Commit 86d6f2bf authored by Renzo Davoli's avatar Renzo Davoli Committed by Linus Torvalds

UML on UML fixed: it did not start

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.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: default avatarRenzo Davoli <renzo@cs.unibo.it>
Reviewed-by: default avatarWANG Cong <xiyou.wangcong@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f1c7404e
......@@ -64,6 +64,11 @@ long arch_ptrace(struct task_struct *child, long request, long addr, long data)
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. */
......
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