Commit a4eb355a authored by Will Deacon's avatar Will Deacon
Browse files

arm64: vdso: Fix CFI directives in sigreturn trampoline 



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 <tamas.zsoldos@arm.com>
Reported-by: default avatarDave Martin <dave.martin@arm.com>
Reported-by: default avatarDaniel Kiss <daniel.kiss@arm.com>
Tested-by: default avatarDaniel Kiss <daniel.kiss@arm.com>
Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent 9a964285

arch/arm64/kernel/vdso/sigreturn.S

+37 −7
Original line number Diff line number Diff line 
/* 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

arch/arm64/kernel/vdso32/sigreturn.S

+3 −0
Original line number Diff line number Diff line
@@ -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

 */

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