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Subject[PATCHv3 6/7] symbol lookup: use new kernel and module dereference functions
Call appropriate function descriptor dereference ARCH callbacks:
- dereference_kernel_function_descriptor() if the pointer is a
kernel symbol;

- dereference_module_function_descriptor() if the pointer is a
module symbol.

This patch also removes dereference_function_descriptor() from
'%pF/%pf' vsprintf handler, because it has the same behavior with
'%pS/%ps' now.

Signed-off-by: Sergey Senozhatsky <>
Tested-by: Helge Deller <> # parisc64
Tested-by: Santosh Sivaraj <> # powerpc64
Acked-by: Michael Ellerman <> # powerpc64
Tested-by: Tony Luck <> # ia64
Documentation/printk-formats.txt | 20 ++++++++++----------
kernel/kallsyms.c | 1 +
kernel/module.c | 1 +
lib/vsprintf.c | 5 +----
4 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 361789df51ec..3adbc4fdd482 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -50,26 +50,28 @@ Symbols/Function Pointers


+ %pS versatile_init+0x0/0x110
+ %ps versatile_init
%pF versatile_init+0x0/0x110
%pf versatile_init
- %pS versatile_init+0x0/0x110
%pSR versatile_init+0x9/0x110
(with __builtin_extract_return_addr() translation)
- %ps versatile_init
%pB prev_fn_of_versatile_init+0x88/0x88

-The ``F`` and ``f`` specifiers are for printing function pointers,
-for example, f->func, &gettimeofday. They have the same result as
-``S`` and ``s`` specifiers. But they do an extra conversion on
-ia64, ppc64 and parisc64 architectures where the function pointers
-are actually function descriptors.
The ``S`` and ``s`` specifiers can be used for printing symbols
from direct addresses, for example, __builtin_return_address(0),
(void *)regs->ip. They result in the symbol name with (``S``) or
without (``s``) offsets. If KALLSYMS are disabled then the symbol
address is printed instead.

+Note, that the ``F`` and ``f`` specifiers are identical to ``S`` (``s``)
+and thus deprecated. We have ``F`` and ``f`` because on ia64, ppc64 and
+parisc64 function pointers are indirect and, in fact, are function
+descriptors, which require additional dereferencing before we can lookup
+the symbol. As of now, ``S`` and ``s`` perform dereferencing on those
+platforms (when needed), so ``F`` and ``f`` exist for compatibility
+reasons only.
The ``B`` specifier results in the symbol name with offsets and should be
used when printing stack backtraces. The specifier takes into
consideration the effect of compiler optimisations which may occur
@@ -77,8 +79,6 @@ when tail-call``s are used and marked with the noreturn GCC attribute.


- printk("Going to call: %pF\n", gettimeofday);
- printk("Going to call: %pF\n", p->func);
printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
printk("%s: called from %pS\n", __func__,
(void *)__builtin_return_address(0));
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 127e7cfafa55..e2fc09ea9509 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -322,6 +322,7 @@ const char *kallsyms_lookup(unsigned long addr,
if (is_ksym_addr(addr)) {
unsigned long pos;

+ addr = dereference_kernel_function_descriptor(addr);
pos = get_symbol_pos(addr, symbolsize, offset);
/* Grab name */
diff --git a/kernel/module.c b/kernel/module.c
index b792e814150a..63361de377ad 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3948,6 +3948,7 @@ const char *module_address_lookup(unsigned long addr,
mod = __module_address(addr);
if (mod) {
+ addr = dereference_module_function_descriptor(mod, addr);
if (modname)
*modname = mod->name;
ret = get_ksymbol(mod, addr, size, offset);
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index bcd906a39010..bf04b4f5d8e7 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -40,7 +40,6 @@
#include "../mm/internal.h" /* For the trace_print_flags arrays */

#include <asm/page.h> /* for PAGE_SIZE */
-#include <asm/sections.h> /* for dereference_function_descriptor() */
#include <asm/byteorder.h> /* cpu_to_le16 */

#include <linux/string_helpers.h>
@@ -1721,10 +1720,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,

switch (*fmt) {
- case 'F':
+ case 'F': /* %pF and %pf are kept for compatibility reasons only */
case 'f':
- ptr = (void *)dereference_function_descriptor((unsigned long)ptr);
- /* Fallthrough */
case 'S':
case 's':
case 'B':
 \ /
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