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 <[email protected]>
---
diff -Naur linux-2.6.29-rc7/arch/um/kernel/ptrace.c linux-2.6.29-rc7-umluml/arch/um/kernel/ptrace.c
--- linux-2.6.29-rc7/arch/um/kernel/ptrace.c 2008-12-25 00:26:37.000000000 +0100
+++ linux-2.6.29-rc7-umluml/arch/um/kernel/ptrace.c 2009-03-06 20:27:51.000000000 +0100
@@ -64,6 +64,11 @@
ret = poke_user(child, addr, data);
break;
+ case PTRACE_SYSEMU:
+ case PTRACE_SYSEMU_SINGLESTEP:
+ ret=-EIO;
+ break;
+
/* continue and stop at next (return from) syscall */
case PTRACE_SYSCALL:
/* restart after signal. */
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
Thanks, renzo! Good work!
>
>Signed-off-by: Renzo Davoli <[email protected]>
>---
>diff -Naur linux-2.6.29-rc7/arch/um/kernel/ptrace.c linux-2.6.29-rc7-umluml/arch/um/kernel/ptrace.c
>--- linux-2.6.29-rc7/arch/um/kernel/ptrace.c 2008-12-25 00:26:37.000000000 +0100
>+++ linux-2.6.29-rc7-umluml/arch/um/kernel/ptrace.c 2009-03-06 20:27:51.000000000 +0100
>@@ -64,6 +64,11 @@
> ret = poke_user(child, addr, data);
> break;
>
>+ case PTRACE_SYSEMU:
>+ case PTRACE_SYSEMU_SINGLESTEP:
>+ ret=-EIO;
>+ break;
>+
I think this is OK. Please feel free to add my reviewed-by:
Reviewed-by: WANG Cong <[email protected]>
Jeff, what do you think?
--
Do what you love, f**k the rest! F**k the regulations!
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 <[email protected]>
Cc: Andrew Morton <[email protected]>
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