[PATCH v8 4/4] vsprintf: Add command line option debug_boot_weak_hash

Tobin C. Harding
2018-06-20T04:21:26Z

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.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Documentation/admin-guide/kernel-parameters.txt | 9 +++++++++
lib/vsprintf.c | 17 +++++++++++++++++
2 files changed, 26 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 638342d0a095..a116fc0366b0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -748,6 +748,15 @@

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 c445f9f28760..17ebe076ae41 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1651,6 +1651,17 @@ 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_boot_weak_hash __ro_after_init;
+
+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 DEFINE_STATIC_KEY_TRUE(not_filled_random_ptr_key);
static siphash_key_t ptr_key __read_mostly;

@@ -1703,6 +1714,12 @@ 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_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 = 2 * sizeof(ptr);
/* string length must be less than default_width */
--
2.17.1