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360 lines
15 KiB
360 lines
15 KiB
Compile-time stack metadata validation |
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====================================== |
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Overview |
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-------- |
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The kernel CONFIG_STACK_VALIDATION option enables a host tool named |
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objtool which runs at compile time. It has a "check" subcommand which |
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analyzes every .o file and ensures the validity of its stack metadata. |
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It enforces a set of rules on asm code and C inline assembly code so |
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that stack traces can be reliable. |
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For each function, it recursively follows all possible code paths and |
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validates the correct frame pointer state at each instruction. |
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It also follows code paths involving special sections, like |
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.altinstructions, __jump_table, and __ex_table, which can add |
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alternative execution paths to a given instruction (or set of |
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instructions). Similarly, it knows how to follow switch statements, for |
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which gcc sometimes uses jump tables. |
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(Objtool also has an 'orc generate' subcommand which generates debuginfo |
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for the ORC unwinder. See Documentation/x86/orc-unwinder.rst in the |
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kernel tree for more details.) |
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Why do we need stack metadata validation? |
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----------------------------------------- |
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Here are some of the benefits of validating stack metadata: |
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a) More reliable stack traces for frame pointer enabled kernels |
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Frame pointers are used for debugging purposes. They allow runtime |
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code and debug tools to be able to walk the stack to determine the |
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chain of function call sites that led to the currently executing |
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code. |
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For some architectures, frame pointers are enabled by |
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CONFIG_FRAME_POINTER. For some other architectures they may be |
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required by the ABI (sometimes referred to as "backchain pointers"). |
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For C code, gcc automatically generates instructions for setting up |
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frame pointers when the -fno-omit-frame-pointer option is used. |
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But for asm code, the frame setup instructions have to be written by |
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hand, which most people don't do. So the end result is that |
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CONFIG_FRAME_POINTER is honored for C code but not for most asm code. |
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For stack traces based on frame pointers to be reliable, all |
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functions which call other functions must first create a stack frame |
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and update the frame pointer. If a first function doesn't properly |
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create a stack frame before calling a second function, the *caller* |
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of the first function will be skipped on the stack trace. |
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For example, consider the following example backtrace with frame |
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pointers enabled: |
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[<ffffffff81812584>] dump_stack+0x4b/0x63 |
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[<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30 |
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[<ffffffff8127f568>] seq_read+0x108/0x3e0 |
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[<ffffffff812cce62>] proc_reg_read+0x42/0x70 |
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[<ffffffff81256197>] __vfs_read+0x37/0x100 |
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[<ffffffff81256b16>] vfs_read+0x86/0x130 |
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[<ffffffff81257898>] SyS_read+0x58/0xd0 |
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[<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76 |
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It correctly shows that the caller of cmdline_proc_show() is |
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seq_read(). |
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If we remove the frame pointer logic from cmdline_proc_show() by |
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replacing the frame pointer related instructions with nops, here's |
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what it looks like instead: |
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[<ffffffff81812584>] dump_stack+0x4b/0x63 |
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[<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30 |
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[<ffffffff812cce62>] proc_reg_read+0x42/0x70 |
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[<ffffffff81256197>] __vfs_read+0x37/0x100 |
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[<ffffffff81256b16>] vfs_read+0x86/0x130 |
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[<ffffffff81257898>] SyS_read+0x58/0xd0 |
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[<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76 |
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Notice that cmdline_proc_show()'s caller, seq_read(), has been |
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skipped. Instead the stack trace seems to show that |
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cmdline_proc_show() was called by proc_reg_read(). |
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The benefit of objtool here is that because it ensures that *all* |
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functions honor CONFIG_FRAME_POINTER, no functions will ever[*] be |
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skipped on a stack trace. |
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[*] unless an interrupt or exception has occurred at the very |
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beginning of a function before the stack frame has been created, |
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or at the very end of the function after the stack frame has been |
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destroyed. This is an inherent limitation of frame pointers. |
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b) ORC (Oops Rewind Capability) unwind table generation |
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An alternative to frame pointers and DWARF, ORC unwind data can be |
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used to walk the stack. Unlike frame pointers, ORC data is out of |
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band. So it doesn't affect runtime performance and it can be |
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reliable even when interrupts or exceptions are involved. |
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For more details, see Documentation/x86/orc-unwinder.rst. |
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c) Higher live patching compatibility rate |
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Livepatch has an optional "consistency model", which is needed for |
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more complex patches. In order for the consistency model to work, |
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stack traces need to be reliable (or an unreliable condition needs to |
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be detectable). Objtool makes that possible. |
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For more details, see the livepatch documentation in the Linux kernel |
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source tree at Documentation/livepatch/livepatch.rst. |
