Date: Tue, 11 Jan 2005 12:10:15 +0100
From: Andries Brouwer <aebr@win.tue.nl>
To: Alex LIU <alex.liu@st.com>
Cc: "'Andries Brouwer'" <aebr@win.tue.nl>, "'Pavel Machek'" <pavel@ucw.cz>,
       linux-kernel@vger.kernel.org
Subject: Re: The purpose of PT_TRACESYSGOOD
Message-ID: <20050111111015.GC2760@pclin040.win.tue.nl>
References: <20050111023003.GA2760@pclin040.win.tue.nl> <00ac01c4f7a6$c79e03f0$ac655e0a@sha.st.com>
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>> /*
>>  * A child stopped at a syscall has status as if it received SIGTRAP.
>>  * In order to distinguish between SIGTRAP and syscall, some kernel
>>  * versions have the PTRACE_O_TRACESYSGOOD option, that sets an extra
>>  * bit 0x80 in the syscall case.
>>  */

> Then I think the tracing thread should call the ptrace_request to set
> PTRACE_O_TRACESYSGOOD flag of the traced thread first before
> ptrace(PTRACE_SYSCALL...) ,right?

Yes.

>From a baby ptrace demo:

#define SIGSYSTRAP      (SIGTRAP | sysgood_bit)

int sysgood_bit = 0;

void set_sysgood(pid_t p) {
#ifdef PTRACE_O_TRACESYSGOOD
        int i = ptrace(PTRACE_SETOPTIONS, p, 0, (void*) PTRACE_O_TRACESYSGOOD);
        if (i == 0)
                sysgood_bit = 0x80;
        else
                perror("PTRACE_O_TRACESYSGOOD");
#endif
}

and now the signal SIGSYSTRAP signifies a system call when the sysgood bit
was implemented, anything different from SIGSYSTRAP is guaranteed to be a signal.

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