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Pull livepatching updates from Jiri Kosina:

 - remove of our own implementation of architecture-specific relocation
   code and leveraging existing code in the module loader to perform
   arch-dependent work, from Jessica Yu.

   The relevant patches have been acked by Rusty (for module.c) and
   Heiko (for s390).

 - live patching support for ppc64le, which is a joint work of Michael
   Ellerman and Torsten Duwe.  This is coming from topic branch that is
   share between livepatching.git and ppc tree.

 - addition of livepatching documentation from Petr Mladek

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/livepatching:
  livepatch: make object/func-walking helpers more robust
  livepatch: Add some basic livepatch documentation
  powerpc/livepatch: Add live patching support on ppc64le
  powerpc/livepatch: Add livepatch stack to struct thread_info
  powerpc/livepatch: Add livepatch header
  livepatch: Allow architectures to specify an alternate ftrace location
  ftrace: Make ftrace_location_range() global
  livepatch: robustify klp_register_patch() API error checking
  Documentation: livepatch: outline Elf format and requirements for patch modules
  livepatch: reuse module loader code to write relocations
  module: s390: keep mod_arch_specific for livepatch modules
  module: preserve Elf information for livepatch modules
  Elf: add livepatch-specific Elf constants
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394 changes: 394 additions & 0 deletions Documentation/livepatch/livepatch.txt
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311 changes: 311 additions & 0 deletions Documentation/livepatch/module-elf-format.txt
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===========================
Livepatch module Elf format
===========================

This document outlines the Elf format requirements that livepatch modules must follow.

-----------------
Table of Contents
-----------------
0. Background and motivation
1. Livepatch modinfo field
2. Livepatch relocation sections
2.1 What are livepatch relocation sections?
2.2 Livepatch relocation section format
2.2.1 Required flags
2.2.2 Required name format
2.2.3 Example livepatch relocation section names
2.2.4 Example `readelf --sections` output
2.2.5 Example `readelf --relocs` output
3. Livepatch symbols
3.1 What are livepatch symbols?
3.2 A livepatch module's symbol table
3.3 Livepatch symbol format
3.3.1 Required flags
3.3.2 Required name format
3.3.3 Example livepatch symbol names
3.3.4 Example `readelf --symbols` output
4. Symbol table and Elf section access

----------------------------
0. Background and motivation
----------------------------

Formerly, livepatch required separate architecture-specific code to write
relocations. However, arch-specific code to write relocations already
exists in the module loader, so this former approach produced redundant
code. So, instead of duplicating code and re-implementing what the module
loader can already do, livepatch leverages existing code in the module
loader to perform the all the arch-specific relocation work. Specifically,
livepatch reuses the apply_relocate_add() function in the module loader to
write relocations. The patch module Elf format described in this document
enables livepatch to be able to do this. The hope is that this will make
livepatch more easily portable to other architectures and reduce the amount
of arch-specific code required to port livepatch to a particular
architecture.

Since apply_relocate_add() requires access to a module's section header
table, symbol table, and relocation section indices, Elf information is
preserved for livepatch modules (see section 4). Livepatch manages its own
relocation sections and symbols, which are described in this document. The
Elf constants used to mark livepatch symbols and relocation sections were
selected from OS-specific ranges according to the definitions from glibc.

0.1 Why does livepatch need to write its own relocations?
---------------------------------------------------------
A typical livepatch module contains patched versions of functions that can
reference non-exported global symbols and non-included local symbols.
Relocations referencing these types of symbols cannot be left in as-is
since the kernel module loader cannot resolve them and will therefore
reject the livepatch module. Furthermore, we cannot apply relocations that
affect modules not yet loaded at patch module load time (e.g. a patch to a
driver that is not loaded). Formerly, livepatch solved this problem by
embedding special "dynrela" (dynamic rela) sections in the resulting patch
module Elf output. Using these dynrela sections, livepatch could resolve
symbols while taking into account its scope and what module the symbol
belongs to, and then manually apply the dynamic relocations. However this
approach required livepatch to supply arch-specific code in order to write
these relocations. In the new format, livepatch manages its own SHT_RELA
relocation sections in place of dynrela sections, and the symbols that the
relas reference are special livepatch symbols (see section 2 and 3). The
arch-specific livepatch relocation code is replaced by a call to
apply_relocate_add().

================================
PATCH MODULE FORMAT REQUIREMENTS
================================

--------------------------
1. Livepatch modinfo field
--------------------------

Livepatch modules are required to have the "livepatch" modinfo attribute.
See the sample livepatch module in samples/livepatch/ for how this is done.

Livepatch modules can be identified by users by using the 'modinfo' command
and looking for the presence of the "livepatch" field. This field is also
used by the kernel module loader to identify livepatch modules.

