1 /* SPDX-License-Identifier: GPL-2.0-only */
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.
7 * It's also fragile as hell, so please think twice before changing anything
10 * Copyright (C) 2012 ARM Limited
12 * Author: Will Deacon <will.deacon@arm.com>
15 #include <linux/linkage.h>
16 #include <asm/assembler.h>
17 #include <asm/unistd.h>
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.
32 /* Ensure that the mysterious NOP can be associated with a function. */
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.
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.
52 // .cfi_def_cfa x29, 0
53 // .cfi_offset x29, 0 * 8
54 // .cfi_offset x30, 1 * 8
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.
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.
71 SYM_CODE_START(__kernel_rt_sigreturn)
72 // PLEASE DO NOT MODIFY
73 mov x8, #__NR_rt_sigreturn
74 // PLEASE DO NOT MODIFY
76 // PLEASE DO NOT MODIFY
78 SYM_CODE_END(__kernel_rt_sigreturn)
80 emit_aarch64_feature_1_and