[PATCH v6 4/4] vsprintf: Add command line option debug_boot_weak_hash

["Tobin C. Harding" <me@tobin.cc>]

Currently printing [hashed] pointers requires enough entropy to be
available.  Early in the boot sequence this may not be the case
resulting in a dummy string '(____ptrval____)' being printed.  This
makes debugging the early boot sequence difficult.  We can relax the
requirement to use cryptographically secure hashing during debugging.
This enables debugging while keeping development/production kernel
behaviour the same.

If new command line option debug_boot_weak_hash is enabled use
cryptographically insecure hashing and hash pointer value immediately.

Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
 lib/vsprintf.c                                  | 20 ++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2040d46f095..8a86d895343e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -753,6 +753,15 @@
 
 	debug		[KNL] Enable kernel debugging (events log level).
 
+	debug_boot_weak_hash
+			[KNL] Enable printing pointers early in the boot
+			sequence.  If enabled, we use a weak hash instead of
+			siphash to hash pointers.  Use this option if you need
+			to see pointer values during early boot (i.e you are
+			seeing instances of '(___ptrval___)').
+			Cryptographically insecure, please do not use on
+			production kernels.
+
 	debug_locks_verbose=
 			[KNL] verbose self-tests
 			Format=<0|1>
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 1545a8aa26a9..369623205e2c 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1670,6 +1670,20 @@ char *pointer_string(char *buf, char *end, const void *ptr,
 }
 
 static DEFINE_STATIC_KEY_TRUE(not_filled_random_ptr_key);
+
+/* Make pointers available for printing early in the boot sequence. */
+static int debug_boot_weak_hash __ro_after_init;
+EXPORT_SYMBOL(debug_boot_weak_hash);
+
+static int __init debug_boot_weak_hash_enable(char *str)
+{
+	debug_boot_weak_hash = 1;
+	pr_info("debug_boot_weak_hash enabled\n");
+	return 0;
+}
+early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable);
+
+static bool have_filled_random_ptr_key __read_mostly;
 static siphash_key_t ptr_key __read_mostly;
 
 static void enable_ptr_key_workfn(struct work_struct *work)
@@ -1721,6 +1735,12 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
 	unsigned long hashval;
 	const int default_width = 2 * sizeof(ptr);
 
+	/* When debugging early boot use non-cryptographically secure hash */
+	if (unlikely(debug_boot_weak_hash)) {
+		hashval = hash_long((unsigned long)ptr, 32);
+		return pointer_string(buf, end, (const void *)hashval, spec);
+	}
+
 	if (static_branch_unlikely(&not_filled_random_ptr_key)) {
 		spec.field_width = default_width;
 		/* string length must be less than default_width */
-- 
2.7.4