question about prctl return value type

question about prctl return value type

[Yang Xu]

Hi All

Since kernel syscall use long value for return, why we set int in sys/prctl.h(glibc). as below:

/include/sys/prctl.h

   1 #ifndef _SYS_PRCTL_H
   2 #include_next <sys/prctl.h>
   3 
   4 # ifndef _ISOMAC
   5 
   6 extern int __prctl (int __option, ...);
   7 
   8 # endif /* !_ISOMAC */
   9 #endif

If we calling prctl(PR_SET_TIMERSLACK, ULONG_MAX) and then calling prctl(PR_GET_TIMERSLACK), the value
will be truncated into INT_MAX(or convered into unsinged,it is UINT_MAX) on 64bit machine. Or, this is a 
glibc limitation for prctl interfaces?

ps:After kernel commit da8b44d5a9f ("timer: convert timer_slack_ns from unsigned long to u64"),
we limit under ULONG_MAX when calling PR_GET_TIMERSLACK. I think we can use long instead of int in sys/prctl.h.

Thanks
Yang Xu 

Re: question about prctl return value type

[Florian Weimer]

* Yang Xu:

> Since kernel syscall use long value for return, why we set int in
> sys/prctl.h(glibc). as below:
>
> /include/sys/prctl.h
>
>    1 #ifndef _SYS_PRCTL_H
>    2 #include_next <sys/prctl.h>
>    3 
>    4 # ifndef _ISOMAC
>    5 
>    6 extern int __prctl (int __option, ...);
>    7 
>    8 # endif /* !_ISOMAC */
>    9 #endif

Note that the actual user-visible header is
sysdeps/unix/sysv/linux/sys/prctl.h, but it has the same issue.

> If we calling prctl(PR_SET_TIMERSLACK, ULONG_MAX) and then calling prctl(PR_GET_TIMERSLACK), the value
> will be truncated into INT_MAX(or convered into unsinged,it is UINT_MAX) on 64bit machine. Or, this is a 
> glibc limitation for prctl interfaces?

I think this is a glibc limitation/bug.  The return type should probably
be __register_t (a type that is currently woefully underused).  We
cannot simply change the return time because once applications are built
against the new prototype, we need to ensure that they also use the new
glibc.

Would you please file a bug for this?
<https://sourceware.org/bugzilla/> is our bug tracker.

Thanks,
Florian

Re: question about prctl return value type

[Yang Xu]

on 2019/07/24 18:48, Florian Weimer wrote:

> * Yang Xu:
>
>> Since kernel syscall use long value for return, why we set int in
>> sys/prctl.h(glibc). as below:
>>
>> /include/sys/prctl.h
>>
>>    1 #ifndef _SYS_PRCTL_H
>>    2 #include_next <sys/prctl.h>
>>    3 
>>    4 # ifndef _ISOMAC
>>    5 
>>    6 extern int __prctl (int __option, ...);
>>    7 
>>    8 # endif /* !_ISOMAC */
>>    9 #endif
> Note that the actual user-visible header is
> sysdeps/unix/sysv/linux/sys/prctl.h, but it has the same issue.
>
>> If we calling prctl(PR_SET_TIMERSLACK, ULONG_MAX) and then calling prctl(PR_GET_TIMERSLACK), the value
>> will be truncated into INT_MAX(or convered into unsinged,it is UINT_MAX) on 64bit machine. Or, this is a 
>> glibc limitation for prctl interfaces?
> I think this is a glibc limitation/bug.  The return type should probably
> be __register_t (a type that is currently woefully underused).  We
> cannot simply change the return time because once applications are built
> against the new prototype, we need to ensure that they also use the new
> glibc.
>
> Would you please file a bug for this?
> <https://sourceware.org/bugzilla/> is our bug tracker.

Hi Florian

I create a bug and cc you, url as below:
https://sourceware.org/bugzilla/show_bug.cgi?id=24850


Thanks 
Yang Xu

> Thanks,
> Florian
>
>
>