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From: "carlos at redhat dot com" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug libc/29115] vfork()-based posix_spawn() has more failure modes
 than fork()-based one
Date: Mon, 02 May 2022 21:24:00 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29115

--- Comment #11 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Florian Weimer from comment #9)
> (In reply to Carlos O'Donell from comment #8)
> > (In reply to Florian Weimer from comment #6)
> > > CLONE_NEWTIME is as specified today fundamentally incompatible with r=
eal
> > > vfork and the vDSO. It just does not work. Entering the new namespace
> > > requires a new vDSO data mapping, and that conflicts with vfork using=
 the
> > > same address space.
> >=20
> > The kernel already has per-cpu data in the vDSO.
>=20
> Uh, since when? I thought that Linux didn't do per-CPU page tables.

So, this is a stretch, but on x86 you use GDT to get the per-CPU data.

Is this not what we could call per-cpu data in a distinct address space?

> > The vDSO doesn't follow any concept of a single address space for the
> > process.
> >=20
> > The vDSO is not a part of POSIX and so doesn't have to follow any vfork
> > semantic requirements.
> >=20
> > What prevents the kernel from making a new vDSO data mapping?
>=20
> It requires creating a new VM for the vfork process, while preserving
> existing shared VM semantics in other regards. That seems difficult?

I don't know until a kernel developer tells me this is difficult :-)

--=20
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