Example modinfo output:
-----------------------
% modinfo livepatch-meminfo.ko
filename: livepatch-meminfo.ko
livepatch: Y
license: GPL
depends:
vermagic: 4.3.0+ SMP mod_unload

--------------------------------
2. Livepatch relocation sections
--------------------------------

-------------------------------------------
2.1 What are livepatch relocation sections?
-------------------------------------------
A livepatch module manages its own Elf relocation sections to apply
relocations to modules as well as to the kernel (vmlinux) at the
appropriate time. For example, if a patch module patches a driver that is
not currently loaded, livepatch will apply the corresponding livepatch
relocation section(s) to the driver once it loads.

Each "object" (e.g. vmlinux, or a module) within a patch module may have
multiple livepatch relocation sections associated with it (e.g. patches to
multiple functions within the same object). There is a 1-1 correspondence
between a livepatch relocation section and the target section (usually the
text section of a function) to which the relocation(s) apply. It is
also possible for a livepatch module to have no livepatch relocation
sections, as in the case of the sample livepatch module (see
samples/livepatch).

Since Elf information is preserved for livepatch modules (see Section 4), a
livepatch relocation section can be applied simply by passing in the
appropriate section index to apply_relocate_add(), which then uses it to
access the relocation section and apply the relocations.

Every symbol referenced by a rela in a livepatch relocation section is a
livepatch symbol. These must be resolved before livepatch can call
apply_relocate_add(). See Section 3 for more information.

---------------------------------------
2.2 Livepatch relocation section format
---------------------------------------

2.2.1 Required flags
--------------------
Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
section flag. See include/uapi/linux/elf.h for the definition. The module
loader recognizes this flag and will avoid applying those relocation sections
at patch module load time. These sections must also be marked with SHF_ALLOC,
so that the module loader doesn't discard them on module load (i.e. they will
be copied into memory along with the other SHF_ALLOC sections).

2.2.2 Required name format
--------------------------
The name of a livepatch relocation section must conform to the following format:

.klp.rela.objname.section_name
^ ^^ ^ ^ ^
|________||_____| |__________|
[A] [B] [C]

[A] The relocation section name is prefixed with the string ".klp.rela."
[B] The name of the object (i.e. "vmlinux" or name of module) to
which the relocation section belongs follows immediately after the prefix.
[C] The actual name of the section to which this relocation section applies.

2.2.3 Example livepatch relocation section names:
-------------------------------------------------
.klp.rela.ext4.text.ext4_attr_store
.klp.rela.vmlinux.text.cmdline_proc_show

2.2.4 Example `readelf --sections` output for a patch
module that patches vmlinux and modules 9p, btrfs, ext4:
--------------------------------------------------------
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ snip ]
[29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
[30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
[ snip ]
[34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
[35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
[36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
[37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
[ snip ] ^ ^
| |
[*] [*]
[*] Livepatch relocation sections are SHT_RELA sections but with a few special
characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
not be discarded when the module is loaded into memory, as well as with the
SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).

2.2.5 Example `readelf --relocs` output for a patch module:
-----------------------------------------------------------
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
[ snip ] ^
|
[*]
[*] Every symbol referenced by a relocation is a livepatch symbol.

--------------------
3. Livepatch symbols
--------------------

-------------------------------
3.1 What are livepatch symbols?
-------------------------------
Livepatch symbols are symbols referred to by livepatch relocation sections.
These are symbols accessed from new versions of functions for patched
objects, whose addresses cannot be resolved by the module loader (because
they are local or unexported global syms). Since the module loader only
resolves exported syms, and not every symbol referenced by the new patched
functions is exported, livepatch symbols were introduced. They are used
also in cases where we cannot immediately know the address of a symbol when
a patch module loads. For example, this is the case when livepatch patches
a module that is not loaded yet. In this case, the relevant livepatch
symbols are resolved simply when the target module loads. In any case, for
any livepatch relocation section, all livepatch symbols referenced by that
section must be resolved before livepatch can call apply_relocate_add() for
that reloc section.

Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
loader can identify and ignore them. Livepatch modules keep these symbols
in their symbol tables, and the symbol table is made accessible through
module->symtab.

-------------------------------------
3.2 A livepatch module's symbol table
-------------------------------------
Normally, a stripped down copy of a module's symbol table (containing only
"core" symbols) is made available through module->symtab (See layout_symtab()
in kernel/module.c). For livepatch modules, the symbol table copied into memory
on module load must be exactly the same as the symbol table produced when the
patch module was compiled. This is because the relocations in each livepatch
relocation section refer to their respective symbols with their symbol indices,
and the original symbol indices (and thus the symtab ordering) must be
preserved in order for apply_relocate_add() to find the right symbol.

