[RFC PATCH for 4.18 09/16] powerpc: Add syscall detection for restartable sequences

[Mathieu Desnoyers]

From: Boqun Feng <boqun.f...@gmail.com>

Syscalls are not allowed inside restartable sequences, so add a call to
rseq_syscall() at the very beginning of system call exiting path for
CONFIG_DEBUG_RSEQ=y kernel. This could help us to detect whether there
is a syscall issued inside restartable sequences.

[ Tested on 64-bit powerpc kernel by Mathieu Desnoyers. Still needs to
  be tested on 32-bit powerpc kernel. ]

Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
CC: Benjamin Herrenschmidt <b...@kernel.crashing.org>
CC: Paul Mackerras <pau...@samba.org>
CC: Michael Ellerman <m...@ellerman.id.au>
CC: Peter Zijlstra <pet...@infradead.org>
CC: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
CC: linuxppc-dev@lists.ozlabs.org
---
 arch/powerpc/kernel/entry_32.S | 7 +++++++
 arch/powerpc/kernel/entry_64.S | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index eb8d01bae8c6..973577f2141c 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -365,6 +365,13 @@ syscall_dotrace_cont:
        blrl                    /* Call handler */
        .globl  ret_from_syscall
 ret_from_syscall:
+#ifdef CONFIG_DEBUG_RSEQ
+       /* Check whether the syscall is issued inside a restartable sequence */
+       stw     r3,GPR3(r1)
+       addi    r3,r1,STACK_FRAME_OVERHEAD
+       bl      rseq_syscall
+       lwz     r3,GPR3(r1)
+#endif
        mr      r6,r3
        CURRENT_THREAD_INFO(r12, r1)
        /* disable interrupts so current_thread_info()->flags can't change */
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 51695608c68b..1c374387656a 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -184,6 +184,14 @@ system_call:                       /* label this so stack 
traces look sane */
 
 .Lsyscall_exit:
        std     r3,RESULT(r1)
+
+#ifdef CONFIG_DEBUG_RSEQ
+       /* Check whether the syscall is issued inside a restartable sequence */
+       addi    r3,r1,STACK_FRAME_OVERHEAD
+       bl      rseq_syscall
+       ld      r3,RESULT(r1)
+#endif
+
        CURRENT_THREAD_INFO(r12, r1)
 
        ld      r8,_MSR(r1)
-- 
2.11.0