prctl(PR_SET_PDEATHSIG, ...) should be called after fork(), not before

[igor-anferov]

From the official Linux documentation:

The parent-death signal setting is cleared for the child of a fork(2).

So, calling prctl(PR_SET_PDEATHSIG, ...) before spawn(), which is currently the case, doesn't make sense.

[krallin]

The commit that introduced this explains the use case:

https://github.com/krallin/tini/commit/eb0f6de3a5efc57bba81992ad8463494a6f45c4a

Their goal was indeed to signal Tini when its parent dies, not to signal Tini’s child when Tini dies.

On Tue, 13 Jun 2023 at 16:24, Igor Anferov @.***> wrote:

From the official Linux documentation https://man7.org/linux/man-pages/man2/prctl.2.html:

The parent-death signal setting is cleared for the child of a fork(2).

So, calling prctl(PR_SET_PDEATHSIG, ...) before spawn(), which is currently the case, doesn't make sense.

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[igor-anferov]

Aaah, I see, thanks for your explanation. But I still don't understand the last phrase from the commit message:

    $ unshare --pid --fork setpriv --reuid user tini -s -p SIGKILL -- <prog>

As soon as unshare is getting killed, tini will get signalled SIGKILL
and exit as well, tearing down <prog> with it.

Why would be torn down in this case?

[krallin]

When you’re in a PID namespace and PID1 in that namespace exits the rest of the namespace will be killed too

On Wed, 14 Jun 2023 at 01:40, Igor Anferov @.***> wrote:

Aaah, I see, thanks for your explanation. But I still don't understand the last phrase from the commit message:

$ unshare --pid --fork setpriv --reuid user tini -s -p SIGKILL -- <prog>

As soon as unshare is getting killed, tini will get signalled SIGKILL and exit as well, tearing down with it.

Why would be torn down in this case?

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[igor-anferov]

Got it, thank you!