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/* SPDX-License-Identifier: GPL-2.0-only */
/*
* Sigreturn trampoline for returning from a signal when the SA_RESTORER
* flag is not set. It serves primarily as a hall of shame for crappy
* unwinders and features an exciting but mysterious NOP instruction.
*
* It's also fragile as hell, so please think twice before changing anything
* in here.
*
* Copyright (C) 2012 ARM Limited
*
* Author: Will Deacon <will.deacon@arm.com>
*/
#include <linux/linkage.h>
#include <asm/assembler.h>
#include <asm/unistd.h>
.text
/* Ensure that the mysterious NOP can be associated with a function. */
.cfi_startproc
/*
* .cfi_signal_frame causes the corresponding Frame Description Entry in the
* .eh_frame section to be annotated as a signal frame. This allows DWARF
* unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo(), which permits
* unwinding out of the signal trampoline without the need for the mysterious
* NOP.
*/
.cfi_signal_frame
/*
* Tell the unwinder where to locate the frame record linking back to the
* interrupted context. We don't provide unwind info for registers other
* than the frame pointer and the link register here; in practice, this
* is sufficient for unwinding in C/C++ based runtimes and the values in
* the sigcontext may have been modified by this point anyway. Debuggers
* already have baked-in strategies for attempting to unwind out of signals.
*/
.cfi_def_cfa x29, 0
.cfi_offset x29, 0 * 8
.cfi_offset x30, 1 * 8
/*
* This mysterious NOP is required for some unwinders (e.g. libc++) that
* unconditionally subtract one from the result of _Unwind_GetIP() in order to
* identify the calling function.
* Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
*/
nop // Mysterious NOP
/*
* GDB relies on being able to identify the sigreturn instruction sequence to
* unwind from signal handlers. We cannot, therefore, use SYM_FUNC_START()
* here, as it will emit a BTI C instruction and break the unwinder. Thankfully,
* this function is only ever called from a RET and so omitting the landing pad
* is perfectly fine.
*/
SYM_CODE_START(__kernel_rt_sigreturn)
mov x8, #__NR_rt_sigreturn
svc #0
.cfi_endproc
SYM_CODE_END(__kernel_rt_sigreturn)
emit_aarch64_feature_1_and
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