The purpose of PT_TRACESYSGOOD

The purpose of PT_TRACESYSGOOD

[Alex LIU]

Hi:

What's the effect of PT_TRACESYSGOOD flag? I found it's used only set in ptrace_setoptions,which is called in the function ptrace_request. And the PT_TRACESYSGOOD flag will be requested in do_syscall_trace. What's the purpose of that flag? Thanks! 

Alex

Re: The purpose of PT_TRACESYSGOOD

[Pavel Machek]

Hi!

> What's the effect of PT_TRACESYSGOOD flag? I found it's used only set in ptrace_setoptions,which is called in the function ptrace_request. And the PT_TRACESYSGOOD flag will be requested in do_syscall_trace. What's the purpose of that flag? Thanks! 
> 

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Re: The purpose of PT_TRACESYSGOOD

[Andries Brouwer]

>> What's the effect of PT_TRACESYSGOOD flag? I found it's used only set
>> in ptrace_setoptions, which is called in the function
>> ptrace_request. And the PT_TRACESYSGOOD flag will be requested in
>> do_syscall_trace. What's the purpose of that flag?


/*
 * 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.
 */

RE: The purpose of PT_TRACESYSGOOD

[Alex LIU]

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?
Thanks a lot! 

Alex

Andries Brouwer wrote:

>> What's the effect of PT_TRACESYSGOOD flag? I found it's used only set 
>> in ptrace_setoptions, which is called in the function ptrace_request. 
>> And the PT_TRACESYSGOOD flag will be requested in do_syscall_trace. 
>> What's the purpose of that flag?

>/*
> * 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.
> */

Re: The purpose of PT_TRACESYSGOOD

[Andries Brouwer]

>> /*
>>  * 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.