[PATCH v2 4/4] vsprintf: Add command line option debug_early_boot

From: Tobin C. Harding
Date: Wed May 02 2018 - 18:51:23 EST


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@xxxxxxxx>
---
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..6c139b442267 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;
+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