For example, take this particular rela from a livepatch module:
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4

This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
symbol index 94.
And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
[ snip ]
94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
[ snip ]

---------------------------
3.3 Livepatch symbol format
---------------------------

3.3.1 Required flags
--------------------
Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
that the module loader can identify them and not attempt to resolve them.
See include/uapi/linux/elf.h for the actual definitions.

3.3.2 Required name format
--------------------------
Livepatch symbol names must conform to the following format:

.klp.sym.objname.symbol_name,sympos
^ ^^ ^ ^ ^ ^
|_______||_____| |_________| |
[A] [B] [C] [D]

[A] The symbol name is prefixed with the string ".klp.sym."
[B] The name of the object (i.e. "vmlinux" or name of module) to
which the symbol belongs follows immediately after the prefix.
[C] The actual name of the symbol.
[D] The position of the symbol in the object (as according to kallsyms)
This is used to differentiate duplicate symbols within the same
object. The symbol position is expressed numerically (0, 1, 2...).
The symbol position of a unique symbol is 0.

3.3.3 Example livepatch symbol names:
-------------------------------------
.klp.sym.vmlinux.snprintf,0
.klp.sym.vmlinux.printk,0
.klp.sym.btrfs.btrfs_ktype,0

3.3.4 Example `readelf --symbols` output for a patch module:
------------------------------------------------------------
Symbol table '.symtab' contains 127 entries:
Num: Value Size Type Bind Vis Ndx Name
[ snip ]
73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
[ snip ] ^
|
[*]
[*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
"OS" means OS-specific.

--------------------------------------
4. Symbol table and Elf section access
--------------------------------------
A livepatch module's symbol table is accessible through module->symtab.

Since apply_relocate_add() requires access to a module's section headers,
symbol table, and relocation section indices, Elf information is preserved for
livepatch modules and is made accessible by the module loader through
module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
this struct is filled in by the module loader. Its fields are documented below:

struct klp_modinfo {
Elf_Ehdr hdr; /* Elf header */
Elf_Shdr *sechdrs; /* Section header table */
char *secstrings; /* String table for the section headers */
unsigned int symndx; /* The symbol table section index */
};
1 change: 1 addition & 0 deletions MAINTAINERS
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Original file line number Diff line number Diff line change
Expand Up @@ -6699,6 +6699,7 @@ F: kernel/livepatch/
F: include/linux/livepatch.h
F: arch/x86/include/asm/livepatch.h
F: arch/x86/kernel/livepatch.c
F: Documentation/livepatch/
F: Documentation/ABI/testing/sysfs-kernel-livepatch
F: samples/livepatch/
L: [email protected]
Expand Down
3 changes: 3 additions & 0 deletions arch/powerpc/Kconfig
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Expand Up @@ -160,6 +160,7 @@ config PPC
select HAVE_ARCH_SECCOMP_FILTER
select ARCH_HAS_UBSAN_SANITIZE_ALL
select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
select HAVE_LIVEPATCH if HAVE_DYNAMIC_FTRACE_WITH_REGS

config GENERIC_CSUM
def_bool CPU_LITTLE_ENDIAN
Expand Down Expand Up @@ -1107,3 +1108,5 @@ config PPC_LIB_RHEAP
bool

source "arch/powerpc/kvm/Kconfig"

source "kernel/livepatch/Kconfig"
62 changes: 62 additions & 0 deletions arch/powerpc/include/asm/livepatch.h
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/*
* livepatch.h - powerpc-specific Kernel Live Patching Core
*
* Copyright (C) 2015-2016, SUSE, IBM Corp.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
#ifndef _ASM_POWERPC_LIVEPATCH_H
#define _ASM_POWERPC_LIVEPATCH_H

#include <linux/module.h>
#include <linux/ftrace.h>

#ifdef CONFIG_LIVEPATCH
static inline int klp_check_compiler_support(void)
{
return 0;
}

static inline int klp_write_module_reloc(struct module *mod, unsigned long
type, unsigned long loc, unsigned long value)
{
/* This requires infrastructure changes; we need the loadinfos. */
return -ENOSYS;
}

static inline void klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
{
regs->nip = ip;
}

#define klp_get_ftrace_location klp_get_ftrace_location
static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
{
/*
* Live patch works only with -mprofile-kernel on PPC. In this case,
* the ftrace location is always within the first 16 bytes.
*/
return ftrace_location_range(faddr, faddr + 16);
}

static inline void klp_init_thread_info(struct thread_info *ti)
{
/* + 1 to account for STACK_END_MAGIC */
ti->livepatch_sp = (unsigned long *)(ti + 1) + 1;
}
#else
static void klp_init_thread_info(struct thread_info *ti) { }
#endif /* CONFIG_LIVEPATCH */

#endif /* _ASM_POWERPC_LIVEPATCH_H */
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