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Diffstat (limited to 'tools/objtool/Documentation/stack-validation.txt')
| -rw-r--r-- | tools/objtool/Documentation/stack-validation.txt | 153 |
1 files changed, 65 insertions, 88 deletions
diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt index 55a60d331f47..17c1195f11f4 100644 --- a/tools/objtool/Documentation/stack-validation.txt +++ b/tools/objtool/Documentation/stack-validation.txt @@ -127,28 +127,13 @@ b) 100% reliable stack traces for DWARF enabled kernels 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 + 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 ----- @@ -201,80 +186,84 @@ To achieve the validation, objtool enforces the following rules: return normally. -Errors in .S files ------------------- +Objtool warnings +---------------- -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. +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. asm_file.o: warning: objtool: func()+0x128: call without frame pointer save/setup +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. - - 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. + updating the frame pointer, and CONFIG_FRAME_POINTER is enabled. - If you're getting this error in a .c file, see the "Errors in .c - files" section. + 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: -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. + register void *__sp asm("rsp"); + asm volatile("call func" : "+r" (__sp)); - If the return instruction is inside (or reachable from) a callable - function, the function needs to be annotated with the ENTRY/ENDPROC - macros. + Otherwise the stack frame may not get created before the call. - 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. +2. file.o: warning: objtool: .text+0x53: unreachable instruction -3. asm_file.o: warning: objtool: func()+0x9: function has unreachable instruction + Objtool couldn't find a code path to reach the 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 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 the instruction is actually needed, and it's actually in a - callable function, ensure that its function is properly annotated - with ENTRY/ENDPROC. + 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. asm_file.o: warning: objtool: func(): can't find starting instruction +4. file.o: warning: objtool: func(): can't find starting instruction or - asm_file.o: warning: objtool: func()+0x11dd: can't decode instruction + file.o: warning: objtool: func()+0x11dd: can't decode instruction - Did you put data in a text section? If so, that can confuse + 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. 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. +5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function - If the instruction isn't actually in a callable function, change - ENDPROC to END. + 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. asm_file.o: warning: objtool: func()+0x26: sibling call from callable instruction with changed frame pointer +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. Stacktool + 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. @@ -282,24 +271,28 @@ they mean, and suggestions for how to fix them. 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. + kernel entry code), change ENDPROC to END and annotate manually with + the CFI hint macros in asm/undwarf.h. -7. asm_file: warning: objtool: func()+0x5c: frame pointer state mismatch +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 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. + 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. -Errors in .c files ------------------- -1. c_file.o: warning: objtool: funcA() falls through to next function funcB() +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 @@ -318,22 +311,6 @@ Errors in .c files might be corrupt due to a gcc bug. For more details, see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646 -2. If you're getting any other objtool error in a compiled .c file, it - may be because 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. - -3. 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. |