linux-kernel - Re: [RFC PATCH] ptrace: add PTRACE_GET_SYSCALL

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Date:   Wed, 7 Nov 2018 12:21:01 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Elvira Khabirova <lineprinter@...linux.org>
Cc:     rostedt@...dmis.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, ldv@...linux.org, esyr@...hat.com,
        luto@...nel.org, strace-devel@...ts.strace.io
Subject: Re: [RFC PATCH] ptrace: add PTRACE_GET_SYSCALL_INFO request

On 11/07, Elvira Khabirova wrote:
>
> In short, if a 64-bit task performs a syscall through int 0x80, its tracer
> has no reliable means to find out that the syscall was, in fact,
> a compat syscall, and misidentifies it.
> * Syscall-enter-stop and syscall-exit-stop look the same for the tracer.

Yes, this was discussed many times...

So perhaps it makes sense to encode compat/is_enter in ->ptrace_message,
debugger can use PTRACE_GETEVENTMSG to get this info.

> Secondly, ptracers also have to support a lot of arch-specific code for
> obtaining information about the tracee. For some architectures, this
> requires a ptrace(PTRACE_PEEKUSER, ...) invocation for every syscall
> argument and return value.

I am not sure about this change... I won't really argue, but imo this
needs a separate patch.

> +#define PT_IN_SYSCALL_STOP	0x00000004	/* task is in a syscall-stop */
...
> -static inline int ptrace_report_syscall(struct pt_regs *regs)
> +static inline int ptrace_report_syscall(struct pt_regs *regs,
> +					unsigned long message)
>  {
>  	int ptrace = current->ptrace;
>  
>  	if (!(ptrace & PT_PTRACED))
>  		return 0;
> +	current->ptrace |= PT_IN_SYSCALL_STOP;
>  
> +	current->ptrace_message = message;
>  	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
>  
>  	/*
> @@ -76,6 +79,7 @@ static inline int ptrace_report_syscall(struct pt_regs *regs)
>  		current->exit_code = 0;
>  	}
>  
> +	current->ptrace &= ~PT_IN_SYSCALL_STOP;
>  	return fatal_signal_pending(current);
...

> +	case PTRACE_GET_SYSCALL_INFO:
> +		if (child->ptrace & PT_IN_SYSCALL_STOP)
> +			ret = ptrace_get_syscall(child, datavp);
> +		break;

Why? If debugger uses PTRACE_O_TRACESYSGOOD it can know if the tracee reported
syscall entry/exit or not. PTRACE_GET_SYSCALL_INFO is pointless if not, but
nothing bad can happen.

Oleg.