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 
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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