arm64: entry: always restore x0 from the stack on syscall return

We have a micro-optimisation on the fast syscall return path where we
take care to keep x0 live with the return value from the syscall so that
we can avoid restoring it from the stack. The benefit of doing this is
fairly suspect, since we will be restoring x1 from the stack anyway
(which lives adjacent in the pt_regs structure) and the only additional
cost is saving x0 back to pt_regs after the syscall handler, which could
be seen as a poor man's prefetch.

More importantly, this causes issues with the context tracking code.

The ct_user_enter macro ends up branching into C code, which is free to
use x0 as a scratch register and consequently leads to us returning junk
back to userspace as the syscall return value. Rather than special case
the context-tracking code, this patch removes the questionable
optimisation entirely.

Cc: <stable@vger.kernel.org>
Cc: Larry Bassel <larry.bassel@linaro.org>
Cc: Kevin Hilman <khilman@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Hanjun Guo <hanjun.guo@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 6 insertions, 11 deletions

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d8a523600a4c..4306c937b1ff 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -116,7 +116,7 @@
         */
         .endm
 
-        .macro  kernel_exit, el, ret = 0
+        .macro  kernel_exit, el
         ldp     x21, x22, [sp, #S_PC]           // load ELR, SPSR
         .if     \el == 0
         ct_user_enter
@@ -143,11 +143,7 @@ alternative_endif
         .endif
         msr     elr_el1, x21                    // set up the return data
         msr     spsr_el1, x22
-        .if     \ret
-        ldr     x1, [sp, #S_X1]                 // preserve x0 (syscall return)
-        .else
         ldp     x0, x1, [sp, #16 * 0]
-        .endif
         ldp     x2, x3, [sp, #16 * 1]
         ldp     x4, x5, [sp, #16 * 2]
         ldp     x6, x7, [sp, #16 * 3]
@@ -610,22 +606,21 @@ ENDPROC(cpu_switch_to)
  */
 ret_fast_syscall:
         disable_irq                             // disable interrupts
+        str     x0, [sp, #S_X0]                 // returned x0
         ldr     x1, [tsk, #TI_FLAGS]            // re-check for syscall tracing
         and     x2, x1, #_TIF_SYSCALL_WORK
         cbnz    x2, ret_fast_syscall_trace
         and     x2, x1, #_TIF_WORK_MASK
-        cbnz    x2, fast_work_pending
+        cbnz    x2, work_pending
         enable_step_tsk x1, x2
-        kernel_exit 0, ret = 1
+        kernel_exit 0
 ret_fast_syscall_trace:
         enable_irq                              // enable interrupts
-        b       __sys_trace_return
+        b       __sys_trace_return_skipped      // we already saved x0
 
 /*
  * Ok, we need to do extra processing, enter the slow path.
  */
-fast_work_pending:
-        str     x0, [sp, #S_X0]                 // returned x0
 work_pending:
         tbnz    x1, #TIF_NEED_RESCHED, work_resched
         /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
@@ -649,7 +644,7 @@ ret_to_user:
         cbnz    x2, work_pending
         enable_step_tsk x1, x2
 no_work_pending:
-        kernel_exit 0, ret = 0
+        kernel_exit 0
 ENDPROC(ret_to_user)
 
 /*