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Subject: Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Helge Deller <deller@gmx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian@brauner.io>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Roman Gushchin <guro@fb.com>,
        Marcos Paulo de Souza <marcos.souza.org@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "Victor Kamensky (kamensky)" <kamensky@cisco.com>,
        xe-linux-external@cisco.com, Stefan Strogin <sstrogin@cisco.com>,
        Enke Chen <enkechen@cisco.com>
References: <e7ed306f-8992-9d00-bcab-5131159e8d89@cisco.com>
 <458c04d8-d189-4a26-729a-bb1d1d751534@cisco.com>
 <87sh0vpj5q.fsf@xmission.com>
 <df08c74f-1027-c635-fb99-a12d23f60947@cisco.com>
 <87zhv2md04.fsf@xmission.com>
From:   Enke Chen <enkechen@cisco.com>
Message-ID: <3da93f7c-972d-14e0-dae2-f0217ba03dc1@cisco.com>
Date:   Thu, 25 Oct 2018 14:24:19 -0700
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Hi, Eric:

I have a couple comments inlined.

>> On Wed, Oct 24, 2018 at 3:30 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>> Enke Chen <enkechen@cisco.com> writes:
>>>> For simplicity and consistency, this patch provides an implementation
>>>> for signal-based fault notification prior to the coredump of a child
>>>> process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can
>>>> be used by an application to express its interest and to specify the
>>>> signal (SIGCHLD or SIGUSR1 or SIGUSR2) for such a notification. A new
>>>> signal code (si_code), CLD_PREDUMP, is also defined for SIGCHLD.
>>>>
>>>> Changes to prctl(2):
>>>>
>>>>    PR_SET_PREDUMP_SIG (since Linux 4.20.x)
>>>>           Set the child pre-coredump signal of the calling process to
>>>>           arg2 (either SIGUSR1, or SIUSR2, or SIGCHLD, or 0 to clear).
>>>>           This is the signal that the calling process will get prior to
>>>>           the coredump of a child process. This value is cleared across
>>>>           execve(2), or for the child of a fork(2).
>>>>
>>>>           When SIGCHLD is specified, the signal code will be set to
>>>>           CLD_PREDUMP in such an SIGCHLD signal.
>> [...]
>>> Ugh.  Your test case is even using signalfd.  So you don't even want
>>> this signal to be delivered as a signal.
>>
>> Just to make sure everyone's on the same page: You're suggesting that
>> it might make sense to deliver the pre-dump notification via a new
>> type of file instead (along the lines of signalfd, timerfd, eventfd
>> and so on)?
> 
> My real complaint was that the API was not being tested in the way it
> is expected to be used.  Which makes a test pretty much useless as some
> aspect userspace could regress and the test would not notice because it
> is testing something different.
> 
> 

As I stated in a prior email, I have test code for both sigaction/waipid(),
and signefd(). As the sigaction/waitpid is more widely used and that is
what you prefer, I will change the selftest code to reflect that in the
next version.  Actually I should separate out the selftest code.

> 
> I do think that a file descriptor based API might be a good alternative
> to a signal based API.  The proc connector and signals are not the only
> API solution.
> 
> The common solution to this problem is that distributions defailt the
> rlimit core file size to 0.

We do need coredumps in order to have the bugs fixed.

Thanks.  -- Enke