ARC: entry: fix potential EFA clobber when TIF_SYSCALL_TRACE

Trap handler for syscall tracing reads EFA (Exception Fault Address),
in case strace wants PC of trap instruction (EFA is not part of pt_regs
as of current code).

However this EFA read is racy as it happens after dropping to pure
kernel mode (re-enabling interrupts). A taken interrupt could
context-switch, trigger a different task's trap, clobbering EFA for this
execution context.

Fix this by reading EFA early, before re-enabling interrupts. A slight
side benefit is de-duplication of FAKE_RET_FROM_EXCPN in trap handler.
The trap handler is common to both ARCompact and ARCv2 builds too.

This just came out of code rework/review and no real problem was reported
but is clearly a potential problem specially for strace.

Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index 60406ec..ea00c8a 100644 (file)
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -165,7 +165,6 @@ END(EV_Extension)
 tracesys:
        ; save EFA in case tracer wants the PC of traced task
        ; using ERET won't work since next-PC has already committed
-       lr  r12, [efa]
        GET_CURR_TASK_FIELD_PTR   TASK_THREAD, r11
        st  r12, [r11, THREAD_FAULT_ADDR]       ; thread.fault_address
 
@@ -208,15 +207,9 @@ tracesys_exit:
 ; Breakpoint TRAP
 ; ---------------------------------------------
 trap_with_param:
-
-       ; stop_pc info by gdb needs this info
-       lr  r0, [efa]
+       mov r0, r12     ; EFA in case ptracer/gdb wants stop_pc
        mov r1, sp
 
-       ; Now that we have read EFA, it is safe to do "fake" rtie
-       ;   and get out of CPU exception mode
-       FAKE_RET_FROM_EXCPN
-
        ; Save callee regs in case gdb wants to have a look
        ; SP will grow up by size of CALLEE Reg-File
        ; NOTE: clobbers r12
@@ -243,6 +236,10 @@ ENTRY(EV_Trap)
 
        EXCEPTION_PROLOGUE
 
+       lr  r12, [efa]
+
+       FAKE_RET_FROM_EXCPN
+
        ;============ TRAP 1   :breakpoints
        ; Check ECR for trap with arg (PROLOGUE ensures r10 has ECR)
        bmsk.f 0, r10, 7
@@ -250,9 +247,6 @@ ENTRY(EV_Trap)
 
        ;============ TRAP  (no param): syscall top level
 
-       ; First return from Exception to pure K mode (Exception/IRQs renabled)
-       FAKE_RET_FROM_EXCPN
-
        ; If syscall tracing ongoing, invoke pre-post-hooks
        GET_CURR_THR_INFO_FLAGS   r10
        btst r10, TIF_SYSCALL_TRACE