[PULL 3/7] linux-user: Fix Coverity CID 1430271 / CID 1430272

[Laurent Vivier]

In new functions print_ioctl() and print_syscall_ret_ioctl(), we don't
check if lock_user() returns NULL and this would cause a segfault in
thunk_print().

If lock_user() returns NULL don't call thunk_print() but prints only the
value of the (invalid) pointer.

Tested with:

    # cat ioctl.c
    #include <unistd.h>
    #include <sys/ioctl.h>

    int main(void)
    {
        int ret;

        ret = ioctl(STDOUT_FILENO, TCGETS, 0xdeadbeef);
        ret = ioctl(STDOUT_FILENO, TCSETSF, 0xdeadbeef);
        return 0;
    }
    # QEMU_STRACE= ./ioctl
    ...
    578 ioctl(1,TCGETS,0xdeadbeef) = -1 errno=2 (Bad address)
    578 ioctl(1,TCSETSF,0xdeadbeef) = -1 errno=2 (Bad address)
    ...
    # QEMU_STRACE= passwd
    ...
    623 ioctl(0,TCGETS,0x3fffed04) = 0 ({})
    623 ioctl(0,TCSETSF,{}) = 0
    ...

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 79482e5987c8 ("linux-user: Add strace support for printing arguments of 
ioctl()")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
---
 linux-user/strace.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/linux-user/strace.c b/linux-user/strace.c
index 5235b2260cdd..39554d903911 100644
--- a/linux-user/strace.c
+++ b/linux-user/strace.c
@@ -889,8 +889,12 @@ print_syscall_ret_ioctl(const struct syscallname *name, 
abi_long ret,
             arg_type++;
             target_size = thunk_type_size(arg_type, 0);
             argptr = lock_user(VERIFY_READ, arg2, target_size, 1);
-            thunk_print(argptr, arg_type);
-            unlock_user(argptr, arg2, target_size);
+            if (argptr) {
+                thunk_print(argptr, arg_type);
+                unlock_user(argptr, arg2, target_size);
+            } else {
+                print_pointer(arg2, 1);
+            }
             qemu_log(")");
         }
     }
@@ -3119,8 +3123,12 @@ print_ioctl(const struct syscallname *name,
                     arg_type++;
                     target_size = thunk_type_size(arg_type, 0);
                     argptr = lock_user(VERIFY_READ, arg2, target_size, 1);
-                    thunk_print(argptr, arg_type);
-                    unlock_user(argptr, arg2, target_size);
+                    if (argptr) {
+                        thunk_print(argptr, arg_type);
+                        unlock_user(argptr, arg2, target_size);
+                    } else {
+                        print_pointer(arg2, 1);
+                    }
                     break;
                 }
                 break;
-- 
2.26.2