master: Improve "core not dumped" error messages with Linux

Recommend setting /proc/fs/suid_dumpable to 2 and
/proc/sys/kernel/core_pattern to absolute path, if they already aren't.

src/master/common.h

@@ -5,9 +5,14 @@
 #include "master-interface.h"
 #include "master-settings.h"
 
+#define LINUX_PROC_FS_SUID_DUMPABLE "/proc/sys/fs/suid_dumpable"
+#define LINUX_PROC_SYS_KERNEL_CORE_PATTERN "/proc/sys/kernel/core_pattern"
+
 extern uid_t master_uid;
 extern gid_t master_gid;
 extern bool core_dumps_disabled;
+extern bool have_proc_fs_suid_dumpable;
+extern bool have_proc_sys_kernel_core_pattern;
 extern const char *ssl_manual_key_password;
 extern int global_master_dead_pipe_fd[2];
 extern struct service_list *services;

src/master/main.c

@@ -46,6 +46,8 @@ struct master_delayed_error {
 uid_t master_uid;
 gid_t master_gid;
 bool core_dumps_disabled;
+bool have_proc_fs_suid_dumpable;
+bool have_proc_sys_kernel_core_pattern;
 const char *ssl_manual_key_password;
 int global_master_dead_pipe_fd[2];
 struct service_list *services;
@@ -471,6 +473,7 @@ static void main_log_startup(char **protocols)
 #define STARTUP_STRING PACKAGE_NAME" v"DOVECOT_VERSION_FULL" starting up"
 	string_t *str = t_str_new(128);
 	rlim_t core_limit;
+	struct stat st;
 
 	str_append(str, STARTUP_STRING);
 	if (protocols[0] == NULL)
@@ -484,6 +487,10 @@ static void main_log_startup(char **protocols)
 		core_limit == 0;
 	if (core_dumps_disabled)
 		str_append(str, " (core dumps disabled)");
+	if (stat(LINUX_PROC_FS_SUID_DUMPABLE, &st) == 0)
+		have_proc_fs_suid_dumpable = TRUE;
+	if (stat(LINUX_PROC_SYS_KERNEL_CORE_PATTERN, &st) == 0)
+		have_proc_sys_kernel_core_pattern = TRUE;
 	i_info("%s", str_c(str));
 }
 

src/master/service-process.c

@@ -444,12 +444,57 @@ get_exit_status_message(struct service *service, enum fatal_exit_status status)
 	return NULL;
 }
 
+static bool linux_proc_fs_suid_is_dumpable(unsigned int *value_r)
+{
+	int fd = open(LINUX_PROC_FS_SUID_DUMPABLE, O_RDONLY);
+	if (fd == -1) {
+		/* we already checked that it exists - shouldn't get here */
+		i_error("open(%s) failed: %m", LINUX_PROC_FS_SUID_DUMPABLE);
+		have_proc_fs_suid_dumpable = FALSE;
+		return FALSE;
+	}
+	char buf[10];
+	ssize_t ret = read(fd, buf, sizeof(buf)-1);
+	if (ret < 0) {
+		i_error("read(%s) failed: %m", LINUX_PROC_FS_SUID_DUMPABLE);
+		have_proc_fs_suid_dumpable = FALSE;
+		*value_r = 0;
+	} else {
+		buf[ret] = '\0';
+		if (str_to_uint(buf, value_r) < 0)
+			*value_r = 0;
+	}
+	i_close_fd(&fd);
+	return *value_r != 0;
+}
+
+static bool linux_is_absolute_core_pattern(void)
+{
+	int fd = open(LINUX_PROC_SYS_KERNEL_CORE_PATTERN, O_RDONLY);
+	if (fd == -1) {
+		/* we already checked that it exists - shouldn't get here */
+		i_error("open(%s) failed: %m", LINUX_PROC_SYS_KERNEL_CORE_PATTERN);
+		have_proc_sys_kernel_core_pattern = FALSE;
+		return FALSE;
+	}
+	char buf[10];
+	ssize_t ret = read(fd, buf, sizeof(buf)-1);
+	if (ret < 0) {
+		i_error("read(%s) failed: %m", LINUX_PROC_SYS_KERNEL_CORE_PATTERN);
+		have_proc_sys_kernel_core_pattern = FALSE;
+		buf[0] = '\0';
+	}
+	i_close_fd(&fd);
+	return buf[0] == '/' || buf[0] == '|';
+}
+
 static void
 log_coredump(struct service *service, string_t *str, int status)
 {
 #define CORE_DUMP_URL "https://dovecot.org/bugreport.html#coredumps"
 #ifdef WCOREDUMP
 	int signum = WTERMSIG(status);
+	unsigned int dumpable;
 
 	if (WCOREDUMP(status) != 0) {
 		str_append(str, " (core dumped)");
@@ -466,6 +511,24 @@ log_coredump(struct service *service, string_t *str, int status)
 	}
 	str_append(str, " (core not dumped - "CORE_DUMP_URL);
 
+	/* If we're running on Linux, the best way to get core dumps is to set
+	   fs.suid_dumpable=2 and sys.kernel.core_pattern to be an absolute
+	   path. */
+	if (!have_proc_fs_suid_dumpable)
+		;
+	else if (!linux_proc_fs_suid_is_dumpable(&dumpable)) {
+		str_printfa(str, " - set %s to 2)", LINUX_PROC_FS_SUID_DUMPABLE);
+		return;
+	} else if (dumpable == 2 && have_proc_sys_kernel_core_pattern &&
+		   !linux_is_absolute_core_pattern()) {
+		str_printfa(str, " - set %s to absolute path)",
+			    LINUX_PROC_SYS_KERNEL_CORE_PATTERN);
+		return;
+	} else if (dumpable == 1 || have_proc_sys_kernel_core_pattern) {
+		str_append(str, " - core wasn't writable?)");
+		return;
+	}
+
 #ifndef HAVE_PR_SET_DUMPABLE
 	if (!service->set->drop_priv_before_exec && service->uid != 0) {
 		str_printfa(str, " - set service %s "