objtool.txt

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

- 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 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
   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 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 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% 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.