Q: Can we get rid of __copy_siginfo_to_user32?

Q: Can we get rid of __copy_siginfo_to_user32?

[Eric W. Biederman]

Andy,

I am looking at copy_siginfo_to_user32 and find it very unfortunate
that x86 with _sigchld_x32 needs to be the odd man out.  I am looking
at ways to simplify the special case.

The core of the special case comes from:
exit_to_usermode_loop
  do_signal
    handle_signal
       setup_rt_frame


In setup_rt_frame the code looks at ksig to see which kind of signal
frame should be written for the signal.

This leads to the one case in the kernel where copy_siginfo_to_user32
does not use is_ia32_syscall() or is_x32_syscall() to see which kind of
signal frame it needs to create.

Andy, since you have been all over the entry point code in recent years
do you know if we allow tasks that can do both ia32 and x86_64 system
calls?  That seems to be what we the testing of ksig to see which kind
of signal frame to setup is all about.

If we don't allow mixed abi's on x86_64 then can I see if I have a ia32
task in setup_rt_frame by just calling is_ia32_syscall()?

If we do allow mixed abi's do you know if it would be safe to
temporarily play with orig_ax or current_thread_info()->status?

My goal is to write two wrappers: copy_siginfo_to_user32_ia32, and
copy_siginfo_to_user32_x32 around the ordinary copy_siginfo_to_user32.
With only a runtime test to see which ABI we need to implement.

Aka change:
> 	case SIL_CHLD:
> 		to->si_pid    = from->si_pid;
> 		to->si_uid    = from->si_uid;
> 		to->si_status = from->si_status;
> #ifdef CONFIG_X86_X32_ABI
> 		if (x32_ABI) {
> 			to->_sifields._sigchld_x32._utime = from->si_utime;
> 			to->_sifields._sigchld_x32._stime = from->si_stime;
> 		} else
> #endif
> 		{
> 			to->si_utime = from->si_utime;
> 			to->si_stime = from->si_stime;
> 		}
> 		break;
to something like:                
> 	case SIL_CHLD:
> 		to->si_pid    = from->si_pid;
> 		to->si_uid    = from->si_uid;
> 		to->si_status = from->si_status;
> #ifdef CONFIG_X86_X32_ABI
> 		if (!is_ia32_syscall()) {
> 			to->_sifields._sigchld_x32._utime = from->si_utime;
> 			to->_sifields._sigchld_x32._stime = from->si_stime;
> 		} else
> #endif
> 		{
> 			to->si_utime = from->si_utime;
> 			to->si_stime = from->si_stime;
> 		}
> 		break;

I just don't understand the introdcacies of the ia32 and x32 emulation
to really guess which test I need to substitute in there.  So any help
or ideas would really be appreciated.

Eric

Re: Q: Can we get rid of __copy_siginfo_to_user32?

[Andy Lutomirski]

> On Apr 10, 2018, at 6:26 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
>
> Andy,
>
> I am looking at copy_siginfo_to_user32 and find it very unfortunate
> that x86 with _sigchld_x32 needs to be the odd man out.  I am looking
> at ways to simplify the special case.
>
> The core of the special case comes from:
> exit_to_usermode_loop
>  do_signal
>    handle_signal
>       setup_rt_frame
>
>
> In setup_rt_frame the code looks at ksig to see which kind of signal
> frame should be written for the signal.
>
> This leads to the one case in the kernel where copy_siginfo_to_user32
> does not use is_ia32_syscall() or is_x32_syscall() to see which kind of
> signal frame it needs to create.
>
> Andy, since you have been all over the entry point code in recent years
> do you know if we allow tasks that can do both ia32 and x86_64 system
> calls?  That seems to be what we the testing of ksig to see which kind
> of signal frame to setup is all about.

We do :(

> If we don't allow mixed abi's on x86_64 then can I see if I have a ia32
> task in setup_rt_frame by just calling is_ia32_syscall()?
>
> If we do allow mixed abi's do you know if it would be safe to
> temporarily play with orig_ax or current_thread_info()->status?

Maybe, but it’s a real minefield. I think the right fix is to use
sa_flags's SA_X32_ABI bit instead for the sigchld bit.  In general,
the is_..._syscall() helpers can't be expected to return anything
valid in any context other than a syscall, and handle_signal() is not
a syscall.

