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Re: [PATCH] posix: Do not use WNOHANG in waitpid call for Linux posix_spawn


On 10/23/2017 12:38 PM, Szabolcs Nagy wrote:
On 23/10/17 07:32, Florian Weimer wrote:
On 10/22/2017 10:51 PM, Adhemerval Zanella wrote:
As shown in some buildbot issues on aarch64 and powerpc, calling
clone (VFORK) and waitpid (WNOHANG) does not guarantee the child
is ready to be collected.  This patch changes the call back to 0
as before fe05e1cb6d64 fix.

I see it on x86-64, too.  It does look like a kernel bug.

This change can lead to the scenario 4.3 described in the commit,
where the waitpid call can hang undefinitely on the call.  However
this is also a very unlikely and also undefinied situation where
both the caller is trying to terminate a pid before posix_spawn
returns and the race pid reuse is triggered.  I don't see how to
correct handle this specific situation within posix_spawn.

Agreed.  I wish we could do better here, but it seems we can't.


musl writes a close-on-exec pipe in the child on error,
reading it in the parent tells if the child died before
exec or not. (so the waitpid can be made precise)

How so? A wildcard wait can still collect the PID, which is the problem we were trying to defend against with the WNOHANG flag.

Thanks,
Florian


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