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arm64: vdso: Fix CFI directives in sigreturn trampoline

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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: Dave Martin <dave.martin@arm.com>
Reported-by: Daniel Kiss <daniel.kiss@arm.com>
Tested-by: Daniel Kiss <daniel.kiss@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Will Deacon 2020-05-19 12:56:05 +01:00
parent 9a96428557
commit a4eb355a3f
2 changed files with 40 additions and 7 deletions
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44
arch/arm64/kernel/vdso/sigreturn.S
View File

@ -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

3
arch/arm64/kernel/vdso32/sigreturn.S
View File

@ -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
*/