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Merge branches 'for-4.7/core', 'for-4.7/livepatching-doc' and 'for-4.7/livepatching-ppc64' into for-linus

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=========
Livepatch
=========
This document outlines basic information about kernel livepatching.
Table of Contents:
1. Motivation
2. Kprobes, Ftrace, Livepatching
3. Consistency model
4. Livepatch module
4.1. New functions
4.2. Metadata
4.3. Livepatch module handling
5. Livepatch life-cycle
5.1. Registration
5.2. Enabling
5.3. Disabling
5.4. Unregistration
6. Sysfs
7. Limitations
1. Motivation
=============
There are many situations where users are reluctant to reboot a system. It may
be because their system is performing complex scientific computations or under
heavy load during peak usage. In addition to keeping systems up and running,
users want to also have a stable and secure system. Livepatching gives users
both by allowing for function calls to be redirected; thus, fixing critical
functions without a system reboot.
2. Kprobes, Ftrace, Livepatching
================================
There are multiple mechanisms in the Linux kernel that are directly related
to redirection of code execution; namely: kernel probes, function tracing,
and livepatching:
+ The kernel probes are the most generic. The code can be redirected by
putting a breakpoint instruction instead of any instruction.
+ The function tracer calls the code from a predefined location that is
close to the function entry point. This location is generated by the
compiler using the '-pg' gcc option.
+ Livepatching typically needs to redirect the code at the very beginning
of the function entry before the function parameters or the stack
are in any way modified.
All three approaches need to modify the existing code at runtime. Therefore
they need to be aware of each other and not step over each other's toes.
Most of these problems are solved by using the dynamic ftrace framework as
a base. A Kprobe is registered as a ftrace handler when the function entry
is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
a live patch is called with the help of a custom ftrace handler. But there are
some limitations, see below.
3. Consistency model
====================
Functions are there for a reason. They take some input parameters, get or
release locks, read, process, and even write some data in a defined way,
have return values. In other words, each function has a defined semantic.
Many fixes do not change the semantic of the modified functions. For
example, they add a NULL pointer or a boundary check, fix a race by adding
a missing memory barrier, or add some locking around a critical section.
Most of these changes are self contained and the function presents itself
the same way to the rest of the system. In this case, the functions might
be updated independently one by one.
But there are more complex fixes. For example, a patch might change
ordering of locking in multiple functions at the same time. Or a patch
might exchange meaning of some temporary structures and update
all the relevant functions. In this case, the affected unit
(thread, whole kernel) need to start using all new versions of
the functions at the same time. Also the switch must happen only
when it is safe to do so, e.g. when the affected locks are released
or no data are stored in the modified structures at the moment.
The theory about how to apply functions a safe way is rather complex.
The aim is to define a so-called consistency model. It attempts to define
conditions when the new implementation could be used so that the system
stays consistent. The theory is not yet finished. See the discussion at
http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189
The current consistency model is very simple. It guarantees that either
the old or the new function is called. But various functions get redirected
one by one without any synchronization.
In other words, the current implementation _never_ modifies the behavior
in the middle of the call. It is because it does _not_ rewrite the entire
function in the memory. Instead, the function gets redirected at the
very beginning. But this redirection is used immediately even when
some other functions from the same patch have not been redirected yet.
See also the section "Limitations" below.
4. Livepatch module
===================
Livepatches are distributed using kernel modules, see
samples/livepatch/livepatch-sample.c.
The module includes a new implementation of functions that we want
to replace. In addition, it defines some structures describing the
relation between the original and the new implementation. Then there
is code that makes the kernel start using the new code when the livepatch
module is loaded. Also there is code that cleans up before the
livepatch module is removed. All this is explained in more details in
the next sections.
4.1. New functions
------------------
New versions of functions are typically just copied from the original
sources. A good practice is to add a prefix to the names so that they
can be distinguished from the original ones, e.g. in a backtrace. Also
they can be declared as static because they are not called directly
and do not need the global visibility.
