1/* SPDX-License-Identifier: GPL-2.0-only */
2/*
3 * Sigreturn trampoline for returning from a signal when the SA_RESTORER
4 * flag is not set. It serves primarily as a hall of shame for crappy
5 * unwinders and features an exciting but mysterious NOP instruction.
6 *
7 * It's also fragile as hell, so please think twice before changing anything
8 * in here.
9 *
10 * Copyright (C) 2012 ARM Limited
11 *
12 * Author: Will Deacon <will.deacon@arm.com>
13 */
14
15#include <linux/linkage.h>
16#include <asm/assembler.h>
17#include <asm/unistd.h>
18
19	.text
20
21/*
22 * NOTE!!!  You may notice that all of the .cfi directives in this file have
23 * been commented out. This is because they have been shown to trigger segfaults
24 * in libgcc when unwinding out of a SIGCANCEL handler to invoke pthread
25 * cleanup handlers during the thread cancellation dance. By omitting the
26 * directives, we trigger an arm64-specific fallback path in the unwinder which
27 * recognises the signal frame and restores many of the registers directly from
28 * the sigcontext. Re-enabling the cfi directives here therefore needs to be
29 * much more comprehensive to reduce the risk of further regressions.
30 */
31
32/* Ensure that the mysterious NOP can be associated with a function. */
33//	.cfi_startproc
34
35/*
36 * .cfi_signal_frame causes the corresponding Frame Description Entry (FDE) in
37 * the .eh_frame section to be annotated as a signal frame. This allows DWARF
38 * unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo() and identify
39 * the next frame using the unmodified return address instead of subtracting 1,
40 * which may yield the wrong FDE.
41 */
42//	.cfi_signal_frame
43
44/*
45 * Tell the unwinder where to locate the frame record linking back to the
46 * interrupted context. We don't provide unwind info for registers other than
47 * the frame pointer and the link register here; in practice, this is likely to
48 * be insufficient for unwinding in C/C++ based runtimes, especially without a
49 * means to restore the stack pointer. Thankfully, unwinders and debuggers
50 * already have baked-in strategies for attempting to unwind out of signals.
51 */
52//	.cfi_def_cfa    x29, 0
53//	.cfi_offset     x29, 0 * 8
54//	.cfi_offset     x30, 1 * 8
55
56/*
57 * This mysterious NOP is required for some unwinders (e.g. libc++) that
58 * unconditionally subtract one from the result of _Unwind_GetIP() in order to
59 * identify the calling function.
60 * Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
61 */
62	nop	// Mysterious NOP
63
64/*
65 * GDB, libgcc and libunwind rely on being able to identify the sigreturn
66 * instruction sequence to unwind from signal handlers. We cannot, therefore,
67 * use SYM_FUNC_START() here, as it will emit a BTI C instruction and break the
68 * unwinder. Thankfully, this function is only ever called from a RET and so
69 * omitting the landing pad is perfectly fine.
70 */
71SYM_CODE_START(__kernel_rt_sigreturn)
72//	PLEASE DO NOT MODIFY
73	mov	x8, #__NR_rt_sigreturn
74//	PLEASE DO NOT MODIFY
75	svc	#0
76//	PLEASE DO NOT MODIFY
77//	.cfi_endproc
78SYM_CODE_END(__kernel_rt_sigreturn)
79
80emit_aarch64_feature_1_and
81