[PATCH v2] common: cli_hush: fix console_buffer overflow on boot retry

[Ngo Luong Thanh Tra]

Add const_strcpy() macro to <linux/build_bug.h> that enforces at
compile time that the destination is a char array (not a pointer),
the source is a string literal, and the source fits in the
destination including the NUL terminator. It uses __builtin_strcpy()
so the compiler can optimize the copy.

Fix the console_buffer extern declaration in <console.h> to include
the array size so that sizeof(console_buffer) is valid at call sites.

Replace the unbounded strcpy() in cli_hush.c with const_strcpy() to
catch at compile time any configuration where CONFIG_SYS_CBSIZE is
smaller than the boot retry command string.

Fixes: 657e19f8f2dd ("cli_hush: support running bootcmd on boot retry")
Signed-off-by: Ngo Luong Thanh Tra <S4210155 at student.rmit.edu.au>
To: u-boot at lists.denx.de
---

Changes in v2:
- Replace strlcpy() + BUILD_BUG_ON approach with a new const_strcpy()
  macro that enforces compile-time safety for string literal copies
- Fix console_buffer extern declaration in <console.h> to include
  array size so sizeof(console_buffer) is valid at call sites
- Address review from Rasmus Villemoes

 common/cli_hush.c         |  3 ++-
 include/console.h         |  3 ++-
 include/linux/build_bug.h | 23 +++++++++++++++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/common/cli_hush.c b/common/cli_hush.c
index 7bd6943d3ed..048577cb40a 100644
--- a/common/cli_hush.c
+++ b/common/cli_hush.c
@@ -84,6 +84,7 @@
 #include <cli_hush.h>
 #include <command.h>        /* find_cmd */
 #include <asm/global_data.h>
+#include <linux/build_bug.h>
 #endif
 #ifndef __U_BOOT__
 #include <ctype.h>     /* isalpha, isdigit */
@@ -1029,7 +1030,7 @@ static void get_user_input(struct in_str *i)
 #  ifdef CONFIG_RESET_TO_RETRY
 	  do_reset(NULL, 0, 0, NULL);
 #  elif IS_ENABLED(CONFIG_RETRY_BOOTCMD)
-	strcpy(console_buffer, "run bootcmd\n");
+	const_strcpy(console_buffer, "run bootcmd\n");
 # else
 #	error "This only works with CONFIG_RESET_TO_RETRY or CONFIG_BOOT_RETRY_COMMAND enabled"
 #  endif
diff --git a/include/console.h b/include/console.h
index 8d0d7bb8a4c..97ccf5e5f6a 100644
--- a/include/console.h
+++ b/include/console.h
@@ -10,8 +10,9 @@
 #include <stdbool.h>
 #include <stdio_dev.h>
 #include <linux/errno.h>
+#include <config.h>
 
-extern char console_buffer[];
+extern char console_buffer[CONFIG_SYS_CBSIZE + 1];
 
 /* common/console.c */
 int console_init_f(void);	/* Before relocation; uses the serial  stuff */
diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
index 20c2dc7f4bd..d775bf1bf91 100644
--- a/include/linux/build_bug.h
+++ b/include/linux/build_bug.h
@@ -76,4 +76,27 @@
 #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
 #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
 
+/**
+ * const_strcpy - Copy a string literal to a char array with compile-time checks
+ * @d: destination char array (must be a char array, not a pointer)
+ * @s: source string literal
+ *
+ * Enforces at compile time that:
+ * (a) @d is a char array, not a pointer
+ * (b) @s is a string literal (adjacent string concatenation trick)
+ * (c) @s fits in @d including the NUL terminator
+ *
+ * Uses __builtin_strcpy() so the compiler can optimize the copy into
+ * immediate stores rather than emitting a function call.
+ *
+ * Note: @s is used twice in the macro expansion but this is intentional
+ * and safe: the ("" s "") trick enforces at compile time that @s is a
+ * string literal, and string literals have no side effects.
+ */
+#define const_strcpy(d, s) ({						\
+	BUILD_BUG_ON(__builtin_types_compatible_p(typeof(d), char *));	\
+	BUILD_BUG_ON(sizeof(d) < sizeof("" s ""));			\
+	__builtin_strcpy(d, s);						\
+})
+
 #endif	/* _LINUX_BUILD_BUG_H */
-- 
2.53.0

base-commit: 42571a36b6eac469ee94e305c7e7062306fde423
branch: fix/sys-cbsize-overflow-series