[PATCH 4.4.y-cip] signal/m68k: Use force_sigsegv(SIGSEGV) in fpsp040_die

[Finn Thain <fthain@linux-m68k.org>]

From: "Eric W. Biederman" <ebiederm@xmission.com>

[ Upstream commit a3616a3c02722d1edb95acc7fceade242f6553ba ]

In the fpsp040 code when copyin or copyout fails call
force_sigsegv(SIGSEGV) instead of do_exit(SIGSEGV).

This solves a couple of problems.  Because do_exit embeds the ptrace
stop PTRACE_EVENT_EXIT a complete stack frame needs to be present for
that to work correctly.  There is always the information needed for a
ptrace stop where get_signal is called.  So exiting with a signal
solves the ptrace issue.

Further exiting with a signal ensures that all of the threads in a
process are killed not just the thread that malfunctioned.  Which
avoids confusing userspace.

To make force_sigsegv(SIGSEGV) work in fpsp040_die modify the code to
save all of the registers and jump to ret_from_exception (which
ultimately calls get_signal) after fpsp040_die returns.

v2: Updated the branches to use gas's pseudo ops that automatically
    calculate the best branch instruction to use for the purpose.

v1: https://lkml.kernel.org/r/87a6m8kgtx.fsf_-_@disp2133
Link: https://lkml.kernel.org/r/87tukghjfs.fsf_-_@disp2133
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
[ Pass 'current' to force_sigsegv() as was done prior to v5.3 ]
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
 arch/m68k/fpsp040/skeleton.S | 3 ++-
 arch/m68k/kernel/traps.c     | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/m68k/fpsp040/skeleton.S b/arch/m68k/fpsp040/skeleton.S
index 31a9c634c81e..081922c72daa 100644
--- a/arch/m68k/fpsp040/skeleton.S
+++ b/arch/m68k/fpsp040/skeleton.S
@@ -502,7 +502,8 @@ in_ea:
 	.section .fixup,"ax"
 	.even
 1:
-	jbra	fpsp040_die
+	jbsr	fpsp040_die
+	jbra	.Lnotkern
 
 	.section __ex_table,"a"
 	.align	4
diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index 35f706d836c5..c6f18dc5884b 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -1155,7 +1155,7 @@ asmlinkage void set_esp0(unsigned long ssp)
  */
 asmlinkage void fpsp040_die(void)
 {
-	do_exit(SIGSEGV);
+	force_sigsegv(SIGSEGV, current);
 }
 
 #ifdef CONFIG_M68KFPU_EMU