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    SubjectRe: [PATCH 03/14] arm: Add restartable sequences support
    ----- On May 16, 2018, at 12:18 PM, Peter Zijlstra peterz@infradead.org wrote:

    > On Mon, Apr 30, 2018 at 06:44:22PM -0400, Mathieu Desnoyers wrote:
    >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
    >> index a7f8e7f4b88f..4f5c386631d4 100644
    >> --- a/arch/arm/Kconfig
    >> +++ b/arch/arm/Kconfig
    >> @@ -91,6 +91,7 @@ config ARM
    >> select HAVE_PERF_USER_STACK_DUMP
    >> select HAVE_RCU_TABLE_FREE if (SMP && ARM_LPAE)
    >> select HAVE_REGS_AND_STACK_ACCESS_API
    >> + select HAVE_RSEQ
    >> select HAVE_SYSCALL_TRACEPOINTS
    >> select HAVE_UID16
    >> select HAVE_VIRT_CPU_ACCOUNTING_GEN
    >> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
    >> index bd8810d4acb3..5879ab3f53c1 100644
    >> --- a/arch/arm/kernel/signal.c
    >> +++ b/arch/arm/kernel/signal.c
    >> @@ -541,6 +541,12 @@ static void handle_signal(struct ksignal *ksig, struct
    >> pt_regs *regs)
    >> int ret;
    >>
    >> /*
    >> + * Increment event counter and perform fixup for the pre-signal
    >> + * frame.
    >> + */
    >> + rseq_signal_deliver(regs);
    >> +
    >> + /*
    >> * Set up the stack frame
    >> */
    >> if (ksig->ka.sa.sa_flags & SA_SIGINFO)
    >> @@ -660,6 +666,7 @@ do_work_pending(struct pt_regs *regs, unsigned int
    >> thread_flags, int syscall)
    >> } else {
    >> clear_thread_flag(TIF_NOTIFY_RESUME);
    >> tracehook_notify_resume(regs);
    >> + rseq_handle_notify_resume(regs);
    >> }
    >> }
    >> local_irq_disable();
    >
    > I think you forgot to hook up rseq_syscall() checking.

    Considering that rseq_syscall is implemented as follows:

    +void rseq_syscall(struct pt_regs *regs)
    +{
    + unsigned long ip = instruction_pointer(regs);
    + struct task_struct *t = current;
    + struct rseq_cs rseq_cs;
    +
    + if (!t->rseq)
    + return;
    + if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
    + rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
    + force_sig(SIGSEGV, t);
    +}

    and that x86 calls it from syscall_return_slowpath() (which AFAIU is
    now used in the fast-path since KPTI), I wonder where we should call
    this on ARM ? I was under the impression that ARM return to userspace
    fast-path was not calling C code unless work flags were set, but I might
    be wrong.

    Thoughts ?

    Thanks,

    Mathieu

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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