Re: [PATCH] UML on UML fixed: it did not start

["Américo Wang" <xiyou.wangcong@gmail.com>]

On Fri, Mar 06, 2009 at 08:49:22PM +0100, Renzo Davoli wrote:
>It is currently impossible to run a user-mode linux machine inside another user-mode 
>linux (UML on UML). It breaks after a few instructions. When it tries to check
>whether SYSEMU is installed (the inner) UML receives an inconsistent result 
>(from the outer UML).
>
>This is the output of a broken attempt:
>$ ./linux mem=256m ubd0=cow
>Locating the bottom of the address space ... 0x0
>Locating the top of the address space ... 0xc0000000
>Core dump limits :
>        soft - 0
>        hard - NONE
>Checking that ptrace can change system call numbers...OK
>Checking ptrace new tags for syscall emulation...unsupported
>Checking syscall emulation patch for ptrace...check_sysemu : expected SIGTRAP, got status = 256
>$
>
>The problem is the following:
>PTRACE_SYSCALL/SINGLESTEP is currently managed inside arch_ptrace for ARCH=um.
>
>PTRACE_SYSEMU/SUSEMU_SINGLESTEP is not captured in arch_ptrace's switch, therefore
>it is erroneously passed back to ptrace_request (in kernel/ptrace).
>
>This simple patch simply forces ptrace to return an error on PTRACE_SYSEMU/SUSEMU_SINGLESTEP
>as it is unsupported on ARCH=um, and fixes the problem.
>
>I posted the same patch one month ago. I just tested it again against the latest kernel.
>
>	renzo
>
>Signed-off-by: Renzo Davoli <renzo@cs.unibo.it>

Cc: Andrew Morton <akpm@osdl.org>

Andrew, would you like to put this patch into -mm? It looks fine for me.

Renzo, I remember you sent some patches about PTRACE_SYSEMU support
for UML some days ago, could you please rebase them against the latest
-git tree and resend?

Thank you very much! Have a nice day!


Yours,
Cong