< Internal module state.*/
 	enum shell_receive_state receive_state;/*!< Escape sequence indicator.*/

subsys/shell/Kconfig

@@ -55,6 +55,24 @@ config SHELL_BACKSPACE_MODE_DELETE
 	  Some terminals send code: 0x08 (backspace) other 0x7F (delete). When
 	  this option is set shell will expect 0x7F for backspace key.
 
+config SHELL_PROMPT_CHANGE
+	bool "Allow prompt change in runtime"
+	default y if !SHELL_MINIMAL
+	help
+	  Allow for the modification of the shell prompt at runtime.
+	  Enabling this will allocate additional RAM memory where
+	  the string of the prompt will be stored.
+
+config SHELL_PROMPT_BUFF_SIZE
+	int "Shell prompt buffer size"
+	depends on SHELL_PROMPT_CHANGE
+	range 2 40
+	default 10 if SHELL_MINIMAL
+	default 20
+	help
+	  Maximum prompt size in bytes. One byte is reserved for the string
+	  terminator character.
+
 config SHELL_CMD_BUFF_SIZE
 	int "Shell command buffer size"
 	default 128 if SHELL_MINIMAL

subsys/shell/shell.c

@@ -1208,7 +1208,6 @@ static int instance_init(const struct shell *sh,
 			(sh->shell_flag == SHELL_FLAG_OLF_CRLF));
 
 	memset(sh->ctx, 0, sizeof(*sh->ctx));
-	sh->ctx->prompt = sh->default_prompt;
 	if (CONFIG_SHELL_CMD_ROOT[0]) {
 		sh->ctx->selected_cmd = root_cmd_find(CONFIG_SHELL_CMD_ROOT);
 	}
@@ -1235,7 +1234,13 @@ static int instance_init(const struct shell *sh,
 					CONFIG_SHELL_DEFAULT_TERMINAL_WIDTH;
 	sh->ctx->vt100_ctx.cons.terminal_hei =
 					CONFIG_SHELL_DEFAULT_TERMINAL_HEIGHT;
+
+#if defined(CONFIG_SHELL_PROMPT_CHANGE) && CONFIG_SHELL_PROMPT_CHANGE
+	shell_prompt_change(sh, sh->default_prompt);
+#else
+	sh->ctx->prompt = sh->default_prompt;
 	sh->ctx->vt100_ctx.cons.name_len = z_shell_strlen(sh->ctx->prompt);
+#endif
 
 	/* Configure backend according to enabled shell features and backend
 	 * specific settings.
@@ -1614,15 +1619,35 @@ void shell_hexdump(const struct shell *sh, const uint8_t *data, size_t len)
 
 int shell_prompt_change(const struct shell *sh, const char *prompt)
 {
+#if defined(CONFIG_SHELL_PROMPT_CHANGE) && CONFIG_SHELL_PROMPT_CHANGE
 	__ASSERT_NO_MSG(sh);
 
 	if (prompt == NULL) {
 		return -EINVAL;
 	}
-	sh->ctx->prompt = prompt;
+
-	sh->ctx->vt100_ctx.cons.name_len = z_shell_strlen(prompt);
+	static const size_t mtx_timeout_ms = 20;
+	size_t prompt_length = z_shell_strlen(prompt);
+
+	if (k_mutex_lock(&sh->ctx->wr_mtx, K_MSEC(mtx_timeout_ms))) {
+		return -EBUSY;
+	}
+
+	if ((prompt_length + 1 > CONFIG_SHELL_PROMPT_BUFF_SIZE) || (prompt_length == 0)) {
+		k_mutex_unlock(&sh->ctx->wr_mtx);
+		return -EINVAL;
+	}
+
+	strcpy(sh->ctx->prompt, prompt);
+
+	sh->ctx->vt100_ctx.cons.name_len = prompt_length;
+
+	k_mutex_unlock(&sh->ctx->wr_mtx);
 
 	return 0;
+#else
+	return -EPERM;
+#endif
 }
 
 void shell_help(const struct shell *sh)