v4.19.13 snapshot.
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+Compile-time stack metadata validation
+======================================
+
+
+Overview
+--------
+
+The kernel CONFIG_STACK_VALIDATION option enables a host tool named
+objtool which runs at compile time. It has a "check" subcommand which
+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.
+
+(Objtool also has an 'orc generate' subcommand which generates debuginfo
+for the ORC unwinder. See Documentation/x86/orc-unwinder.txt in the
+kernel tree for more details.)
+
+
+Why do we need stack metadata validation?
+-----------------------------------------
+
+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/x86/orc-unwinder.txt.
+
+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.txt.
+
+Rules
+-----
+
+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
+----------------
+
+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.
+
+
+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.
+
+
+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
+
+
+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 := n
+
+ to the Makefile.
+
+- To skip validation of a directory, add
+
+ OBJECT_FILES_NON_STANDARD := y
+
+ to the Makefile.