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-rw-r--r--Documentation/printk-formats.txt31
1 files changed, 28 insertions, 3 deletions
diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 361789df51ec..aa0a776c817a 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -5,7 +5,6 @@ How to get printk format specifiers right
:Author: Randy Dunlap <rdunlap@infradead.org>
:Author: Andrew Murray <amurray@mpc-data.co.uk>
-
Integer types
=============
@@ -45,6 +44,18 @@ return from vsnprintf.
Raw pointer value SHOULD be printed with %p. The kernel supports
the following extended format specifiers for pointer types:
+Pointer Types
+=============
+
+Pointers printed without a specifier extension (i.e unadorned %p) are
+hashed to give a unique identifier without leaking kernel addresses to user
+space. On 64 bit machines the first 32 bits are zeroed. If you _really_
+want the address see %px below.
+
+::
+
+ %p abcdef12 or 00000000abcdef12
+
Symbols/Function Pointers
=========================
@@ -85,18 +96,32 @@ Examples::
printk("Faulted at %pS\n", (void *)regs->ip);
printk(" %s%pB\n", (reliable ? "" : "? "), (void *)*stack);
-
Kernel Pointers
===============
::
- %pK 0x01234567 or 0x0123456789abcdef
+ %pK 01234567 or 0123456789abcdef
For printing kernel pointers which should be hidden from unprivileged
users. The behaviour of ``%pK`` depends on the ``kptr_restrict sysctl`` - see
Documentation/sysctl/kernel.txt for more details.
+Unmodified Addresses
+====================
+
+::
+
+ %px 01234567 or 0123456789abcdef
+
+For printing pointers when you _really_ want to print the address. Please
+consider whether or not you are leaking sensitive information about the
+Kernel layout in memory before printing pointers with %px. %px is
+functionally equivalent to %lx. %px is preferred to %lx because it is more
+uniquely grep'able. If, in the future, we need to modify the way the Kernel
+handles printing pointers it will be nice to be able to find the call
+sites.
+
Struct Resources
================