linux-user: Fix syslog() syscall support

There are currently several problems related to syslog() support.

For example, if the second argument "bufp" of target syslog() syscall
is NULL, the current implementation always returns error code EFAULT.
However, NULL is a perfectly valid value for the second argument for
many use cases of this syscall. This is, for example, visible from
this excerpt of man page for syslog(2):

> EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is
>        NULL, or len is less than zero; or for type 8, the level is
>        outside the range 1 to 8).

Moreover, the argument "bufp" is ignored for all cases of values of the
first argument, except 2, 3 and 4. This means that for such cases
(the first argument is not 2, 3 or 4), there is no need to pass "buf"
between host and target, and it can be set to NULL while calling host's
syslog(), without loss of emulation accuracy.

Note also that if "bufp" is NULL and the first argument is 2, 3 or 4, the
correct returned error code is EINVAL, not EFAULT.

All these details are reflected in this patch.

"#ifdef TARGET_NR_syslog" is also proprerly inserted when needed.

Support for Qemu's "-strace" switch for syslog() syscall is included too.

LTP tests syslog11 and syslog12 pass with this patch (while fail without
it), on any platform.

Changes to original patch by Riku Voipio:

 fixed error paths in TARGET_SYSLOG_ACTION_READ_ALL to match

http://lxr.free-electrons.com/source/kernel/printk/printk.c?v=4.7#L1335

Should fix also the build error in:

https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg03721.html

Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/strace.c

@@ -1827,6 +1827,78 @@ print_rt_sigprocmask(const struct syscallname *name,
 }
 #endif
 
+#ifdef TARGET_NR_syslog
+static void
+print_syslog_action(abi_ulong arg, int last)
+{
+    const char *type;
+
+    switch (arg) {
+        case TARGET_SYSLOG_ACTION_CLOSE: {
+            type = "SYSLOG_ACTION_CLOSE";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_OPEN: {
+            type = "SYSLOG_ACTION_OPEN";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_READ: {
+            type = "SYSLOG_ACTION_READ";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_READ_ALL: {
+            type = "SYSLOG_ACTION_READ_ALL";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_READ_CLEAR: {
+            type = "SYSLOG_ACTION_READ_CLEAR";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_CLEAR: {
+            type = "SYSLOG_ACTION_CLEAR";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_CONSOLE_OFF: {
+            type = "SYSLOG_ACTION_CONSOLE_OFF";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_CONSOLE_ON: {
+            type = "SYSLOG_ACTION_CONSOLE_ON";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_CONSOLE_LEVEL: {
+            type = "SYSLOG_ACTION_CONSOLE_LEVEL";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_SIZE_UNREAD: {
+            type = "SYSLOG_ACTION_SIZE_UNREAD";
+            break;
+        }
+        case TARGET_SYSLOG_ACTION_SIZE_BUFFER: {
+            type = "SYSLOG_ACTION_SIZE_BUFFER";
+            break;
+        }
+        default: {
+            print_raw_param("%ld", arg, last);
+            return;
+        }
+    }
+    gemu_log("%s%s", type, get_comma(last));
+}
+
+static void
+print_syslog(const struct syscallname *name,
+    abi_long arg0, abi_long arg1, abi_long arg2,
+    abi_long arg3, abi_long arg4, abi_long arg5)
+{
+    print_syscall_prologue(name);
+    print_syslog_action(arg0, 0);
+    print_pointer(arg1, 0);
+    print_raw_param("%d", arg2, 1);
+    print_syscall_epilogue(name);
+}
+#endif
+
 #ifdef TARGET_NR_mknod
 static void
 print_mknod(const struct syscallname *name,

linux-user/strace.list

@@ -1486,7 +1486,7 @@
 { TARGET_NR_sys_kexec_load, "sys_kexec_load" , NULL, NULL, NULL },
 #endif
 #ifdef TARGET_NR_syslog
-{ TARGET_NR_syslog, "syslog" , NULL, NULL, NULL },
+{ TARGET_NR_syslog, "syslog" , NULL, print_syslog, NULL },
 #endif
 #ifdef TARGET_NR_sysmips
 { TARGET_NR_sysmips, "sysmips" , NULL, NULL, NULL },

linux-user/syscall.c

@@ -9320,14 +9320,52 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         ret = do_setsockopt(arg1, arg2, arg3, arg4, (socklen_t) arg5);
         break;
 #endif
-
+#if defined(TARGET_NR_syslog)
     case TARGET_NR_syslog:
-        if (!(p = lock_user_string(arg2)))
-            goto efault;
-        ret = get_errno(sys_syslog((int)arg1, p, (int)arg3));
-        unlock_user(p, arg2, 0);
-        break;
+        {
+            int len = arg2;
 
