Objtool
=======
The kernel CONFIG_OBJTOOL option enables a host tool named 'objtool'
which runs at compile time. It can do various validations and
transformations on .o files.
Objtool has become an integral part of the x86-64 kernel toolchain. The
kernel depends on it for a variety of security and performance features
(and other types of features as well).
Features
--------
Objtool has the following features:
- Stack unwinding metadata validation -- useful for helping to ensure
stack traces are reliable for live patching
- ORC unwinder metadata generation -- a faster and more precise
alternative to frame pointer based unwinding
- Retpoline validation -- ensures that all indirect calls go through
retpoline thunks, for Spectre v2 mitigations
- Retpoline call site annotation -- annotates all retpoline thunk call
sites, enabling the kernel to patch them inline, to prevent "thunk
funneling" for both security and performance reasons
- Non-instrumentation validation -- validates non-instrumentable
("noinstr") code rules, preventing instrumentation in low-level C
entry code
- Static call annotation -- annotates static call sites, enabling the
kernel to implement inline static calls, a faster alternative to some
indirect branches
- Uaccess validation -- validates uaccess rules for a proper
implementation of Supervisor Mode Access Protection (SMAP)
- Straight Line Speculation validation -- validates certain SLS
mitigations
- Indirect Branch Tracking validation -- validates Intel CET IBT rules
to ensure that all functions referenced by function pointers have
corresponding ENDBR instructions
- Indirect Branch Tracking annotation -- annotates unused ENDBR
instruction sites, enabling the kernel to "seal" them (replace them
with NOPs) to further harden IBT
- Function entry annotation -- annotates function entries, enabling
kernel function tracing
- Other toolchain hacks which will go unmentioned at this time...
Each feature can be enabled individually or in combination using the
objtool cmdline.
Objects
-------
Typically, objtool runs on every translation unit (TU, aka ".o file") in
the kernel. If a TU is part of a kernel module, the '--module' option
is added.
However:
- If noinstr validation is enabled, it also runs on vmlinux.o, with all
options removed and '--noinstr' added.
- If IBT or LTO is enabled, it doesn't run on TUs at all. Instead it
runs on vmlinux.o and linked modules, with all options.
In summary:
A) Legacy mode:
TU: objtool [--module] <options>
vmlinux: N/A
module: N/A
B) CONFIG_NOINSTR_VALIDATION=y && !(CONFIG_X86_KERNEL_IBT=y || CONFIG_LTO=y):
TU: objtool [--module] <options> // no --noinstr
vmlinux: objtool --noinstr // other options removed
module: N/A
C) CONFIG_X86_KERNEL_IBT=y || CONFIG_LTO=y:
TU: N/A
vmlinux: objtool --noinstr <options>
module: objtool --module --noinstr <options>
Stack validation
----------------
Objtool's stack validation feature analyzes every .o file and ensures
the validity of its stack metadata. It enforces a set of rules on asm
code and C inline assembly code so that stack traces can be reliable.
For each function, it recursively follows all possible code paths and
validates the correct frame pointer state at each instruction.
It also follows code paths involving special sections, like
.altinstructions, __jump_table, and __ex_table, which can add
alternative execution paths to a given instruction (or set of
instructions). Similarly, it knows how to follow switch statements, for
which gcc sometimes uses jump tables.
Here are some of the benefits of validating stack metadata:
a) More reliable stack traces for frame pointer enabled kernels
Frame pointers are used for debugging purposes. They allow runtime
code and debug tools to be able to walk the stack to determine the
chain of function call sites that led to the currently executing
code.
For some architectures, frame pointers are enabled by
CONFIG_FRAME_POINTER. For some other architectures they may be
required by the ABI (sometimes referred to as "backchain pointers").
For C code, gcc automatically generates instructions for setting up
frame pointers when the -fno-omit-frame-pointer option is used.
But for asm code, the frame setup instructions have to be written by
hand, which most people don't do. So the end result is that
CONFIG_FRAME_POINTER is honored for C code but not for most asm code.
For stack traces based on frame pointers to be reliable, all
functions which call other functions must first create a stack frame
and update the frame pointer. If a first function doesn't properly
create a stack frame before calling a second function, the *caller*
of the first function will be skipped on the stack trace.
For example, consider the following example backtrace with frame
pointers enabled:
[<ffffffff81812584>] dump_stack+0x4b/0x63
[<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
[<ffffffff8127f568>] seq_read+0x108/0x3e0
[<ffffffff812cce62>] proc_reg_read+0x42/0x70
[<ffffffff81256197>] __vfs_read+0x37/0x100
[<ffffffff81256b16>] vfs_read+0x86/0x130
[<ffffffff81257898>] SyS_read+0x58/0xd0
[<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
It correctly shows that the caller of cmdline_proc_show() is
seq_read().
