diff options
Diffstat (limited to 'arch/arm64/kernel/vdso/sigreturn.S')
| -rw-r--r-- | arch/arm64/kernel/vdso/sigreturn.S | 80 |
1 files changed, 80 insertions, 0 deletions
diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S new file mode 100644 index 000000000..0e18729ab --- /dev/null +++ b/arch/arm64/kernel/vdso/sigreturn.S @@ -0,0 +1,80 @@ +/* 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 |