samples: Add fs error monitoring example

Introduce an example of a FAN_ERROR fanotify user to track filesystem
errors.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

samples/Kconfig

@@ -120,6 +120,13 @@ config SAMPLE_CONNECTOR
 	  with it.
 	  See also Documentation/driver-api/connector.rst
 
+config SAMPLE_FANOTIFY_ERROR
+	bool "Build fanotify error monitoring sample"
+	depends on FANOTIFY
+	help
+	  When enabled, this builds an example code that uses the FAN_ERROR
+	  fanotify mechanism to monitor filesystem errors.
+
 config SAMPLE_HIDRAW
 	bool "hidraw sample"
 	depends on CC_CAN_LINK && HEADERS_INSTALL

samples/Makefile

@@ -5,6 +5,7 @@ subdir-$(CONFIG_SAMPLE_AUXDISPLAY)	+= auxdisplay
 subdir-$(CONFIG_SAMPLE_ANDROID_BINDERFS) += binderfs
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
+obj-$(CONFIG_SAMPLE_FANOTIFY_ERROR)	+= fanotify/
 subdir-$(CONFIG_SAMPLE_HIDRAW)		+= hidraw
 obj-$(CONFIG_SAMPLE_HW_BREAKPOINT)	+= hw_breakpoint/
 obj-$(CONFIG_SAMPLE_KDB)		+= kdb/

samples/fanotify/Makefile

@@ -0,0 +1,3 @@
+userprogs-always-y += fs-monitor
+
+userccflags += -I usr/include

samples/fanotify/fs-monitor.c

@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2021, Collabora Ltd.
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <err.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <sys/fanotify.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#ifndef FAN_ERROR
+
+#define FAN_ERROR		0x00100000
+#define FAN_PREALLOC_QUEUE      0x00000080
+
+#define FAN_EVENT_INFO_TYPE_LOCATION	4
+#define FAN_EVENT_INFO_TYPE_ERROR	5
+#define FAN_EVENT_INFO_TYPE_FSDATA	6
+
+struct fanotify_event_info_error {
+	struct fanotify_event_info_header hdr;
+	int version;
+	int error;
+	long long unsigned fsid;
+};
+
+struct fanotify_event_info_location {
+	struct fanotify_event_info_header hdr;
+	int line;
+	char function[0];
+};
+
+struct fanotify_event_info_fsdata {
+	struct fanotify_event_info_header hdr;
+	char data[0];
+};
+
+struct ext4_error_inode_report {
+	unsigned long long inode;
+	unsigned long long block;
+	char desc[40];
+};
+#endif
+
+static void handle_notifications(char *buffer, int len)
+{
+	struct fanotify_event_metadata *metadata;
+	struct fanotify_event_info_header *hdr = 0;
+	char *off, *next;
+
+	for (metadata = (struct fanotify_event_metadata *) buffer;
+	     FAN_EVENT_OK(metadata, len); metadata = FAN_EVENT_NEXT(metadata, len)) {
+		next = (char*)metadata  + metadata->event_len;
+		if (!(metadata->mask == FAN_ERROR)) {
+			printf("unexpected FAN MARK: %llx\n", metadata->mask);
+			continue;
+		}
+		if (metadata->fd != FAN_NOFD) {
+			printf("bizar fd != FAN_NOFD\n");
+			continue;;
+		}
+
+		printf("FAN_ERROR found len=%d\n", metadata->event_len);
+
+		for (off = (char*)(metadata+1); off < next; off = off + hdr->len) {
+			hdr = (struct fanotify_event_info_header*)(off);
+
+			if (hdr->info_type == FAN_EVENT_INFO_TYPE_ERROR) {
+				struct fanotify_event_info_error *error =
+					(struct fanotify_event_info_error*) hdr;
+
+				printf("  Generic Error Record: len=%d\n", hdr->len);
+				printf("      version: %d\n", error->version);
+				printf("      error: %d\n", error->error);
+				printf("      fsid: %llx\n", error->fsid);
+
+			} else if(hdr->info_type == FAN_EVENT_INFO_TYPE_LOCATION) {
+				struct fanotify_event_info_location *loc =
+					(struct fanotify_event_info_location*) hdr;
+
+				printf("  Location Record Size = %d\n", loc->hdr.len);
+				printf("      loc=%s:%d\n", loc->function, loc->line);
+
+			} else if(hdr->info_type == FAN_EVENT_INFO_TYPE_FSDATA) {
+				struct fanotify_event_info_fsdata *data =
+					(struct fanotify_event_info_fsdata *)hdr;
+				struct ext4_error_inode_report *fsdata =
+					(struct ext4_error_inode_report*) ((char*)data->data);
+
+				printf("  Fsdata Record: len=%d\n", hdr->len);
+				printf("      inode=%llu\n", fsdata->inode);
+				if (fsdata->block != -1L)
+					printf("      block=%llu\n", fsdata->block);
+				printf("      desc=%s\n", fsdata->desc);
+			}
+		}
+	}
+}
+
+int main(int argc, char **argv)
+{
+	int fd;
+	char buffer[BUFSIZ];
+
+	if (argc < 2) {
+		printf("Missing path argument\n");
+		return 1;
+	}
+
+	fd = fanotify_init(FAN_CLASS_NOTIF|FAN_PREALLOC_QUEUE, O_RDONLY);
+	if (fd < 0)
+		errx(1, "fanotify_init");
+
+	if (fanotify_mark(fd, FAN_MARK_ADD|FAN_MARK_FILESYSTEM,
+			  FAN_ERROR, AT_FDCWD, argv[1])) {
+		errx(1, "fanotify_mark");
+	}
+
+	while (1) {
+		int n = read(fd, buffer, BUFSIZ);
+		if (n < 0)
+			errx(1, "read");
+
+		handle_notifications(buffer, n);
+	}
+
+	return 0;
+}