arm64: entry: always restore x0 from the stack on syscall return (412fcb6c) · Commits · 戴 / test

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>

arch/arm64/kernel/entry.S

+6 −11

Original line number	Diff line number	Diff line
		@@ -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)

		/*

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