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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Arnd Bergmann <arnd@arndb.de>, christian@brauner.io,
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Date:   Sat, 01 Dec 2018 18:06:49 -0600
In-Reply-To: <F53D6D38-3521-4C20-9034-5AF447DF62FF@amacapital.net> (Andy
        Lutomirski's message of "Sat, 1 Dec 2018 07:52:37 -0800")
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Subject: Re: [PATCH v2] signal: add procfd_signal() syscall
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Andy Lutomirski <luto@amacapital.net> writes:

>> On Dec 1, 2018, at 7:28 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>> 
>> 
>> It just occurs to me that the simple way to implement
>> procfd_sigqueueinfo info is like:
>> 
>> int copy_siginfo_from_user_any(kernel_siginfo_t *info, siginfo_t *uinfo)
>> {
>> #ifdef CONFIG_COMPAT
>>    if (in_compat_syscall)
>>        return copy_siginfo_from_user32(info, uinfo);
>> #endif
>>    return copy_siginfo_from_user(info, uinfo);                        
>> }
>> 
>> long procfd_sigqueueinfo(int fd, siginfo_t *uinfo)
>> {
>>    kernel_siginfo info;
>> 
>>        if (copy_siginfo_from_user_any(&info, uinfo))
>>            return -EFAULT;
>>    ...;                
>> }
>> 
>> It looks like there is already a place in ptrace.c that already
>> hand rolls copy_siginfo_from_user_any.
>> 
>> So while I would love to figure out the subset of siginfo_t tha we can
>> just pass through, as I think that would make a better more forward
>> compatible copy_siginfo_from_user32.
>
> Seems reasonable to me. It’s less code overall than any other suggestion, too.
>
>>  I think for this use case we just
>> add the in_compat_syscall test and then we just need to ensure this new
>> system call is placed in the proper places in the syscall table.
>> 
>> Because we will need 3 call sights: x86_64, x32 and ia32.  As the layout
>> changes between those three subarchitecuters.
>> 
>> 
>
> If it’s done this way, it can just be “common” in the 64-bit
> table. And we kick the can a bit farther down the road :)
>
> I’m working on patches to clean up x86’s syscall mess. It’s slow
> because I keep finding new messes.  So far I have rt_sigreturn working
> like every other syscall — whee.
>
> Also, Eric, for your edification, I have a draft patch set to
> radically simplify x86’s signal delivery and return.  Once that’s
> done, I can trivially speed up delivery by a ton by using sysret.

Nice.

Do we care about the performance of synchronous signal delivery (AKA
hardware exceptions) vs ordinary signal delivery.  I get the feeling
there are serious simplifications to be had in that case.

Eric