If we remove the frame pointer logic from cmdline_proc_show() by
replacing the frame pointer related instructions with nops, here's
what it looks like instead:
[<ffffffff81812584>] dump_stack+0x4b/0x63
[<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
[<ffffffff812cce62>] proc_reg_read+0x42/0x70
[<ffffffff81256197>] __vfs_read+0x37/0x100
[<ffffffff81256b16>] vfs_read+0x86/0x130
[<ffffffff81257898>] SyS_read+0x58/0xd0
[<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
Notice that cmdline_proc_show()'s caller, seq_read(), has been
skipped. Instead the stack trace seems to show that
cmdline_proc_show() was called by proc_reg_read().
The benefit of objtool here is that because it ensures that *all*
functions honor CONFIG_FRAME_POINTER, no functions will ever[*] be
skipped on a stack trace.
[*] unless an interrupt or exception has occurred at the very
beginning of a function before the stack frame has been created,
or at the very end of the function after the stack frame has been
destroyed. This is an inherent limitation of frame pointers.
b) ORC (Oops Rewind Capability) unwind table generation
An alternative to frame pointers and DWARF, ORC unwind data can be
used to walk the stack. Unlike frame pointers, ORC data is out of
band. So it doesn't affect runtime performance and it can be
reliable even when interrupts or exceptions are involved.
For more details, see Documentation/arch/x86/orc-unwinder.rst.
c) Higher live patching compatibility rate
Livepatch has an optional "consistency model", which is needed for
more complex patches. In order for the consistency model to work,
stack traces need to be reliable (or an unreliable condition needs to
be detectable). Objtool makes that possible.
For more details, see the livepatch documentation in the Linux kernel
source tree at Documentation/livepatch/livepatch.rst.
To achieve the validation, objtool enforces the following rules:
1. Each callable function must be annotated as such with the ELF
function type. In asm code, this is typically done using the
ENTRY/ENDPROC macros. If objtool finds a return instruction
outside of a function, it flags an error since that usually indicates
callable code which should be annotated accordingly.
This rule is needed so that objtool can properly identify each
callable function in order to analyze its stack metadata.
2. Conversely, each section of code which is *not* callable should *not*
be annotated as an ELF function. The ENDPROC macro shouldn't be used
in this case.
This rule is needed so that objtool can ignore non-callable code.
Such code doesn't have to follow any of the other rules.
3. Each callable function which calls another function must have the
correct frame pointer logic, if required by CONFIG_FRAME_POINTER or
the architecture's back chain rules. This can by done in asm code
with the FRAME_BEGIN/FRAME_END macros.
This rule ensures that frame pointer based stack traces will work as
designed. If function A doesn't create a stack frame before calling
function B, the _caller_ of function A will be skipped on the stack
trace.
4. Dynamic jumps and jumps to undefined symbols are only allowed if:
a) the jump is part of a switch statement; or
b) the jump matches sibling call semantics and the frame pointer has
the same value it had on function entry.
This rule is needed so that objtool can reliably analyze all of a
function's code paths. If a function jumps to code in another file,
and it's not a sibling call, objtool has no way to follow the jump
because it only analyzes a single file at a time.
5. A callable function may not execute kernel entry/exit instructions.
The only code which needs such instructions is kernel entry code,
which shouldn't be be in callable functions anyway.
This rule is just a sanity check to ensure that callable functions
return normally.
Objtool warnings
----------------
NOTE: When requesting help with an objtool warning, please recreate with
OBJTOOL_VERBOSE=1 (e.g., "make OBJTOOL_VERBOSE=1") and send the full
output, including any disassembly or backtrace below the warning, to the
objtool maintainers.
For asm files, if you're getting an error which doesn't make sense,
first make sure that the affected code follows the above rules.
For C files, the common culprits are inline asm statements and calls to
"noreturn" functions. See below for more details.
Another possible cause for errors in C code is if the Makefile removes
-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.
Here are some examples of common warnings reported by objtool, what
they mean, and suggestions for how to fix them. When in doubt, ping
the objtool maintainers.
1. file.o: warning: objtool: func()+0x128: call without frame pointer save/setup
The func() function made a function call without first saving and/or
updating the frame pointer, and CONFIG_FRAME_POINTER is enabled.
If the error is for an asm file, and func() is indeed a callable
function, add proper frame pointer logic using the FRAME_BEGIN and
FRAME_END macros. Otherwise, if it's not a callable function, remove
its ELF function annotation by changing ENDPROC to END, and instead
use the manual unwind hint macros in asm/unwind_hints.h.
If it's a GCC-compiled .c file, the error may be because the function
uses an inline asm() statement which has a "call" instruction. An
asm() statement with a call instruction must declare the use of the
stack pointer in its output operand. On x86_64, this means adding
the ASM_CALL_CONSTRAINT as an output constraint:
asm volatile("call func" : ASM_CALL_CONSTRAINT);
Otherwise the stack frame may not get created before the call.
2. file.o: warning: objtool: .text+0x53: unreachable instruction
Objtool couldn't find a code path to reach the instruction.
If the error is for an asm file, and the instruction is inside (or
reachable from) a callable function, the function should be annotated
with the ENTRY/ENDPROC macros (ENDPROC is the important one).
Otherwise, the code should probably be annotated with the unwind hint
macros in asm/unwind_hints.h so objtool and the unwinder can know the
stack state associated with the code.