>
> My goal is to write two wrappers: copy_siginfo_to_user32_ia32, and
> copy_siginfo_to_user32_x32 around the ordinary copy_siginfo_to_user32.
> With only a runtime test to see which ABI we need to implement.
>
> Aka change:
>>    case SIL_CHLD:
>>        to->si_pid    = from->si_pid;
>>        to->si_uid    = from->si_uid;
>>        to->si_status = from->si_status;
>> #ifdef CONFIG_X86_X32_ABI
>>        if (x32_ABI) {
>>            to->_sifields._sigchld_x32._utime = from->si_utime;
>>            to->_sifields._sigchld_x32._stime = from->si_stime;
>>        } else
>> #endif
>>        {
>>            to->si_utime = from->si_utime;
>>            to->si_stime = from->si_stime;
>>        }
>>        break;
> to something like:
>>    case SIL_CHLD:
>>        to->si_pid    = from->si_pid;
>>        to->si_uid    = from->si_uid;
>>        to->si_status = from->si_status;
>> #ifdef CONFIG_X86_X32_ABI
>>        if (!is_ia32_syscall()) {
>>            to->_sifields._sigchld_x32._utime = from->si_utime;
>>            to->_sifields._sigchld_x32._stime = from->si_stime;
>>        } else
>> #endif
>>        {
>>            to->si_utime = from->si_utime;
>>            to->si_stime = from->si_stime;
>>        }
>>        break;
>

Makes sense, but can you get to sa_flags in there instead?

FWIW, I have a branch here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=execve

that contains a *massive* cleanup of some other x86 signal stuff.  I
need to dust it off and test it better.

Q: Do si_time and si_utime need to be 64bit for y2038?

[Eric W. Biederman]

Arnd,

I am looking at the siginfo si_utime and si_stime fields of type clock_t
on 32bit architectures except for x32 these are 32bit fields.  For y2038
do we want to extend these fields to 64bit like x32 does?  Or is it not
a problem for these fields to be 32bit?

I care right now because I am trying to figure out how
copy_siginfo_to_user32 and copy_siginfo_to_user need to evolve.

If we are going to extend existing architectures with 64bit variations
of si_utime and si_stime copy_siginfo_to_user and copy_siginfo_to_user32
needs an additional parameter describing which variant they should be
copying.

It looks like posix does not define si_stime and and si_utime so we only
have to be backwards compatible with ourselves for whatever that is
worth.

I am wondering if perhaps the general solution might be to just add
two extra fields si_stime64 and si_utime64 and always fill those in.

Arnd do you have any ideas?


Andy Lutomirski <luto@amacapital.net> writes:

>> On Apr 10, 2018, at 6:26 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>
>>
>> Andy,
>>
>> I am looking at copy_siginfo_to_user32 and find it very unfortunate
>> that x86 with _sigchld_x32 needs to be the odd man out.  I am looking
>> at ways to simplify the special case.
>>
>> The core of the special case comes from:
>> exit_to_usermode_loop
>>  do_signal
>>    handle_signal
>>       setup_rt_frame
>>
>>
>> In setup_rt_frame the code looks at ksig to see which kind of signal
>> frame should be written for the signal.
>>
>> This leads to the one case in the kernel where copy_siginfo_to_user32
>> does not use is_ia32_syscall() or is_x32_syscall() to see which kind of
>> signal frame it needs to create.
>>
>> Andy, since you have been all over the entry point code in recent years
>> do you know if we allow tasks that can do both ia32 and x86_64 system
>> calls?  That seems to be what we the testing of ksig to see which kind
>> of signal frame to setup is all about.
>
> We do :(
>
>> If we don't allow mixed abi's on x86_64 then can I see if I have a ia32
>> task in setup_rt_frame by just calling is_ia32_syscall()?
>>
>> If we do allow mixed abi's do you know if it would be safe to
>> temporarily play with orig_ax or current_thread_info()->status?
>
> Maybe, but it’s a real minefield. I think the right fix is to use
> sa_flags's SA_X32_ABI bit instead for the sigchld bit.  In general,
> the is_..._syscall() helpers can't be expected to return anything
> valid in any context other than a syscall, and handle_signal() is not
> a syscall.
>
>>
>> My goal is to write two wrappers: copy_siginfo_to_user32_ia32, and
>> copy_siginfo_to_user32_x32 around the ordinary copy_siginfo_to_user32.
>> With only a runtime test to see which ABI we need to implement.
>>
>> Aka change:
>>>    case SIL_CHLD:
>>>        to->si_pid    = from->si_pid;
>>>        to->si_uid    = from->si_uid;
>>>        to->si_status = from->si_status;
>>> #ifdef CONFIG_X86_X32_ABI
>>>        if (x32_ABI) {
>>>            to->_sifields._sigchld_x32._utime = from->si_utime;
>>>            to->_sifields._sigchld_x32._stime = from->si_stime;
>>>        } else
>>> #endif
>>>        {
>>>            to->si_utime = from->si_utime;
>>>            to->si_stime = from->si_stime;
>>>        }
>>>        break;
>> to something like:
>>>    case SIL_CHLD:
>>>        to->si_pid    = from->si_pid;
>>>        to->si_uid    = from->si_uid;
>>>        to->si_status = from->si_status;
>>> #ifdef CONFIG_X86_X32_ABI
>>>        if (!is_ia32_syscall()) {
>>>            to->_sifields._sigchld_x32._utime = from->si_utime;
>>>            to->_sifields._sigchld_x32._stime = from->si_stime;
>>>        } else
>>> #endif
>>>        {
>>>            to->si_utime = from->si_utime;
>>>            to->si_stime = from->si_stime;
>>>        }
>>>        break;
>>
>
> Makes sense, but can you get to sa_flags in there instead?

