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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
/*
* NOTE!!! You may notice that all of the .cfi directives in this file have
* been commented out. This is because they have been shown to trigger segfaults
* in libgcc when unwinding out of a SIGCANCEL handler to invoke pthread
* cleanup handlers during the thread cancellation dance. By omitting the
* directives, we trigger an arm64-specific fallback path in the unwinder which
* recognises the signal frame and restores many of the registers directly from
* the sigcontext. Re-enabling the cfi directives here therefore needs to be
* much more comprehensive to reduce the risk of further regressions.
*/
/* Ensure that the mysterious NOP can be associated with a function. */
// .cfi_startproc
/*
* .cfi_signal_frame causes the corresponding Frame Description Entry (FDE) in
* the .eh_frame section to be annotated as a signal frame. This allows DWARF
* unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo() and identify
* the next frame using the unmodified return address instead of subtracting 1,
* which may yield the wrong FDE.
*/
// .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 likely to
* be insufficient for unwinding in C/C++ based runtimes, especially without a
* means to restore the stack pointer. Thankfully, unwinders and 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, libgcc and libunwind rely 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)
// PLEASE DO NOT MODIFY
mov x8, #__NR_rt_sigreturn
// PLEASE DO NOT MODIFY
svc #0
// PLEASE DO NOT MODIFY
// .cfi_endproc
SYM_CODE_END(__kernel_rt_sigreturn)
emit_aarch64_feature_1_and
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