Re: [Xen-devel] [PATCH v2 2/2] libxl: fix build (missing CLONE_NEWIPC) on astonishingly old systems

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.01.19 at 20:23, <ian.jackson@xxxxxxxxxxxxx> wrote:
> CLONE_NEWIPC was introduced in Linux 2.6.19, on the 29th of November
> 2006, which was 12 years, 1 month, and 14 days ago.

Thanks for the very precise counting, the latter part which will be
wrong - even if just slightly - by the time you commit it ;-)

> Nevertheless apparently some people are trying to build Xen on systems
> whose kernel headers are that old.  Placate these people by providing
> a fallback #define for CLONE_NEWIPC.
> 
> The actual binary value will of course remain constant, because of the
> kernel API promise, so this is and will be correct on all platforms
> where the CLONE_NEWIPC is supported.  (Even if for some reason we miss
> the right #includes.)
> 
> Of course at runtime this value will not work on older kernels.  It
> will be rejected as unknown.

Sadly the situation is more complicated: The check to disallow
unknown flags was introduced only in 2.6.17 [1], and apparently
never backported to 2.6.16 or older stable trees despite the
description talking about it going into 2.6.16. Since it didn't
matter in my variant of the workaround, I didn't mention this.

Of course a pretty reasonable position to take would be to
consider the 2.6.18-based XenoLinux tree a "baseline", beyond
which we don't care about undesirable behavior here.

Jan

[1] 06f9d4f94a unshare: Error if passed unsupported flags



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