The patch contains only functions that are really modified. But they
might want to access functions or data from the original source file
that may only be locally accessible. This can be solved by a special
relocation section in the generated livepatch module, see
Documentation/livepatch/module-elf-format.txt for more details.
4.2. Metadata
------------
The patch is described by several structures that split the information
into three levels:
+ struct klp_func is defined for each patched function. It describes
the relation between the original and the new implementation of a
particular function.
The structure includes the name, as a string, of the original function.
The function address is found via kallsyms at runtime.
Then it includes the address of the new function. It is defined
directly by assigning the function pointer. Note that the new
function is typically defined in the same source file.
As an optional parameter, the symbol position in the kallsyms database can
be used to disambiguate functions of the same name. This is not the
absolute position in the database, but rather the order it has been found
only for a particular object ( vmlinux or a kernel module ). Note that
kallsyms allows for searching symbols according to the object name.
+ struct klp_object defines an array of patched functions (struct
klp_func) in the same object. Where the object is either vmlinux
(NULL) or a module name.
The structure helps to group and handle functions for each object
together. Note that patched modules might be loaded later than
the patch itself and the relevant functions might be patched
only when they are available.
+ struct klp_patch defines an array of patched objects (struct
klp_object).
This structure handles all patched functions consistently and eventually,
synchronously. The whole patch is applied only when all patched
symbols are found. The only exception are symbols from objects
(kernel modules) that have not been loaded yet. Also if a more complex
consistency model is supported then a selected unit (thread,
kernel as a whole) will see the new code from the entire patch
only when it is in a safe state.
4.3. Livepatch module handling
------------------------------
The usual behavior is that the new functions will get used when
the livepatch module is loaded. For this, the module init() function
has to register the patch (struct klp_patch) and enable it. See the
section "Livepatch life-cycle" below for more details about these
two operations.
Module removal is only safe when there are no users of the underlying
functions. The immediate consistency model is not able to detect this;
therefore livepatch modules cannot be removed. See "Limitations" below.
5. Livepatch life-cycle
=======================
Livepatching defines four basic operations that define the life cycle of each
live patch: registration, enabling, disabling and unregistration. There are
several reasons why it is done this way.
First, the patch is applied only when all patched symbols for already
loaded objects are found. The error handling is much easier if this
check is done before particular functions get redirected.
Second, the immediate consistency model does not guarantee that anyone is not
sleeping in the new code after the patch is reverted. This means that the new
code needs to stay around "forever". If the code is there, one could apply it
again. Therefore it makes sense to separate the operations that might be done
once and those that need to be repeated when the patch is enabled (applied)
again.
Third, it might take some time until the entire system is migrated
when a more complex consistency model is used. The patch revert might
block the livepatch module removal for too long. Therefore it is useful
to revert the patch using a separate operation that might be called
explicitly. But it does not make sense to remove all information
until the livepatch module is really removed.
5.1. Registration
-----------------
Each patch first has to be registered using klp_register_patch(). This makes
the patch known to the livepatch framework. Also it does some preliminary
computing and checks.
In particular, the patch is added into the list of known patches. The
addresses of the patched functions are found according to their names.
The special relocations, mentioned in the section "New functions", are
applied. The relevant entries are created under
/sys/kernel/livepatch/<name>. The patch is rejected when any operation
fails.
5.2. Enabling
-------------
Registered patches might be enabled either by calling klp_enable_patch() or
by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
start using the new implementation of the patched functions at this stage.
In particular, if an original function is patched for the first time, a
function specific struct klp_ops is created and an universal ftrace handler
is registered.
Functions might be patched multiple times. The ftrace handler is registered
only once for the given function. Further patches just add an entry to the
list (see field `func_stack`) of the struct klp_ops. The last added
entry is chosen by the ftrace handler and becomes the active function
replacement.
Note that the patches might be enabled in a different order than they were
registered.
