• Renzo Davoli's avatar
    UML on UML fixed: it did not start · 86d6f2bf
    Renzo Davoli authored
    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>
    86d6f2bf
ptrace.c 6.79 KB