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Commit 3672476e authored by Tobin C. Harding's avatar Tobin C. Harding Committed by Theodore Ts'o
Browse files

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: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: default avatarTobin C. Harding <me@tobin.cc>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 1c4facb8

Documentation/admin-guide/kernel-parameters.txt

+8 −0
Original line number Diff line number Diff line
@@ -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>

lib/vsprintf.c

+17 −0
Original line number Diff line number Diff line
@@ -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 */