5.3. Disabling
--------------
Enabled patches might get disabled either by calling klp_disable_patch() or
by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
either the code from the previously enabled patch or even the original
code gets used.
Here all the functions (struct klp_func) associated with the to-be-disabled
patch are removed from the corresponding struct klp_ops. The ftrace handler
is unregistered and the struct klp_ops is freed when the func_stack list
becomes empty.
Patches must be disabled in exactly the reverse order in which they were
enabled. It makes the problem and the implementation much easier.
5.4. Unregistration
-------------------
Disabled patches might be unregistered by calling klp_unregister_patch().
This can be done only when the patch is disabled and the code is no longer
used. It must be called before the livepatch module gets unloaded.
At this stage, all the relevant sys-fs entries are removed and the patch
is removed from the list of known patches.
6. Sysfs
========
Information about the registered patches can be found under
/sys/kernel/livepatch. The patches could be enabled and disabled
by writing there.
See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
7. Limitations
==============
The current Livepatch implementation has several limitations:
+ The patch must not change the semantic of the patched functions.
The current implementation guarantees only that either the old
or the new function is called. The functions are patched one
by one. It means that the patch must _not_ change the semantic
of the function.
+ Data structures can not be patched.
There is no support to version data structures or anyhow migrate
one structure into another. Also the simple consistency model does
not allow to switch more functions atomically.
Once there is more complex consistency mode, it will be possible to
use some workarounds. For example, it will be possible to use a hole
for a new member because the data structure is aligned. Or it will
be possible to use an existing member for something else.
There are no plans to add more generic support for modified structures
at the moment.
+ Only functions that can be traced could be patched.
Livepatch is based on the dynamic ftrace. In particular, functions
implementing ftrace or the livepatch ftrace handler could not be
patched. Otherwise, the code would end up in an infinite loop. A
potential mistake is prevented by marking the problematic functions
by "notrace".
+ Anything inlined into __schedule() can not be patched.
The switch_to macro is inlined into __schedule(). It switches the
context between two processes in the middle of the macro. It does
not save RIP in x86_64 version (contrary to 32-bit version). Instead,
the currently used __schedule()/switch_to() handles both processes.
Now, let's have two different tasks. One calls the original
__schedule(), its registers are stored in a defined order and it
goes to sleep in the switch_to macro and some other task is restored
using the original __schedule(). Then there is the second task which
calls patched__schedule(), it goes to sleep there and the first task
is picked by the patched__schedule(). Its RSP is restored and now
the registers should be restored as well. But the order is different
in the new patched__schedule(), so...
There is work in progress to remove this limitation.
+ Livepatch modules can not be removed.
The current implementation just redirects the functions at the very
beginning. It does not check if the functions are in use. In other
words, it knows when the functions get called but it does not
know when the functions return. Therefore it can not decide when
the livepatch module can be safely removed.
This will get most likely solved once a more complex consistency model
is supported. The idea is that a safe state for patching should also
mean a safe state for removing the patch.
Note that the patch itself might get disabled by writing zero
to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
code will not longer get called. But it does not guarantee
that anyone is not sleeping anywhere in the new code.
+ Livepatch works reliably only when the dynamic ftrace is located at
the very beginning of the function.
The function need to be redirected before the stack or the function
parameters are modified in any way. For example, livepatch requires
using -fentry gcc compiler option on x86_64.
One exception is the PPC port. It uses relative addressing and TOC.
Each function has to handle TOC and save LR before it could call
the ftrace handler. This operation has to be reverted on return.
Fortunately, the generic ftrace code has the same problem and all
this is is handled on the ftrace level.
+ Kretprobes using the ftrace framework conflict with the patched
functions.
Both kretprobes and livepatches use a ftrace handler that modifies
the return address. The first user wins. Either the probe or the patch
is rejected when the handler is already in use by the other.
+ Kprobes in the original function are ignored when the code is
redirected to the new implementation.
There is a work in progress to add warnings about this situation.

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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 */
};