Almost.  copy_siginfo_to_user32 is called in 3 places: setup_rt_frame32
(or whatever the arch names the function for setting up the 32bit signal
 frame), ptrace, and compat_binfmt_elf.
 
So except for ptrace and compat_binfmt_elf sa_flags are available so
that is a possibility.  And for those we can fake something up if
needed.

Stepping back it really looks like the question is really do
we want/need 64bit time in siginfo for 32bit architectures to
make the code y2038 safe?

If so passing an extra parameter to copy_siginfo_to_user32 and
copy_siginfo_to_user is a no-brainer.  If not we are at x86 and
in particular x32 is weird.  So I am asking Arnd above if he
has any idea which way things should evolve.

> FWIW, I have a branch here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=execve
>
> that contains a *massive* cleanup of some other x86 signal stuff.  I
> need to dust it off and test it better.

It looks interesting, and except for the last patch "Drop the separate
compat signal delivery code" looks orthogonal to what I am doing.

What I have seen other architectures do in that last case are instead of
#ifdefs to #define functions to their compat counterparts on 64bit.
Something like:
#define copy_siginfo_to_user copy_siginfo_to_user32

Eric

Re: Q: Do si_time and si_utime need to be 64bit for y2038?

[Arnd Bergmann]

On Wed, Apr 11, 2018 at 6:11 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> Arnd,
>
> I am looking at the siginfo si_utime and si_stime fields of type clock_t
> on 32bit architectures except for x32 these are 32bit fields.  For y2038
> do we want to extend these fields to 64bit like x32 does?  Or is it not
> a problem for these fields to be 32bit?

Short answer: I think we're fine without changing it, at least for y2038.

> I care right now because I am trying to figure out how
> copy_siginfo_to_user32 and copy_siginfo_to_user need to evolve.
>
> If we are going to extend existing architectures with 64bit variations
> of si_utime and si_stime copy_siginfo_to_user and copy_siginfo_to_user32
> needs an additional parameter describing which variant they should be
> copying.
>
> It looks like posix does not define si_stime and and si_utime so we only
> have to be backwards compatible with ourselves for whatever that is
> worth.
>
> I am wondering if perhaps the general solution might be to just add
> two extra fields si_stime64 and si_utime64 and always fill those in.
>
> Arnd do you have any ideas?

There are generally four areas to be aware of with the data structure
changes required for y2038:

1. Stuff that overflows in the next few decades (either 2038 or some other
    year). si_utime/si_stime counts relative times, so there is no overflow
    happening at a particular date that we have to be aware of. However,
    it does overflow when a 32-bit process runs for more than
    (LONG_MAX / USER_HZ) seconds, which is about 248 days.
    When you have a large SMP system with 256 CPUs and run a single
    task across all of them, the overflow happens within one day of runtime.
    This is a rare use case for 32-bit tasks, but it is an actual limitation
    that we may want to fix regardless of the y2038 changes.

2. Types that don't overflow in a particular interface (because they count
    relative times) but do overflow in others. We have a problem in
    wait4()/waidid() and getrusage() here, since those use 'struct timeval'
    to count process times. Those can count up to 68 years of process
    times (97 days on a 256-core machine running one task), so we
    probably don't care about the overflow, but POSIX requires the
    use of timeval [1] and we have to redefine that structure with an
    incompatible layout.
    We do have a choice between either keeping the existing structure
    and letting the libc translate the 32-bit time_t to a 64-bit time_t,
    or adding replacement syscalls for both getrusage() and waitid().
    IIRC we don't need a new wait4(), since that can be implemented
    using waitid.
    clock_t is used in exactly two places: struct siginfo and struct tms,
    so if we change one of the two, we also have to change the other.

3. In some cases, two structures are almost identical between 32-bit
    and 64-bit architectures. Using the exact same layout simplifies the
    compat syscall handling. I think in x32, this was a factor as it means
    that e.g. times() is shared between x32 and x86-64.

4. If we change an interface, we may want to improve it in more than
   one way, like we did with stat()->stat64()->statx() or time()->
   gettimeofday()->clock_gettime()->clock_gettime64().
   If we introduce a larger range for the 32-bit siginfo and tms
   structures, we could also consider extending the resolution to
   nanoseconds. I wouldn't follow rusage's timeval but use
   timespec64 (__kernel_timespec as seen from user space).
   64-bit nanoseconds are another option, but that again
   overflows after 584 CPU-years or 52 days on a 4096-core
   system.

