[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, 63 insertions, 0 deletions

diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index 1647abb0d1aa..ba671dab8878 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -13,6 +13,7 @@
 #include <sys/wait.h>
 #include <sys/mman.h>
 #include <sys/user.h>
+#include <sys/time.h>
 #include <asm/unistd.h>
 #include <asm/types.h>
 #include "user.h"
@@ -22,6 +23,7 @@
 #include "user_util.h"
 #include "kern_util.h"
 #include "skas.h"
+#include "stub-data.h"
 #include "mm_id.h"
 #include "sysdep/sigcontext.h"
 #include "sysdep/stub.h"
@@ -296,6 +298,67 @@ void userspace(union uml_pt_regs *regs)
 #define INIT_JMP_HALT 3
 #define INIT_JMP_REBOOT 4
 
+
+int copy_context_skas0(unsigned long new_stack, int pid)
+{
+        int err;
+        unsigned long regs[MAX_REG_NR];
+        unsigned long current_stack = current_stub_stack();
+        struct stub_data *data = (struct stub_data *) current_stack;
+        struct stub_data *child_data = (struct stub_data *) new_stack;
+        __u64 new_offset;
+        int new_fd = phys_mapping(to_phys((void *)new_stack), &new_offset);
+
+        /* prepare offset and fd of child's stack as argument for parent's
+         * and child's mmap2 calls
+         */
+        *data = ((struct stub_data) { .offset   = MMAP_OFFSET(new_offset),
+                                      .fd       = new_fd,
+                                      .timer    = ((struct itimerval)
+                                                   { { 0, 1000000 / hz() },
+                                                     { 0, 1000000 / hz() }})});
+        get_safe_registers(regs);
+
+        /* Set parent's instruction pointer to start of clone-stub */
+        regs[REGS_IP_INDEX] = UML_CONFIG_STUB_CODE +
+                                (unsigned long) stub_clone_handler -
+                                (unsigned long) &__syscall_stub_start;
+        regs[REGS_SP_INDEX] = UML_CONFIG_STUB_DATA + PAGE_SIZE -
+                sizeof(void *);
+        err = ptrace_setregs(pid, regs);
+        if(err < 0)
+                panic("copy_context_skas0 : PTRACE_SETREGS failed, "
+                      "pid = %d, errno = %d\n", pid, errno);
+
+        /* set a well known return code for detection of child write failure */
+        child_data->err = 12345678;
+
+        /* Wait, until parent has finished its work: read child's pid from
+         * parent's stack, and check, if bad result.
+         */
+        wait_stub_done(pid, 0, "copy_context_skas0");
+
+        pid = data->err;
+        if(pid < 0)
+                panic("copy_context_skas0 - stub-parent reports error %d\n",
+                      pid);
+
+        /* Wait, until child has finished too: read child's result from
+         * child's stack and check it.
+         */
+        wait_stub_done(pid, -1, "copy_context_skas0");
+        if (child_data->err != UML_CONFIG_STUB_DATA)
+                panic("copy_context_skas0 - stub-child reports error %d\n",
+                      child_data->err);
+
+        if (ptrace(PTRACE_OLDSETOPTIONS, pid, NULL,
+                   (void *)PTRACE_O_TRACESYSGOOD) < 0)
+                panic("copy_context_skas0 : PTRACE_SETOPTIONS failed, "
+                      "errno = %d\n", errno);
+
+        return pid;
+}
+
 void new_thread(void *stack, void **switch_buf_ptr, void **fork_buf_ptr,
                 void (*handler)(int))
 {