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. Currently it only checks frame pointer usage, but there are plans to add CFI validation for C files and CFI generation for asm files. 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. 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: [] dump_stack+0x4b/0x63 [] cmdline_proc_show+0x12/0x30 [] seq_read+0x108/0x3e0 [] proc_reg_read+0x42/0x70 [] __vfs_read+0x37/0x100 [] vfs_read+0x86/0x130 [] SyS_read+0x58/0xd0 [] 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: [] dump_stack+0x4b/0x63 [] cmdline_proc_show+0x12/0x30 [] proc_reg_read+0x42/0x70 [] __vfs_read+0x37/0x100 [] vfs_read+0x86/0x130 [] SyS_read+0x58/0xd0 [] 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) 100% reliable stack traces for DWARF enabled kernels (NOTE: This is not yet implemented) As an alternative to frame pointers, DWARF Call Frame Information (CFI) metadata can be used to walk the stack. Unlike frame pointers, CFI metadata is out of band. So it doesn't affect runtime performance and it can be reliable even when interrupts or exceptions are involved. For C code, gcc automatically generates DWARF CFI metadata. But for asm code, generating CFI is a tedious manual approach which requires manually placed .cfi assembler macros to be scattered throughout the code. It's clumsy and very easy to get wrong, and it makes the real code harder to read. Stacktool will improve this situation in several ways. For code which already has CFI annotations, it will validate them. For code which doesn't have CFI annotations, it will generate them. So an architecture can opt to strip out all the manual .cfi annotations from their asm code and have objtool generate them instead. We might also add a runtime stack validation debug option where we periodically walk the stack from schedule() and/or an NMI to ensure that the stack metadata is sane and that we reach the bottom of the stack. So the benefit of objtool here will be that external tooling should always show perfect stack traces. And the same will be true for kernel warning/oops traces if the architecture has a runtime DWARF unwinder. 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 CFI hint macros in asm/undwarf.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. For example, on x86_64: register void *__sp asm("rsp"); asm volatile("call func" : "+r" (__sp)); 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 CFI hint macros in asm/undwarf.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 CFI hint macros in asm/undwarf.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 CFI hint macros in asm/undwarf.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 CFI hint macros in asm/undwarf.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.