+            switch (arg1) {
+            case TARGET_SYSLOG_ACTION_CLOSE:         /* Close log */
+            case TARGET_SYSLOG_ACTION_OPEN:          /* Open log */
+            case TARGET_SYSLOG_ACTION_CLEAR:         /* Clear ring buffer */
+            case TARGET_SYSLOG_ACTION_CONSOLE_OFF:   /* Disable logging */
+            case TARGET_SYSLOG_ACTION_CONSOLE_ON:    /* Enable logging */
+            case TARGET_SYSLOG_ACTION_CONSOLE_LEVEL: /* Set messages level */
+            case TARGET_SYSLOG_ACTION_SIZE_UNREAD:   /* Number of chars */
+            case TARGET_SYSLOG_ACTION_SIZE_BUFFER:   /* Size of the buffer */
+                {
+                    ret = get_errno(sys_syslog((int)arg1, NULL, (int)arg3));
+                }
+                break;
+            case TARGET_SYSLOG_ACTION_READ:          /* Read from log */
+            case TARGET_SYSLOG_ACTION_READ_CLEAR:    /* Read/clear msgs */
+            case TARGET_SYSLOG_ACTION_READ_ALL:      /* Read last messages */
+                {
+                    ret = -TARGET_EINVAL;
+                    if (len < 0) {
+                        goto fail;
+                    }
+                    ret = 0;
+                    if (len == 0) {
+                        break;
+                    }
+                    p = lock_user(VERIFY_WRITE, arg2, arg3, 0);
+                    if (!p) {
+                        ret = -TARGET_EFAULT;
+                        goto fail;
+                    }
+                    ret = get_errno(sys_syslog((int)arg1, p, (int)arg3));
+                    unlock_user(p, arg2, arg3);
+                }
+                break;
+            default:
+                ret = -EINVAL;
+                break;
+            }
+        }
+        break;
+#endif
     case TARGET_NR_setitimer:
         {
             struct itimerval value, ovalue, *pvalue;

linux-user/syscall_defs.h

@@ -2682,4 +2682,29 @@ struct target_user_cap_data {
     uint32_t inheritable;
 };
 
+/* from kernel's include/linux/syslog.h */
+
+/* Close the log.  Currently a NOP. */
+#define TARGET_SYSLOG_ACTION_CLOSE          0
+/* Open the log. Currently a NOP. */
+#define TARGET_SYSLOG_ACTION_OPEN           1
+/* Read from the log. */
+#define TARGET_SYSLOG_ACTION_READ           2
+/* Read all messages remaining in the ring buffer. */
+#define TARGET_SYSLOG_ACTION_READ_ALL       3
+/* Read and clear all messages remaining in the ring buffer */
+#define TARGET_SYSLOG_ACTION_READ_CLEAR     4
+/* Clear ring buffer. */
+#define TARGET_SYSLOG_ACTION_CLEAR          5
+/* Disable printk's to console */
+#define TARGET_SYSLOG_ACTION_CONSOLE_OFF    6
+/* Enable printk's to console */
+#define TARGET_SYSLOG_ACTION_CONSOLE_ON     7
+/* Set level of messages printed to console */
+#define TARGET_SYSLOG_ACTION_CONSOLE_LEVEL  8
+/* Return number of unread characters in the log buffer */
+#define TARGET_SYSLOG_ACTION_SIZE_UNREAD    9
+/* Return size of the log buffer */
+#define TARGET_SYSLOG_ACTION_SIZE_BUFFER   10
+
 #endif