The patch titled
UML on UML fixed: it did not start
has been added to the -mm tree. Its filename is
uml-on-uml-fixed-it-did-not-start.patch
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Subject: UML on UML fixed: it did not start
From: Renzo Davoli <renzo@xxxxxxxxxxx>
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.
Signed-off-by: Renzo Davoli <renzo@xxxxxxxxxxx>
Reviewed-by: WANG Cong <xiyou.wangcong@xxxxxxxxx>
Cc: Jeff Dike <jdike@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/um/kernel/ptrace.c | 5 +++++
1 file changed, 5 insertions(+)
diff -puN arch/um/kernel/ptrace.c~uml-on-uml-fixed-it-did-not-start arch/um/kernel/ptrace.c
--- a/arch/um/kernel/ptrace.c~uml-on-uml-fixed-it-did-not-start
+++ a/arch/um/kernel/ptrace.c
@@ -64,6 +64,11 @@ long arch_ptrace(struct task_struct *chi
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. */
_
Patches currently in -mm which might be from renzo@xxxxxxxxxxx are
uml-on-uml-fixed-it-did-not-start.patch
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