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Subject[PATCH] vsprintf: Add command line option debug_boot_weak_hash
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 <>

This patch was previously submitted as the last in the set

[PATCH v3 0/4] enable early printing of hashed pointers

Helps debugging using ftrace. Original problem reported by Anna-Maria,
solution requested by Steve.

Changes since above mentioned patch set
- change option name from debug_early_boot -> debug_boot_weak_hash
(suggested by Steve).

I have only tested this by enabling the option and printing some
pointers. This does not _prove_ that it fixes the ftrace issue.


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 3b8032431585..c95dd6704592 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_boot_weak_hash
+ [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.
[KNL] verbose self-tests
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index b82f0c6c2aec..5ff18f8fe3bd 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_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 bool have_filled_random_ptr_key __read_mostly;
static siphash_key_t ptr_key __read_mostly;

@@ -1694,6 +1706,12 @@ 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 (unlikely(!have_filled_random_ptr_key)) {
spec.field_width = 2 * sizeof(ptr);
/* string length must be less than default_width */
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