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The kernel CONFIG_OBJTOOL option enables a host tool named 'objtool'which runs at compile time. It can do various validations andtransformations on .o files.
Objtool has become an integral part of the x86-64 kernel toolchain. Thekernel 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
- Return thunk validation -- validates return thunks are used for certain CPU mitigations including Retbleed and SRSO
- Return thunk annotation -- annotates all return thunk sites so kernel can patch them inline, depending on enabled mitigations
- Return thunk untraining validation -- validate that all entry paths untrain a "safe return" before the first return (or call)
- 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
- Function preamble (prefix) annotation and/or symbol generation -- used for FineIBT and call depth tracking
- Other toolchain hacks which will go unmentioned at this time...
Each feature can be enabled individually or in combination using theobjtool cmdline.
Objects-------
Typically, objtool runs on every translation unit (TU, aka ".o file") inthe kernel. If a TU is part of a kernel module, the '--module' optionis 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 ensuresthe validity of its stack metadata. It enforces a set of rules on asmcode and C inline assembly code so that stack traces can be reliable.
For each function, it recursively follows all possible code paths andvalidates 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 addalternative execution paths to a given instruction (or set ofinstructions). Similarly, it knows how to follow switch statements, forwhich 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 SYM_FUNC_{START,END} 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, or is otherwise doing funny things with the stack or registers, should *not* be annotated as an ELF function. Rather, SYM_CODE_{START,END} should be used along with unwind hints.
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. Indirect 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 withOBJTOOL_VERBOSE=1 (e.g., "make OBJTOOL_VERBOSE=1") and send the fulloutput, including any disassembly or backtrace below the warning, to theobjtool 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, whatthey mean, and suggestions for how to fix them. When in doubt, pingthe 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 using SYM_CODE_{START,END} and 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.
objtool can help with pinpointing the exact function where it happens:
$ OBJTOOL_ARGS="--verbose" make arch/x86/kvm/
arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup arch/x86/kvm/kvm.o: warning: objtool: em_loop.part.0+0x29: (alt) arch/x86/kvm/kvm.o: warning: objtool: em_loop.part.0+0x0: <=== (sym) LD [M] arch/x86/kvm/kvm-intel.o 0000 0000000000028220 <em_loop.part.0>: 0000 28220: 0f b6 47 61 movzbl 0x61(%rdi),%eax 0004 28224: 3c e2 cmp $0xe2,%al 0006 28226: 74 2c je 28254 <em_loop.part.0+0x34> 0008 28228: 48 8b 57 10 mov 0x10(%rdi),%rdx 000c 2822c: 83 f0 05 xor $0x5,%eax 000f 2822f: 48 c1 e0 04 shl $0x4,%rax 0013 28233: 25 f0 00 00 00 and $0xf0,%eax 0018 28238: 81 e2 d5 08 00 00 and $0x8d5,%edx 001e 2823e: 80 ce 02 or $0x2,%dh ...
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 SYM_FUNC_START and SYM_FUNC_END macros.
Otherwise, SYM_CODE_START can be used. In that case the code needs to be annotated with unwind hint macros.
If you're sure the code won't affect the reliability of runtime stack traces and want objtool to ignore it, see "Adding exceptions" below.
3. file.o: warning: objtool: foo+0x48c: bar() missing __noreturn in .c/.h or NORETURN() in noreturns.h
The call from foo() to bar() doesn't return, but bar() is incorrectly annotated. A noreturn function must be marked __noreturn in both its declaration and its definition, and must have a NORETURN() annotation in 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. Such code should probably be placed in a SYM_CODE_{START,END} block with unwind hints.
6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame
This is a branch to an UNDEF symbol. Objtool assumed it's a sibling call and detected that the stack wasn't first restored to its original state.
If it's not really a sibling call, you may need to use unwind hints and/or move the destination code to the local file.
If the instruction is not actually in a callable function (e.g. kernel entry code), use SYM_CODE_{START,END} and unwind hints.
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 SYM_CODE_{START,END} with unwind hints.
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:
a) 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.
b) funcA() uses the unreachable() annotation in a section of code that is actually reachable.
c) Some undefined behavior like divide by zero.
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 objtool maintainers for help.
Adding exceptions-----------------
If you _really_ need objtool to ignore something, and are 100% surethat it won't affect kernel stack traces, you can tell objtool toignore 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 ofvmlinux.o or a linked module. So it should only be used for files whicharen't linked into vmlinux or a module.