Implement Linux syslog(2) syscall; just enough to make Linux dmesg(8)

utility work.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D22465

sys/amd64/linux/linux_dummy.c

@@ -60,7 +60,6 @@ UNIMPLEMENTED(uselib);
 UNIMPLEMENTED(vserver);
 
 DUMMY(sendfile);
-DUMMY(syslog);
 DUMMY(setfsuid);
 DUMMY(setfsgid);
 DUMMY(sysfs);

sys/amd64/linux32/linux32_dummy.c

@@ -63,7 +63,6 @@ UNIMPLEMENTED(vserver);
 
 DUMMY(stime);
 DUMMY(olduname);
-DUMMY(syslog);
 DUMMY(uname);
 DUMMY(vhangup);
 DUMMY(swapoff);

sys/arm64/linux/linux_dummy.c

@@ -65,7 +65,6 @@ UNIMPLEMENTED(uselib);
 UNIMPLEMENTED(vserver);
 
 DUMMY(sendfile);
-DUMMY(syslog);
 DUMMY(setfsuid);
 DUMMY(setfsgid);
 DUMMY(vhangup);

sys/compat/linux/linux_misc.c

@@ -47,6 +47,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/mman.h>
 #include <sys/mount.h>
+#include <sys/msgbuf.h>
 #include <sys/mutex.h>
 #include <sys/namei.h>
 #include <sys/priv.h>
@@ -2289,3 +2290,66 @@ linux_mincore(struct thread *td, struct linux_mincore_args *args)
 		return (EINVAL);
 	return (kern_mincore(td, args->start, args->len, args->vec));
 }
+
+#define	SYSLOG_TAG	"<6>"
+
+int
+linux_syslog(struct thread *td, struct linux_syslog_args *args)
+{
+	char buf[128], *src, *dst;
+	u_int seq;
+	int buflen, error;
+
+	if (args->type != LINUX_SYSLOG_ACTION_READ_ALL) {
+		linux_msg(td, "syslog unsupported type 0x%x", args->type);
+		return (EINVAL);
+	}
+
+	if (args->len < 6) {
+		td->td_retval[0] = 0;
+		return (0);
+	}
+
+	error = priv_check(td, PRIV_MSGBUF);
+	if (error)
+		return (error);
+
+	mtx_lock(&msgbuf_lock);
+	msgbuf_peekbytes(msgbufp, NULL, 0, &seq);
+	mtx_unlock(&msgbuf_lock);
+
+	dst = args->buf;
+	error = copyout(&SYSLOG_TAG, dst, sizeof(SYSLOG_TAG));
+	/* The -1 is to skip the trailing '\0'. */
+	dst += sizeof(SYSLOG_TAG) - 1;
+
+	while (error == 0) {
+		mtx_lock(&msgbuf_lock);
+		buflen = msgbuf_peekbytes(msgbufp, buf, sizeof(buf), &seq);
+		mtx_unlock(&msgbuf_lock);
+
+		if (buflen == 0)
+			break;
+
+		for (src = buf; src < buf + buflen && error == 0; src++) {
+			if (*src == '\0')
+				continue;
+
+			if (dst >= args->buf + args->len)
+				goto out;
+
+			error = copyout(src, dst, 1);
+			dst++;
+
+			if (*src == '\n' && *(src + 1) != '<' &&
+			    dst + sizeof(SYSLOG_TAG) < args->buf + args->len) {
+				error = copyout(&SYSLOG_TAG,
+				    dst, sizeof(SYSLOG_TAG));
+				dst += sizeof(SYSLOG_TAG) - 1;
+			}
+		}
+	}
+out:
+	td->td_retval[0] = dst - args->buf;
+	return (error);
+}

sys/compat/linux/linux_misc.h

@@ -149,6 +149,9 @@ extern int stclohz;
 #define	LINUX_GRND_NONBLOCK	0x0001
 #define	LINUX_GRND_RANDOM	0x0002
 
+/* Linux syslog flags */
+#define	LINUX_SYSLOG_ACTION_READ_ALL	3
+
 #if defined(__amd64__) && !defined(COMPAT_LINUX32)
 int linux_ptrace_status(struct thread *td, int pid, int status);
 #endif

sys/i386/linux/linux_dummy.c

@@ -64,7 +64,6 @@ UNIMPLEMENTED(vserver);
 DUMMY(stime);
 DUMMY(fstat);
 DUMMY(olduname);
-DUMMY(syslog);
 DUMMY(uname);
 DUMMY(vhangup);
 DUMMY(vm86old);