Initial commit.

.gitignore

+/usersyslog

Makefile

+CFLAGS ?= -O2 -s
+CFLAGS += -Wall -fPIE -pie
+LDLIBS = -ldl
+PREFIX = /usr/local
+
+.PHONY: all install uninstall clean
+
+all: usersyslog
+
+usersyslog: usersyslog.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $< $(LDLIBS)
+
+install: all
+	install -Dp usersyslog $(DESTDIR)$(PREFIX)/bin/usersyslog
+
+uninstall:
+	-cd $(DESTDIR)$(PREFIX) && rm -f bin/usersyslog
+
+clean:
+	-rm -f usersyslog

README

+usersyslog - redirect syslog accesses to a specific unix socket.
+
+Rationale
+---------
+
+The need is to customise the socket used to send log lines to syslog, so as
+to direct logging of processes to a user-specified (typically, user-controlled)
+syslog daemon. Useful for software packages only able to log via syslog
+(ex: postfix).
+User syslog daemon must be able to bind to non-default socket path.
+
+Limitations
+-----------
+
+This library relies on LD_PRELOAD mechanism, so it inherits its limitations
+(it will not work on suid binaries, ...).
+
+Requirements
+------------
+
+- make
+- gcc (source uses __attribute__)
+- glibc (source uses RTLD_NEXT)
+
+Building
+--------
+
+Just run "make".
+
+Usage
+-----
+
+$ LOG_SOCKET=/path/to/custom/log/socket LD_PRELOAD=/path/to/usersyslog <command>
+
+`usersyslog` is also an executable wrapper that appends itself to LD_PRELOAD:
+
+$ LOG_SOCKET=/path/to/custom/log/socket usersyslog <command>

usersyslog.c

+/* usersyslog - redirect syslog accesses to a specific logfile */
+/*
+ * Copyright (C) 2015 Vincent Pelletier <vincent@nexedi.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+/* Needed for RTLD_NEXT */
+#define _GNU_SOURCE
+
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <dlfcn.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+
+#ifdef _PATH_LOG
+#define ORIGINAL_LOG_SOCKET _PATH_LOG
+#else
+/* XXX: not BSD-compatible (/var/run/log) */
+#define ORIGINAL_LOG_SOCKET "/dev/log"
+#endif
+
+static int (*original_connect)(int fd, const struct sockaddr *addr,
+                               socklen_t len);
+static struct sockaddr_un replacement_addr_un;
+
+/* Call dlsym(RTLD_NEXT, name), abort()'ing with informative message to
+ * stderr if it cannot be found. */
+static inline void *dlsym_or_abort(const char *name) {
+  char *error;
+  void *symbol;
+  dlerror(); /* Clear any previous error */
+  symbol = dlsym(RTLD_NEXT, name);
+  if (!symbol && (error = dlerror())) {
+    fprintf(stderr, "Error loading '%s': %s\n", name, error);
+    abort();
+  }
+  return symbol;
+}
+
+static void __attribute__ ((constructor)) init(void) {
+  const char *log_path;
+  original_connect = dlsym_or_abort("connect");
+  log_path = getenv("LOG_SOCKET");
+  if (!log_path)
+    /* IDEA: If LOG_SOCKET is not set, try $XDG_CONFIG_HOME/log
+     *       and maybe warn if the latter does not exist. */
+    log_path = ORIGINAL_LOG_SOCKET;
+  if (strnlen(log_path, sizeof(replacement_addr_un.sun_path) + 1) >=
+      sizeof(replacement_addr_un.sun_path)) {
+    fprintf(stderr, "LOG_SOCKET value too long\n");
+    abort();
+  }
+  replacement_addr_un.sun_family = AF_UNIX;
+  (void)strncpy(replacement_addr_un.sun_path, log_path,
+                sizeof(replacement_addr_un.sun_path));
+}
+
+int connect(int fd, const struct sockaddr *addr, socklen_t len) {
+  const struct sockaddr_un *addr_un = (const struct sockaddr_un *) addr;
+  // XXX: len >= sizeof(addr_un) ?
+  if (addr->sa_family == AF_UNIX && len == sizeof(*addr_un) &&
+      !strcmp(addr_un->sun_path, ORIGINAL_LOG_SOCKET))
+    addr = (const struct sockaddr *) &replacement_addr_un;
+  return (*original_connect)(fd, addr, len);
+}
+
+int main(int argc, char *argv[]) {
+  char *ld_preload, *p;
+  int ret = -1;
+
+  if (argc <= 1)
+    fprintf(stderr, "usage: usersyslog <command> [<args>]\n");
+  else {
+    /* In order not to depend on /proc, getauxval(AT_EXECFN) from <sys/auxv.h>
+     * should be used. However, this requires glibc >= 2.16. */
+    ld_preload = realpath("/proc/self/exe", NULL);
+    if (!ld_preload)
+      p = "realpath";
+    else {
+      if ((p = getenv("LD_PRELOAD"))) {
+        size_t n = strlen(ld_preload);
+        ld_preload = realloc(ld_preload, n + strlen(p) + 2);
+        ld_preload[n++] = ':';
+        strcpy(ld_preload + n, p);
+      }
+      ret = setenv("LD_PRELOAD", ld_preload, 1);
+      free(ld_preload);
+      if (ret)
+        p = "setenv";
+      else {
+        ret = execvp(argv[1], &argv[1]);
+        p = "execvp";
+      }
+    }
+    perror(p);
+  }
+  return ret;
+}