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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. [[email protected]: coding-style fixes] Signed-off-by: Renzo Davoli <[email protected]> Reviewed-by: WANG Cong <[email protected]> Cc: Jeff Dike <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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