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

Tobin C. Harding
2018-05-28T01:47:23Z

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