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SubjectRe: [PATCH] arm64/ptrace: add PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP support
On Mon, Sep 03, 2018 at 02:23:17PM +0800, Haibo.Xu wrote:
> Add PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP support on ARM64.
> This copies the x86 semantics for invoking ptrace hooks, and have
> been verified on ARM64 machine.
>
> Signed-off-by: Haibo.Xu <haibo.xu@arm.com>
> Signed-off-by: Bin.Lu <bin.lu@arm.com>
> ---
> arch/arm64/include/asm/thread_info.h | 5 ++++-
> arch/arm64/include/uapi/asm/ptrace.h | 2 ++
> arch/arm64/kernel/ptrace.c | 17 +++++++++++++++++
> 3 files changed, 23 insertions(+), 1 deletion(-)

What is PTRACE_SYSEMU and what is its semantics? Why isn't it done in the
core ptrace code?

> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 46c3b93..5060d2d 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -75,6 +75,7 @@ struct thread_info {
> * TIF_SYSCALL_TRACE - syscall trace active
> * TIF_SYSCALL_TRACEPOINT - syscall tracepoint for ftrace
> * TIF_SYSCALL_AUDIT - syscall auditing
> + * TIF_SYSCALL_EMU - syscall emulation active
> * TIF_SECOMP - syscall secure computing
> * TIF_SIGPENDING - signal pending
> * TIF_NEED_RESCHED - rescheduling necessary
> @@ -91,6 +92,7 @@ struct thread_info {
> #define TIF_SYSCALL_AUDIT 9
> #define TIF_SYSCALL_TRACEPOINT 10
> #define TIF_SECCOMP 11
> +#define TIF_SYSCALL_EMU 12
> #define TIF_MEMDIE 18 /* is terminating due to OOM killer */
> #define TIF_FREEZE 19
> #define TIF_RESTORE_SIGMASK 20
> @@ -106,6 +108,7 @@ struct thread_info {
> #define _TIF_SYSCALL_AUDIT (1 << TIF_SYSCALL_AUDIT)
> #define _TIF_SYSCALL_TRACEPOINT (1 << TIF_SYSCALL_TRACEPOINT)
> #define _TIF_SECCOMP (1 << TIF_SECCOMP)
> +#define _TIF_SYSCALL_EMU (1 << TIF_SYSCALL_EMU)
> #define _TIF_UPROBE (1 << TIF_UPROBE)
> #define _TIF_32BIT (1 << TIF_32BIT)
>
> @@ -115,7 +118,7 @@ struct thread_info {
>
> #define _TIF_SYSCALL_WORK (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
> _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
> - _TIF_NOHZ)
> + _TIF_NOHZ | _TIF_SYSCALL_EMU)
>
> #endif /* __KERNEL__ */
> #endif /* __ASM_THREAD_INFO_H */
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index b5c3933..04ab06f 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -23,6 +23,8 @@
>
> #include <asm/hwcap.h>
>
> +#define PTRACE_SYSEMU 31
> +#define PTRACE_SYSEMU_SINGLESTEP 32
>
> /*
> * PSR bits
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index fc35e06..ff3e322 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -165,6 +165,9 @@ void ptrace_disable(struct task_struct *child)
> * is likely to cause regressions on obscure architectures.
> */
> user_disable_single_step(child);
> +#ifdef TIF_SYSCALL_EMU
> + clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
> +#endif
> }
>
> #ifdef CONFIG_HAVE_HW_BREAKPOINT
> @@ -1351,6 +1354,11 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> if (test_thread_flag(TIF_SYSCALL_TRACE))
> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>
> + if (test_thread_flag(TIF_SYSCALL_EMU)) {
> + tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
> + return -1;
> + }

This looks weird -- are TIF_SYSCALL_EMU and TIF_SYSCALL_TRACE mutually
exclusive, or is it harmless to report this twice? Why do we return early
and skip the seccomp checks?

Will

> +
> /* Do the secure computing after ptrace; failures should be fast. */
> if (secure_computing(NULL) == -1)
> return -1;
> @@ -1373,6 +1381,15 @@ asmlinkage void syscall_trace_exit(struct pt_regs *regs)
>
> if (test_thread_flag(TIF_SYSCALL_TRACE))
> tracehook_report_syscall(regs, PTRACE_SYSCALL_EXIT);
> +
> + /*
> + * We only get here because of TIF_SINGLESTEP,
> + * for PTRACE_SYSEMU_SINGLESTEP, we already reported
> + * the syscall instruction in syscall_trace_enter().
> + */
> + if (test_thread_flag(TIF_SINGLESTEP) &&
> + !test_thread_flag(TIF_SYSCALL_EMU))
> + tracehook_report_syscall_exit(regs, 1);
> }
>
> /*
> --
> 2.7.4
>

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