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.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

2 files changed, 25 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index efc7aa7a0670..0c8f7889efa1 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 printing [hashed] pointers early in the
+                        boot sequence.  If enabled, we use a weak hash instead
+                        of siphash to hash pointers.  Use this option if you are
+                        seeing instances of '(___ptrval___)') and need to see a
+                        value (hashed pointer) instead. 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 6c1fb395bddf..1ee2829f3b54 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 */