um: Store syscall number after syscall_trace_enter()

To support changing syscall numbers we have to store
it after syscall_trace_enter().
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/os.h

@@ -274,6 +274,7 @@ extern void initial_thread_cb_skas(void (*proc)(void *),
 				 void *arg);
 extern void halt_skas(void);
 extern void reboot_skas(void);
+extern int get_syscall(struct uml_pt_regs *regs);
 
 /* irq.c */
 extern int os_waiting_for_events(struct irq_fd *active_fds);

arch/um/kernel/skas/syscall.c

@@ -8,6 +8,7 @@
 #include <kern_util.h>
 #include <sysdep/ptrace.h>
 #include <sysdep/syscalls.h>
+#include <os.h>
 
 extern int syscall_table_size;
 #define NR_SYSCALLS (syscall_table_size / sizeof(void *))
@@ -23,16 +24,8 @@ void handle_syscall(struct uml_pt_regs *r)
 		goto out;
 	}
 
-	/*
-	 * This should go in the declaration of syscall, but when I do that,
-	 * strace -f -c bash -c 'ls ; ls' breaks, sometimes not tracing
-	 * children at all, sometimes hanging when bash doesn't see the first
-	 * ls exit.
-	 * The assembly looks functionally the same to me.  This is
-	 *     gcc version 4.0.1 20050727 (Red Hat 4.0.1-5)
-	 * in case it's a compiler bug.
-	 */
-	syscall = UPT_SYSCALL_NR(r);
+	syscall = get_syscall(r);
+
 	if ((syscall >= NR_SYSCALLS) || (syscall < 0))
 		result = -ENOSYS;
 	else result = EXECUTE_SYSCALL(syscall, regs);

arch/um/os-Linux/skas/process.c

@@ -137,9 +137,6 @@ static void handle_trap(int pid, struct uml_pt_regs *regs,
 	if ((UPT_IP(regs) >= STUB_START) && (UPT_IP(regs) < STUB_END))
 		fatal_sigsegv();
 
-	/* Mark this as a syscall */
-	UPT_SYSCALL_NR(regs) = PT_SYSCALL_NR(regs->gp);
-
 	if (!local_using_sysemu)
 	{
 		err = ptrace(PTRACE_POKEUSER, pid, PT_SYSCALL_NR_OFFSET,
@@ -174,6 +171,13 @@ static void handle_trap(int pid, struct uml_pt_regs *regs,
 	handle_syscall(regs);
 }
 
+int get_syscall(struct uml_pt_regs *regs)
+{
+	UPT_SYSCALL_NR(regs) = PT_SYSCALL_NR(regs->gp);
+
+	return UPT_SYSCALL_NR(regs);
+}
+
 extern char __syscall_stub_start[];
 
 static int userspace_tramp(void *stack)