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-rw-r--r--tools/objtool/Documentation/stack-validation.txt153
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.