Re: [PATCH 03/14] arm: Add restartable sequences support
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: 2018-05-22 18:19:36
Also in:
lkml
Subsystem:
arm port, the rest · Maintainers:
Russell King, Linus Torvalds
----- On May 17, 2018, at 11:30 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: [...]
Or as proposed by Boqun, we can simply call rseq_syscall in a CONFIG_DEBUG_RSEQ ifdef. Given that this is a debug option, is it worth it to add the current->rseq test for NULL in assembly before the call, or do we want to favor simplicity ?
Based on advice from Will Deacon, I alternatively tried to add a new TIF_RSEQ thread flags, but unfortunately bits 1 through 8 are already used, and this is all that fits in an immediate operand on arm32 for the fast-path thread flag syscall work mask check in assembly. So considering that this is a kernel debug option, I took the approach of adding a call at the very beginning of return from syscall fast and slow paths, which is only compiled in if CONFIG_DEBUG_RSEQ=y. Does the following approach make sense ? arm: Add syscall detection for restartable sequences 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. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> ---
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 3c4f887..b427ef8 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S@@ -39,12 +39,13 @@ saved_pc .req lr .section .entry.text,"ax",%progbits .align 5 -#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING)) +#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING) || \ + IS_ENABLED(CONFIG_DEBUG_RSEQ)) /* * This is the fast syscall return path. We do as little as possible here, * such as avoiding writing r0 to the stack. We only use this path if we - * have tracing and context tracking disabled - the overheads from those - * features make this path too inefficient. + * have tracing, context tracking and rseq debug disabled - the overheads + * from those features make this path too inefficient. */ ret_fast_syscall: UNWIND(.fnstart )
@@ -71,14 +72,20 @@ fast_work_pending: /* fall through to work_pending */ #else /* - * The "replacement" ret_fast_syscall for when tracing or context tracking - * is enabled. As we will need to call out to some C functions, we save - * r0 first to avoid needing to save registers around each C function call. + * The "replacement" ret_fast_syscall for when tracing, context tracking, + * or rseq debug is enabled. As we will need to call out to some C functions, + * we save r0 first to avoid needing to save registers around each C function + * call. */ ret_fast_syscall: UNWIND(.fnstart ) UNWIND(.cantunwind ) str r0, [sp, #S_R0 + S_OFF]! @ save returned r0 +#if IS_ENABLED(CONFIG_DEBUG_RSEQ) + /* do_rseq_syscall needs interrupts enabled. */ + mov r0, sp @ 'regs' + bl do_rseq_syscall +#endif disable_irq_notrace @ disable interrupts ldr r2, [tsk, #TI_ADDR_LIMIT] cmp r2, #TASK_SIZE
@@ -113,6 +120,12 @@ ENDPROC(ret_fast_syscall) */ ENTRY(ret_to_user) ret_slow_syscall: +#if IS_ENABLED(CONFIG_DEBUG_RSEQ) + /* do_rseq_syscall needs interrupts enabled. */ + enable_irq_notrace @ enable interrupts + mov r0, sp @ 'regs' + bl do_rseq_syscall +#endif disable_irq_notrace @ disable interrupts ENTRY(ret_to_user_from_irq) ldr r2, [tsk, #TI_ADDR_LIMIT]
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 5879ab3..f09e9d66 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c@@ -710,3 +710,10 @@ asmlinkage void addr_limit_check_failed(void) { addr_limit_user_check(); } + +#ifdef CONFIG_DEBUG_RSEQ +asmlinkage void do_rseq_syscall(struct pt_regs *regs) +{ + rseq_syscall(regs); +} +#endif
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com