| /* 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 |
