From a4eb355a3fdad85d16e4b098e8d56bb28b812ce0 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 19 May 2020 12:56:05 +0100 Subject: [PATCH] arm64: vdso: Fix CFI directives in sigreturn trampoline MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Daniel reports that the .cfi_startproc is misplaced for the sigreturn trampoline, which causes LLVM's unwinder to misbehave: | I run into this with LLVM’s unwinder. | This combination was always broken. This prompted Dave to question our use of CFI directives more generally, and I ended up going down a rabbit hole trying to figure out how this very poorly documented stuff gets used. Move the CFI directives so that the "mysterious NOP" is included in the .cfi_{start,end}proc block and add a bunch of comments so that I can save myself another headache in future. Cc: Tamas Zsoldos Reported-by: Dave Martin Reported-by: Daniel Kiss Tested-by: Daniel Kiss Signed-off-by: Will Deacon --- arch/arm64/kernel/vdso/sigreturn.S | 44 ++++++++++++++++++++++++++++++------ arch/arm64/kernel/vdso32/sigreturn.S | 3 +++ 2 files changed, 40 insertions(+), 7 deletions(-) diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S index 0c921130002a..620a3ef837b7 100644 --- a/arch/arm64/kernel/vdso/sigreturn.S +++ b/arch/arm64/kernel/vdso/sigreturn.S @@ -1,7 +1,11 @@ /* SPDX-License-Identifier: GPL-2.0-only */ /* * Sigreturn trampoline for returning from a signal when the SA_RESTORER - * flag is not set. + * 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 * @@ -14,7 +18,38 @@ .text - nop +/* Ensure that the mysterious NOP can be associated with a function. */ + .cfi_startproc + +/* + * .cfi_signal_frame causes the corresponding Frame Description Entry in the + * .eh_frame section to be annotated as a signal frame. This allows DWARF + * unwinders (e.g. libstdc++) to implement _Unwind_GetIPInfo(), which permits + * unwinding out of the signal trampoline without the need for the mysterious + * NOP. + */ + .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 sufficient for unwinding in C/C++ based runtimes and the values in + * the sigcontext may have been modified by this point anyway. 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 relies on being able to identify the sigreturn instruction sequence to * unwind from signal handlers. We cannot, therefore, use SYM_FUNC_START() @@ -23,11 +58,6 @@ * is perfectly fine. */ SYM_CODE_START(__kernel_rt_sigreturn) - .cfi_startproc - .cfi_signal_frame - .cfi_def_cfa x29, 0 - .cfi_offset x29, 0 * 8 - .cfi_offset x30, 1 * 8 mov x8, #__NR_rt_sigreturn svc #0 .cfi_endproc diff --git a/arch/arm64/kernel/vdso32/sigreturn.S b/arch/arm64/kernel/vdso32/sigreturn.S index b36d4e2267a3..b0091064c3d6 100644 --- a/arch/arm64/kernel/vdso32/sigreturn.S +++ b/arch/arm64/kernel/vdso32/sigreturn.S @@ -3,6 +3,9 @@ * This file provides both A32 and T32 versions, in accordance with the * arm sigreturn code. * + * Please read the comments in arch/arm64/kernel/vdso/sigreturn.S to + * understand some of the craziness in here. + * * Copyright (C) 2018 ARM Limited */ -- 2.11.0