Subject: Re: [PATCH v2 2/6] ptrace: introduce ptrace_syscall_enter to consolidate PTRACE_SYSEMU handling

From: "Dmitry V. Levin" <[email-masked]>
Date: Mon, 18 Mar 2019 17:31:47 +0300

On Mon, Mar 18, 2019 at 10:49:21AM +0000, Sudeep Holla wrote:
> Currently each architecture handles PTRACE_SYSEMU in very similar way.
> It's completely arch independent and can be handled in the code helping
> to consolidate PTRACE_SYSEMU handling.
> 
> Let's introduce a hook 'ptrace_syscall_enter' that arch specific syscall
> entry code can call.
> 
> Cc: Oleg Nesterov <[email-masked]>
> Signed-off-by: Sudeep Holla <[email-masked]>
> ---
>  include/linux/ptrace.h |  1 +
>  kernel/ptrace.c        | 22 ++++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index edb9b040c94c..e30f51e3363e 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -407,6 +407,7 @@ static inline void user_single_step_report(struct pt_regs *regs)
>  #define current_user_stack_pointer() user_stack_pointer(current_pt_regs())
>  #endif
>  
> +extern long ptrace_syscall_enter(struct pt_regs *regs);
>  extern int task_current_syscall(struct task_struct *target, long *callno,
>  				unsigned long args[6], unsigned int maxargs,
>  				unsigned long *sp, unsigned long *pc);
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 4fa3b7f4c3c7..c9c505c483df 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -29,6 +29,7 @@
>  #include <linux/hw_breakpoint.h>
>  #include <linux/cn_proc.h>
>  #include <linux/compat.h>
> +#include <linux/tracehook.h>
>  
>  /*
>   * Access another process' address space via ptrace.
> @@ -557,6 +558,27 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
>  	return 0;
>  }
>  
> +/*
> + * Hook to check and report for PTRACE_SYSEMU, can be called from arch
> + * arch syscall entry code
> + */
> +long ptrace_syscall_enter(struct pt_regs *regs)
> +{
> +#ifdef TIF_SYSCALL_EMU
> +	if (test_thread_flag(TIF_SYSCALL_EMU)) {
> +		if (tracehook_report_syscall_entry(regs))
> +			/*
> +			 * We can ignore the return code here as we need
> +			 * return -1 always for syscall emulation irrespective
> +			 * of whether the tracehook report fails or succeed.
> +			 */
> +			;

This is problematic as it causes build errors with -Werror=empty-body,
see https://lore.kernel.org/lkml/[email-masked]/

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