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diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt new file mode 100644 index 000000000000..5a95896105bc --- /dev/null +++ b/tools/objtool/Documentation/stack-validation.txt @@ -0,0 +1,342 @@ +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: + + [<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) 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 + + (NOTE: This is not yet implemented) + + Currently with CONFIG_LIVEPATCH there's a basic live patching + framework which is safe for roughly 85-90% of "security" fixes. But + patches can't have complex features like function dependency or + prototype changes, or data structure changes. + + There's a strong need to support patches which have the more complex + features so that the patch compatibility rate for security fixes can + eventually approach something resembling 100%. To achieve that, a + "consistency model" is needed, which allows tasks to be safely + transitioned from an unpatched state to a patched state. + + One of the key requirements of the currently proposed livepatch + consistency model [*] is that it needs to walk the stack of each + sleeping task to determine if it can be transitioned to the patched + state. If objtool can ensure that stack traces are reliable, this + consistency model can be used and the live patching compatibility + rate can be improved significantly. + + [*] https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@redhat.com + + +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. + + +Errors in .S files +------------------ + +If you're getting an error in a compiled .S file which you don't +understand, first make sure that the affected code follows the above +rules. + +Here are some examples of common warnings reported by objtool, what +they mean, and suggestions for how to fix them. + + +1. asm_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. + + If func() is indeed a callable function, add proper frame pointer + logic using the FRAME_BEGIN and FRAME_END macros. Otherwise, remove + its ELF function annotation by changing ENDPROC to END. + + If you're getting this error in a .c file, see the "Errors in .c + files" section. + + +2. asm_file.o: warning: objtool: .text+0x53: return instruction outside of a callable function + + A return instruction was detected, but objtool couldn't find a way + for a callable function to reach the instruction. + + If the return instruction is inside (or reachable from) a callable + function, the function needs to be annotated with the ENTRY/ENDPROC + macros. + + If you _really_ need a return instruction outside of a function, and + are 100% sure that it won't affect stack traces, you can tell + objtool to ignore it. See the "Adding exceptions" section below. + + +3. asm_file.o: warning: objtool: func()+0x9: function has unreachable instruction + + The instruction lives inside of a callable function, but there's no + possible control flow path from the beginning of the function to the + instruction. + + If the instruction is actually needed, and it's actually in a + callable function, ensure that its function is properly annotated + with ENTRY/ENDPROC. + + If it's not actually in a callable function (e.g. kernel entry code), + change ENDPROC to END. + + +4. asm_file.o: warning: objtool: func(): can't find starting instruction + or + asm_file.o: warning: objtool: func()+0x11dd: can't decode instruction + + Did you put 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. asm_file.o: warning: objtool: func()+0x6: kernel entry/exit from callable instruction + + This is a kernel entry/exit instruction like sysenter or sysret. + Such instructions aren't allowed in a callable function, and are most + likely part of the kernel entry code. + + If the instruction isn't actually in a callable function, change + ENDPROC to END. + + +6. asm_file.o: warning: objtool: func()+0x26: sibling call from callable instruction with changed frame pointer + + This is a dynamic jump or a jump to an undefined symbol. Stacktool + 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. + + +7. asm_file: warning: objtool: func()+0x5c: frame pointer state mismatch + + The instruction's frame pointer state is inconsistent, depending on + which execution path was taken to reach the instruction. + + Make sure 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. + + +Errors in .c files +------------------ + +If you're getting an objtool error in a compiled .c file, chances are +the file uses an 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. + +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. + +Also see the above section for .S file errors for more information what +the individual error messages mean. + +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. |