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SubjectRe: The purpose of PT_TRACESYSGOOD
>> /*
>> * 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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