318 lines
13 KiB
Plaintext
318 lines
13 KiB
Plaintext
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.txt 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.txt.
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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.txt.
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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. For example, on x86_64:
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register void *__sp asm("rsp");
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asm volatile("call func" : "+r" (__sp));
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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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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 := n
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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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