/* SPDX-License-Identifier: GPL-2.0-only */
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/*
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* Sigreturn trampoline for returning from a signal when the SA_RESTORER
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* flag is not set. It serves primarily as a hall of shame for crappy
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* unwinders and features an exciting but mysterious NOP instruction.
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*
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* It's also fragile as hell, so please think twice before changing anything
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* in here.
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*
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* Copyright (C) 2012 ARM Limited
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*
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* Author: Will Deacon <will.deacon@arm.com>
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*/
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#include <linux/linkage.h>
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#include <asm/assembler.h>
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#include <asm/unistd.h>
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.text
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/*
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* NOTE!!! You may notice that all of the .cfi directives in this file have
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* been commented out. This is because they have been shown to trigger segfaults
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* in libgcc when unwinding out of a SIGCANCEL handler to invoke pthread
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* cleanup handlers during the thread cancellation dance. By omitting the
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* directives, we trigger an arm64-specific fallback path in the unwinder which
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* recognises the signal frame and restores many of the registers directly from
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* the sigcontext. Re-enabling the cfi directives here therefore needs to be
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* much more comprehensive to reduce the risk of further regressions.
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*/
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/* Ensure that the mysterious NOP can be associated with a function. */
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// .cfi_startproc
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/*
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* .cfi_signal_frame causes the corresponding Frame Description Entry (FDE) in
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* the .eh_frame section to be annotated as a signal frame. This allows DWARF
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* unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo() and identify
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* the next frame using the unmodified return address instead of subtracting 1,
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* which may yield the wrong FDE.
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*/
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// .cfi_signal_frame
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/*
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* Tell the unwinder where to locate the frame record linking back to the
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* interrupted context. We don't provide unwind info for registers other than
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* the frame pointer and the link register here; in practice, this is likely to
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* be insufficient for unwinding in C/C++ based runtimes, especially without a
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* means to restore the stack pointer. Thankfully, unwinders and debuggers
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* already have baked-in strategies for attempting to unwind out of signals.
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*/
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// .cfi_def_cfa x29, 0
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// .cfi_offset x29, 0 * 8
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// .cfi_offset x30, 1 * 8
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/*
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* This mysterious NOP is required for some unwinders (e.g. libc++) that
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* unconditionally subtract one from the result of _Unwind_GetIP() in order to
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* identify the calling function.
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* Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
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*/
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nop // Mysterious NOP
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/*
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* GDB, libgcc and libunwind rely on being able to identify the sigreturn
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* instruction sequence to unwind from signal handlers. We cannot, therefore,
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* use SYM_FUNC_START() here, as it will emit a BTI C instruction and break the
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* unwinder. Thankfully, this function is only ever called from a RET and so
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* omitting the landing pad is perfectly fine.
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*/
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SYM_CODE_START(__kernel_rt_sigreturn)
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// PLEASE DO NOT MODIFY
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mov x8, #__NR_rt_sigreturn
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// PLEASE DO NOT MODIFY
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svc #0
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// PLEASE DO NOT MODIFY
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// .cfi_endproc
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SYM_CODE_END(__kernel_rt_sigreturn)
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emit_aarch64_feature_1_and
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