[PATCH] uml: Proper clone support for skas0

This patch implements the clone-stub mechanism, which allows skas0 to run
with proc_mm==0, even if the clib in UML uses modify_ldt.

Note: There is a bug in skas3.v7 host patch, that avoids UML-skas from
running properly on a SMP-box.  In full skas3, I never really saw problems,
but in skas0 they showed up.

More commentary by jdike - What this patch does is makes sure that the host
parent of each new host process matches the UML parent of the corresponding
UML process.  This ensures that any changed LDTs are inherited.  This is
done by having clone actually called by the UML process from its stub,
rather than by the kernel.  We have special syscall stubs that are loaded
onto the stub code page because that code must be completely
self-contained.  These stubs are given C interfaces, and used like normal C
functions, but there are subtleties.  Principally, we have to be careful
about stack variables in stub_clone_handler after the clone.  The code is
written so that there aren't any - everything boils down to a fixed
address.  If there were any locals, references to them after the clone
would be wrong because the stack just changed.

Signed-off-by: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/arch/um/include/sysdep-x86_64/stub.h b/arch/um/include/sysdep-x86_64/stub.h
index 6b5447ad590..f599058d826 100644
--- a/arch/um/include/sysdep-x86_64/stub.h
+++ b/arch/um/include/sysdep-x86_64/stub.h
@@ -11,9 +11,48 @@
 #include <sysdep/ptrace_user.h>
 
 extern void stub_segv_handler(int sig);
+extern void stub_clone_handler(void);
 
 #define STUB_SYSCALL_RET PT_INDEX(RAX)
 #define STUB_MMAP_NR __NR_mmap
 #define MMAP_OFFSET(o) (o)
 
+static inline long stub_syscall2(long syscall, long arg1, long arg2)
+{
+	long ret;
+
+	__asm__("movq %0, %%rsi; " : : "g" (arg2) : "%rsi");
+	__asm__("movq %0, %%rdi; " : : "g" (arg1) : "%rdi");
+	__asm__("movq %0, %%rax; " : : "g" (syscall) : "%rax");
+	__asm__("syscall;" : : : "%rax", "%r11", "%rcx");
+	__asm__ __volatile__("movq %%rax, %0; " : "=g" (ret) :);
+	return(ret);
+}
+
+static inline long stub_syscall3(long syscall, long arg1, long arg2, long arg3)
+{
+	__asm__("movq %0, %%rdx; " : : "g" (arg3) : "%rdx");
+	return(stub_syscall2(syscall, arg1, arg2));
+}
+
+static inline long stub_syscall4(long syscall, long arg1, long arg2, long arg3,
+				 long arg4)
+{
+	__asm__("movq %0, %%r10; " : : "g" (arg4) : "%r10");
+	return(stub_syscall3(syscall, arg1, arg2, arg3));
+}
+
+static inline long stub_syscall6(long syscall, long arg1, long arg2, long arg3,
+				 long arg4, long arg5, long arg6)
+{
+	__asm__("movq %0, %%r9; " : : "g" (arg6) : "%r9");
+	__asm__("movq %0, %%r8; " : : "g" (arg5) : "%r8");
+	return(stub_syscall4(syscall, arg1, arg2, arg3, arg4));
+}
+
+static inline void trap_myself(void)
+{
+	__asm("int3");
+}
+
 #endif