If you're 100% sure the code won't affect stack traces, or if you're
a just a bad person, you can tell objtool to ignore it. See the
"Adding exceptions" section below.
If it's not actually in a callable function (e.g. kernel entry code),
change ENDPROC to END.
3. file.o: warning: objtool: foo+0x48c: bar() is missing a __noreturn annotation
The call from foo() to bar() doesn't return, but bar() is missing the
__noreturn annotation. NOTE: In addition to annotating the function
with __noreturn, please also add it to tools/objtool/noreturns.h.
4. file.o: warning: objtool: func(): can't find starting instruction
or
file.o: warning: objtool: func()+0x11dd: can't decode instruction
Does the file have data in a text section? If so, that can confuse
objtool's instruction decoder. Move the data to a more appropriate
section like .data or .rodata.
5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function
This is a kernel entry/exit instruction like sysenter or iret. Such
instructions aren't allowed in a callable function, and are most
likely part of the kernel entry code. They should usually not have
the callable function annotation (ENDPROC) and should always be
annotated with the unwind hint macros in asm/unwind_hints.h.
6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame
This is a dynamic jump or a jump to an undefined symbol. Objtool
assumed it's a sibling call and detected that the frame pointer
wasn't first restored to its original state.
If it's not really a sibling call, you may need to move the
destination code to the local file.
If the instruction is not actually in a callable function (e.g.
kernel entry code), change ENDPROC to END and annotate manually with
the unwind hint macros in asm/unwind_hints.h.
7. file: warning: objtool: func()+0x5c: stack state mismatch
The instruction's frame pointer state is inconsistent, depending on
which execution path was taken to reach the instruction.
Make sure that, when CONFIG_FRAME_POINTER is enabled, the function
pushes and sets up the frame pointer (for x86_64, this means rbp) at
the beginning of the function and pops it at the end of the function.
Also make sure that no other code in the function touches the frame
pointer.
Another possibility is that the code has some asm or inline asm which
does some unusual things to the stack or the frame pointer. In such
cases it's probably appropriate to use the unwind hint macros in
asm/unwind_hints.h.
8. file.o: warning: objtool: funcA() falls through to next function funcB()
This means that funcA() doesn't end with a return instruction or an
unconditional jump, and that objtool has determined that the function
can fall through into the next function. There could be different
reasons for this:
1) funcA()'s last instruction is a call to a "noreturn" function like
panic(). In this case the noreturn function needs to be added to
objtool's hard-coded global_noreturns array. Feel free to bug the
objtool maintainer, or you can submit a patch.
2) funcA() uses the unreachable() annotation in a section of code
that is actually reachable.
3) If funcA() calls an inline function, the object code for funcA()
might be corrupt due to a gcc bug. For more details, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled
This means that an unexpected call to a non-whitelisted function exists
outside of arch-specific guards.
X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end()
ARM: PAN: uaccess_enable()/uaccess_disable()
These functions should be called to denote a minimal critical section around
access to __user variables. See also: https://lwn.net/Articles/517475/
The intention of the warning is to prevent calls to funcB() from eventually
calling schedule(), potentially leaking the AC flags state, and not
restoring them correctly.
It also helps verify that there are no unexpected calls to funcB() which may
access user space pages with protections against doing so disabled.
To fix, either:
1) remove explicit calls to funcB() from funcA().
2) add the correct guards before and after calls to low level functions like
__get_user_size()/__put_user_size().
3) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
funcB obviously does not call schedule(), and is marked notrace (since
function tracing inserts additional calls, which is not obvious from the
sources).
10. file.o: warning: func()+0x5c: stack layout conflict in alternatives
This means that in the use of the alternative() or ALTERNATIVE()
macro, the code paths have conflicting modifications to the stack.
The problem is that there is only one ORC unwind table, which means
that the ORC unwind entries must be consistent for all possible
instruction boundaries regardless of which code has been patched.
This limitation can be overcome by massaging the alternatives with
NOPs to shift the stack changes around so they no longer conflict.
11. file.o: warning: unannotated intra-function call
This warning means that a direct call is done to a destination which
is not at the beginning of a function. If this is a legit call, you
can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL
directive right before the call.
12. file.o: warning: func(): not an indirect call target
This means that objtool is running with --ibt and a function expected
to be an indirect call target is not. In particular, this happens for
init_module() or cleanup_module() if a module relies on these special
names and does not use module_init() / module_exit() macros to create
them.
If the error doesn't seem to make sense, it could be a bug in objtool.
Feel free to ask the objtool maintainer for help.
Adding exceptions
-----------------
If you _really_ need objtool to ignore something, and are 100% sure
that it won't affect kernel stack traces, you can tell objtool to
ignore it:
- To skip validation of a function, use the STACK_FRAME_NON_STANDARD
macro.
- To skip validation of a file, add
OBJECT_FILES_NON_STANDARD_filename.o := y
to the Makefile.
- To skip validation of a directory, add
OBJECT_FILES_NON_STANDARD := y
to the Makefile.
NOTE: OBJECT_FILES_NON_STANDARD doesn't work for link time validation of
vmlinux.o or a linked module. So it should only be used for files which
aren't linked into vmlinux or a module.