linux-kernel - Re: [PATCH v2] signal: add procfd

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Date:   Sat, 01 Dec 2018 12:52:24 +1300
From:   Christian Brauner <christian@...uner.io>
To:     Florian Weimer <fweimer@...hat.com>
CC:     ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        serge@...lyn.com, jannh@...gle.com, luto@...nel.org,
        akpm@...ux-foundation.org, oleg@...hat.com, cyphar@...har.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, dancol@...gle.com, timmurray@...gle.com,
        linux-man@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall

On November 30, 2018 1:28:15 AM GMT+13:00, Florian Weimer <fweimer@...hat.com> wrote:
>Disclaimer: I'm looking at this patch because Christian requested it.
>I'm not a kernel developer.

Given all your expertise this really doesn't matter. :)
You're the one having to deal with this
in glibc after all.
Thanks for doing this and sorry for the late reply.
I missed that mail.

>
>* Christian Brauner:
>
>> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl
>b/arch/x86/entry/syscalls/syscall_32.tbl
>> index 3cf7b533b3d1..3f27ffd8ae87 100644
>> --- a/arch/x86/entry/syscalls/syscall_32.tbl
>> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
>> @@ -398,3 +398,4 @@
>>  384	i386	arch_prctl		sys_arch_prctl			__ia32_compat_sys_arch_prctl
>> 
>385	i386	io_pgetevents		sys_io_pgetevents		__ia32_compat_sys_io_pgetevents
>>  386	i386	rseq			sys_rseq			__ia32_sys_rseq
>>
>+387	i386	procfd_signal		sys_procfd_signal		__ia32_compat_sys_procfd_signal
>> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl
>b/arch/x86/entry/syscalls/syscall_64.tbl
>> index f0b1709a5ffb..8a30cde82450 100644
>> --- a/arch/x86/entry/syscalls/syscall_64.tbl
>> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
>> @@ -343,6 +343,7 @@
>>  332	common	statx			__x64_sys_statx
>>  333	common	io_pgetevents		__x64_sys_io_pgetevents
>>  334	common	rseq			__x64_sys_rseq
>> +335	64	procfd_signal		__x64_sys_procfd_signal
>>  
>>  #
>>  # x32-specific system call numbers start at 512 to avoid cache
>impact
>> @@ -386,3 +387,4 @@
>>  545	x32	execveat		__x32_compat_sys_execveat/ptregs
>>  546	x32	preadv2			__x32_compat_sys_preadv64v2
>>  547	x32	pwritev2		__x32_compat_sys_pwritev64v2
>> +548	x32	procfd_signal		__x32_compat_sys_procfd_signal
>
>Is there a reason why these numbers have to be different?
>
>(See the recent discussion with Andy Lutomirski.)
>
>> +static int do_procfd_signal(int fd, int sig, kernel_siginfo_t
>*kinfo, int flags,
>> +			    bool had_siginfo)
>> +{
>> +	int ret;
>> +	struct fd f;
>> +	struct pid *pid;
>> +
>> +	/* Enforce flags be set to 0 until we add an extension. */
>> +	if (flags)
>> +		return -EINVAL;
>> +
>> +	f = fdget_raw(fd);
>> +	if (!f.file)
>> +		return -EBADF;
>> +
>> +	/* Is this a process file descriptor? */
>> +	ret = -EINVAL;
>> +	if (!proc_is_tgid_procfd(f.file))
>> +		goto err;
>[…]
>> +	ret = kill_pid_info(sig, kinfo, pid);
>
>I would like to see some comment here what happens to zombie processes.

You'd get ESRCH.
I'm not sure if that has always been the case.
Eric recently did some excellent refactoring of the signal code.
Iirc, part of that involved not delivering signals to zombies.
That's at least how I remember it.
I don't have access to source code though atm.

>
>> +/**
>> + *  sys_procfd_signal - send a signal to a process through a process
>file
>> + *                      descriptor
>> + *  @fd: the file descriptor of the process
>> + *  @sig: signal to be sent
>> + *  @info: the signal info
>> + *  @flags: future flags to be passed
>> + */
>> +SYSCALL_DEFINE4(procfd_signal, int, fd, int, sig, siginfo_t __user
>*, info,
>> +		int, flags)
>
>Sorry, I'm quite unhappy with the name.  “signal” is for signal handler
>management.  procfd_sendsignal, procfd_sigqueueinfo or something like
>that would be fine.  Even procfd_kill would be better IMHO.

Ok. I only have strong opinions to procfd_kill().
Mainly because the new syscall takes
the job of multiple other syscalls
so kill gives the wrong impression.
I'll come up with a better name in the next iteration.

>
>Looking at the rt_tgsigqueueinfo interface, is there a way to implement
>the “tg” part with the current procfd_signal interface?  Would you use
>openat to retrieve the Tgid: line from "status"?

Yes, the tg part can be implemented.
As I pointed out in another mail my
I is to make this work by using file
descriptors for /proc/<pid>/task/<tid>.
I don't want this in the initial patchset though.
I prefer to slowly add those features
once we have gotten the basic functionality
in.


>
>Thanks,
>Florian