[PATCH v3 4/4] vsprintf: Add command line option debug_early_boot

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

Currently printing [hashed] pointers requires either a hw RNG or enough
entropy to be available.  Early in the boot sequence these conditions
may not be met 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_early_boot is enabled use
cryptographically insecure hashing and hash pointer value immediately.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
---
 Documentation/admin-guide/kernel-parameters.txt |  8 ++++++++
 lib/vsprintf.c                                  | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b8d1379aa039..ab619c4ccbf2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -748,6 +748,14 @@
 
 	debug		[KNL] Enable kernel debugging (events log level).
 
+	debug_early_boot
+			[KNL] Enable debugging 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 3697a19c2b25..2c48884f00eb 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1654,6 +1654,18 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
 	return widen_string(buf, buf - buf_start, end, spec);
 }
 
+/* Make pointers available for printing early in the boot sequence. */
+static int debug_early_boot __ro_after_init;
+EXPORT_SYMBOL(debug_early_boot);
+
+static int __init debug_early_boot_enable(char *str)
+{
+	debug_early_boot = 1;
+	pr_info("debug_early_boot enabled\n");
+	return 0;
+}
+early_param("debug_early_boot", debug_early_boot_enable);
+
 static bool have_filled_random_ptr_key __read_mostly;
 static siphash_key_t ptr_key __read_mostly;
 
@@ -1707,6 +1719,12 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
 	const char *str = sizeof(ptr) == 8 ? "(____ptrval____)" : "(ptrval)";
 	unsigned long hashval;
 
+	/* When debugging early boot use non-cryptographically secure hash */
+	if (unlikely(debug_early_boot)) {
+		hashval = hash_long((unsigned long)ptr, 32);
+		return pointer_string(buf, end, (const void *)hashval, spec);
+	}
+
 	if (unlikely(!have_filled_random_ptr_key)) {
 		spec.field_width = 2 * sizeof(ptr);
 		/* string length must be less than default_width */
-- 
2.7.4