UML failing at "Failed to initialize default registers" on kernel 5.10

[Benjamin Berg]

On Mon, 2025-01-20 at 07:07 +0100, Thomas Meyer wrote:
> 
> 
> Am 20. Januar 2025 00:25:35 MEZ schrieb Glenn Washburn
> <development at efficientek.com>:
> > Hi Benjamin,
> > 
> > After applying the close_range patch, I'm now getting a failure at
> > runtime where the last line printed from UML is "Failed to
> > initialize
> > default registers". The host is on Debian 11 at kernel 5.10.216.
> > I've
> > bisected the issue and it looks like the bad commit is: 3f17fed2149
> > ("um: switch to regset API and depend on XSTATE").
> > 
> > On another machine which is on Debian 12 and kernel 6.1.119 the UML
> > binary compiled from v6.13-rc1 runs fine.
> > 
> > I notice that the bad commit appears to be using ptrace in a
> > different
> > way. Perhaps there is a difference in how ptrace operate from 5.10
> > to
> > 6.1 kernels. I've tried debugging this without much success, as GDB
> > can't be used. Any idea on what's going on here?
> 
> What processor does those machines have?

Some relatively old CPU or maybe inside a VM?

I would guess it is the same problem as reported by
https://lore.kernel.org/all/20241203070218.240797-1-sj@kernel.org/

Please try applying
https://patch.msgid.link/20241204074827.1582917-1-benjamin@sipsolutions.net
and check whether that fixes it.

Benjamin