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Rules |
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----- |
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To achieve the validation, objtool enforces the following rules: |
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1. Each callable function must be annotated as such with the ELF |
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function type. In asm code, this is typically done using the |
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ENTRY/ENDPROC macros. If objtool finds a return instruction |
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outside of a function, it flags an error since that usually indicates |
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callable code which should be annotated accordingly. |
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This rule is needed so that objtool can properly identify each |
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callable function in order to analyze its stack metadata. |
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2. Conversely, each section of code which is *not* callable should *not* |
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be annotated as an ELF function. The ENDPROC macro shouldn't be used |
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in this case. |
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This rule is needed so that objtool can ignore non-callable code. |
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Such code doesn't have to follow any of the other rules. |
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3. Each callable function which calls another function must have the |
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correct frame pointer logic, if required by CONFIG_FRAME_POINTER or |
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the architecture's back chain rules. This can by done in asm code |
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with the FRAME_BEGIN/FRAME_END macros. |
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This rule ensures that frame pointer based stack traces will work as |
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designed. If function A doesn't create a stack frame before calling |
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function B, the _caller_ of function A will be skipped on the stack |
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trace. |
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4. Dynamic jumps and jumps to undefined symbols are only allowed if: |
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a) the jump is part of a switch statement; or |
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b) the jump matches sibling call semantics and the frame pointer has |
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the same value it had on function entry. |
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This rule is needed so that objtool can reliably analyze all of a |
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function's code paths. If a function jumps to code in another file, |
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and it's not a sibling call, objtool has no way to follow the jump |
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because it only analyzes a single file at a time. |
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5. A callable function may not execute kernel entry/exit instructions. |
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The only code which needs such instructions is kernel entry code, |
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which shouldn't be be in callable functions anyway. |
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This rule is just a sanity check to ensure that callable functions |
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return normally. |
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Objtool warnings |
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---------------- |
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For asm files, if you're getting an error which doesn't make sense, |
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first make sure that the affected code follows the above rules. |
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For C files, the common culprits are inline asm statements and calls to |
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"noreturn" functions. See below for more details. |
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Another possible cause for errors in C code is if the Makefile removes |
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-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options. |
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Here are some examples of common warnings reported by objtool, what |
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they mean, and suggestions for how to fix them. |
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1. file.o: warning: objtool: func()+0x128: call without frame pointer save/setup |
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The func() function made a function call without first saving and/or |
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updating the frame pointer, and CONFIG_FRAME_POINTER is enabled. |
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If the error is for an asm file, and func() is indeed a callable |
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function, add proper frame pointer logic using the FRAME_BEGIN and |
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FRAME_END macros. Otherwise, if it's not a callable function, remove |
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its ELF function annotation by changing ENDPROC to END, and instead |
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use the manual unwind hint macros in asm/unwind_hints.h. |
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If it's a GCC-compiled .c file, the error may be because the function |
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uses an inline asm() statement which has a "call" instruction. An |
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asm() statement with a call instruction must declare the use of the |
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stack pointer in its output operand. On x86_64, this means adding |
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the ASM_CALL_CONSTRAINT as an output constraint: |
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asm volatile("call func" : ASM_CALL_CONSTRAINT); |
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Otherwise the stack frame may not get created before the call. |
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2. file.o: warning: objtool: .text+0x53: unreachable instruction |
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Objtool couldn't find a code path to reach the instruction. |
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If the error is for an asm file, and the instruction is inside (or |
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reachable from) a callable function, the function should be annotated |
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with the ENTRY/ENDPROC macros (ENDPROC is the important one). |
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Otherwise, the code should probably be annotated with the unwind hint |
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macros in asm/unwind_hints.h so objtool and the unwinder can know the |
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stack state associated with the code. |
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If you're 100% sure the code won't affect stack traces, or if you're |
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a just a bad person, you can tell objtool to ignore it. See the |
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"Adding exceptions" section below. |
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If it's not actually in a callable function (e.g. kernel entry code), |
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change ENDPROC to END. |
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4. file.o: warning: objtool: func(): can't find starting instruction |
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or |
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file.o: warning: objtool: func()+0x11dd: can't decode instruction |
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Does the file have data in a text section? If so, that can confuse |
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objtool's instruction decoder. Move the data to a more appropriate |
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section like .data or .rodata. |
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5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function |
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This is a kernel entry/exit instruction like sysenter or iret. Such |
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instructions aren't allowed in a callable function, and are most |
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likely part of the kernel entry code. They should usually not have |
