Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification

From: Enke Chen
Date: Thu Oct 25 2018 - 17:24:24 EST

Hi, Eric:

I have a couple comments inlined.

>> On Wed, Oct 24, 2018 at 3:30 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>>> Enke Chen <enkechen@xxxxxxxxx> 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