[PATCH 4.2 028/134] arm64: entry: always restore x0 from the stack on syscall return

Greg Kroah-Hartman
2015-09-26T20:58:05Z

From: Will Deacon <will.deacon@arm.com>

commit 412fcb6cebd758d080cacd5a41a0cbc656ea5fce upstream.

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: 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm64/kernel/entry.S | 17 ++++++-----------
1 file changed, 6 insertions(+), 11 deletions(-)

--- 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
@@ -146,11 +146,7 @@
.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]
@@ -613,22 +609,21 @@ 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 */
@@ -652,7 +647,7 @@ 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)

/*