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the callable function annotation (ENDPROC) and should always be |
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annotated with the unwind hint macros in asm/unwind_hints.h. |
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6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame |
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This is a dynamic jump or a jump to an undefined symbol. Objtool |
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assumed it's a sibling call and detected that the frame pointer |
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wasn't first restored to its original state. |
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If it's not really a sibling call, you may need to move the |
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destination code to the local file. |
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If the instruction is not actually in a callable function (e.g. |
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kernel entry code), change ENDPROC to END and annotate manually with |
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the unwind hint macros in asm/unwind_hints.h. |
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7. file: warning: objtool: func()+0x5c: stack state mismatch |
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The instruction's frame pointer state is inconsistent, depending on |
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which execution path was taken to reach the instruction. |
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Make sure that, when CONFIG_FRAME_POINTER is enabled, the function |
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pushes and sets up the frame pointer (for x86_64, this means rbp) at |
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the beginning of the function and pops it at the end of the function. |
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Also make sure that no other code in the function touches the frame |
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pointer. |
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Another possibility is that the code has some asm or inline asm which |
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does some unusual things to the stack or the frame pointer. In such |
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cases it's probably appropriate to use the unwind hint macros in |
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asm/unwind_hints.h. |
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8. file.o: warning: objtool: funcA() falls through to next function funcB() |
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This means that funcA() doesn't end with a return instruction or an |
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unconditional jump, and that objtool has determined that the function |
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can fall through into the next function. There could be different |
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reasons for this: |
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1) funcA()'s last instruction is a call to a "noreturn" function like |
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panic(). In this case the noreturn function needs to be added to |
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objtool's hard-coded global_noreturns array. Feel free to bug the |
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objtool maintainer, or you can submit a patch. |
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2) funcA() uses the unreachable() annotation in a section of code |
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that is actually reachable. |
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3) If funcA() calls an inline function, the object code for funcA() |
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might be corrupt due to a gcc bug. For more details, see: |
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646 |
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9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled |
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This means that an unexpected call to a non-whitelisted function exists |
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outside of arch-specific guards. |
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X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end() |
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ARM: PAN: uaccess_enable()/uaccess_disable() |
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These functions should be called to denote a minimal critical section around |
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access to __user variables. See also: https://lwn.net/Articles/517475/ |
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The intention of the warning is to prevent calls to funcB() from eventually |
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calling schedule(), potentially leaking the AC flags state, and not |
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restoring them correctly. |
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It also helps verify that there are no unexpected calls to funcB() which may |
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access user space pages with protections against doing so disabled. |
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To fix, either: |
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1) remove explicit calls to funcB() from funcA(). |
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2) add the correct guards before and after calls to low level functions like |
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__get_user_size()/__put_user_size(). |
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3) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if |
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funcB obviously does not call schedule(), and is marked notrace (since |
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function tracing inserts additional calls, which is not obvious from the |
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sources). |
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10. file.o: warning: func()+0x5c: stack layout conflict in alternatives |
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This means that in the use of the alternative() or ALTERNATIVE() |
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macro, the code paths have conflicting modifications to the stack. |
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The problem is that there is only one ORC unwind table, which means |
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that the ORC unwind entries must be consistent for all possible |
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instruction boundaries regardless of which code has been patched. |
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This limitation can be overcome by massaging the alternatives with |
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NOPs to shift the stack changes around so they no longer conflict. |
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11. file.o: warning: unannotated intra-function call |
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This warning means that a direct call is done to a destination which |
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is not at the beginning of a function. If this is a legit call, you |
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can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL |
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directive right before the call. |
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If the error doesn't seem to make sense, it could be a bug in objtool. |
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Feel free to ask the objtool maintainer for help. |
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Adding exceptions |
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----------------- |
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If you _really_ need objtool to ignore something, and are 100% sure |
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that it won't affect kernel stack traces, you can tell objtool to |
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ignore it: |
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- To skip validation of a function, use the STACK_FRAME_NON_STANDARD |
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macro. |
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- To skip validation of a file, add |
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OBJECT_FILES_NON_STANDARD_filename.o := y |
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to the Makefile. |
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- To skip validation of a directory, add |
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OBJECT_FILES_NON_STANDARD := y |
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to the Makefile.
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