         Arnd

[1] http://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/resource.h.html

Re: Q: Do si_time and si_utime need to be 64bit for y2038?

[Eric W. Biederman]

Arnd Bergmann <arnd@arndb.de> writes:

> On Wed, Apr 11, 2018 at 6:11 PM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>>
>> Arnd,
>>
>> I am looking at the siginfo si_utime and si_stime fields of type clock_t
>> on 32bit architectures except for x32 these are 32bit fields.  For y2038
>> do we want to extend these fields to 64bit like x32 does?  Or is it not
>> a problem for these fields to be 32bit?
>
> Short answer: I think we're fine without changing it, at least for y2038.
Short acknowledgement: I am going to assume this isn't a generic
32bit/64bit problem that merits a general solution.

>> I care right now because I am trying to figure out how
>> copy_siginfo_to_user32 and copy_siginfo_to_user need to evolve.
>>
>> If we are going to extend existing architectures with 64bit variations
>> of si_utime and si_stime copy_siginfo_to_user and copy_siginfo_to_user32
>> needs an additional parameter describing which variant they should be
>> copying.
>>
>> It looks like posix does not define si_stime and and si_utime so we only
>> have to be backwards compatible with ourselves for whatever that is
>> worth.
>>
>> I am wondering if perhaps the general solution might be to just add
>> two extra fields si_stime64 and si_utime64 and always fill those in.
>>
>> Arnd do you have any ideas?
>
> There are generally four areas to be aware of with the data structure
> changes required for y2038:
>
> 1. Stuff that overflows in the next few decades (either 2038 or some other
>     year). si_utime/si_stime counts relative times, so there is no overflow
>     happening at a particular date that we have to be aware of. However,
>     it does overflow when a 32-bit process runs for more than
>     (LONG_MAX / USER_HZ) seconds, which is about 248 days.
>     When you have a large SMP system with 256 CPUs and run a single
>     task across all of them, the overflow happens within one day of runtime.
>     This is a rare use case for 32-bit tasks, but it is an actual limitation
>     that we may want to fix regardless of the y2038 changes.
>
> 2. Types that don't overflow in a particular interface (because they count
>     relative times) but do overflow in others. We have a problem in
>     wait4()/waidid() and getrusage() here, since those use 'struct timeval'
>     to count process times. Those can count up to 68 years of process
>     times (97 days on a 256-core machine running one task), so we
>     probably don't care about the overflow, but POSIX requires the
>     use of timeval [1] and we have to redefine that structure with an
>     incompatible layout.
>     We do have a choice between either keeping the existing structure
>     and letting the libc translate the 32-bit time_t to a 64-bit time_t,
>     or adding replacement syscalls for both getrusage() and waitid().
>     IIRC we don't need a new wait4(), since that can be implemented
>     using waitid.
>     clock_t is used in exactly two places: struct siginfo and struct tms,
>     so if we change one of the two, we also have to change the other.

Good point.

> 3. In some cases, two structures are almost identical between 32-bit
>     and 64-bit architectures. Using the exact same layout simplifies the
>     compat syscall handling. I think in x32, this was a factor as it means
>     that e.g. times() is shared between x32 and x86-64.

Sort of.  You can get to the ordinary times system call but the x32
system call table has a pointer to compat_times that uses defines
compat_clock_t to be an s32.  Meanwhile x32 makes uses
__kernel_si_clock_t in struct siginfo which is defined as long long.

I don't have a clue what x32 libcs do with that combination.

> 4. If we change an interface, we may want to improve it in more than
>    one way, like we did with stat()->stat64()->statx() or time()->
>    gettimeofday()->clock_gettime()->clock_gettime64().
>    If we introduce a larger range for the 32-bit siginfo and tms
>    structures, we could also consider extending the resolution to
>    nanoseconds. I wouldn't follow rusage's timeval but use
>    timespec64 (__kernel_timespec as seen from user space).
>    64-bit nanoseconds are another option, but that again
>    overflows after 584 CPU-years or 52 days on a 4096-core
>    system.

Enhancing the interface makes sense.

Playing around with debian's code search it does not look like anything
really uses si_utime and si_stime from struct siginfo except for
libraries that make the values accessible to other layers.

QT looks does something with them from the return of waitid but the
kernel does not populate those fields for waitid.  So bug.

I suspect the best enhancement would be to simply deprecate the use of
si_stime si_utime, probably by always setting them to 0.

times() on the other hand is used quite widely.  So it still may be
worth enhancing that side of the interface for 32bit processes someday.
Although it sounds like the truly problematic cases are all on 64bit
machines so we may not care.

Eric