From a597bc04de8990dc66e9641c782d6a9fc80a5abe Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mssola@mssola.com>
Date: Tue, 24 Feb 2026 00:17:26 +0100
Subject: btrfs: add examples on the shutdown experimental feature
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Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
---
 fs/btrfs/shutdown/fserror.c | 77 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 fs/btrfs/shutdown/fserror.c

(limited to 'fs/btrfs/shutdown/fserror.c')

diff --git a/fs/btrfs/shutdown/fserror.c b/fs/btrfs/shutdown/fserror.c
new file mode 100644
index 0000000..f06521c
--- /dev/null
+++ b/fs/btrfs/shutdown/fserror.c
@@ -0,0 +1,77 @@
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/fanotify.h>
+
+int main(int argc, char *argv[])
+{
+    int fd;
+    char buf[4096];
+    ssize_t len;
+    struct fanotify_event_metadata *metadata;
+    struct fanotify_event_info_error *error_info;
+
+    if (argc < 2) {
+        fprintf(stderr, "Usage: %s <mount point>\n", argv[0]);
+        exit(EXIT_FAILURE);
+    }
+
+    /*
+	 * Initialize 'fanotify' with the default value and `FAN_REPORT_FID`, which
+	 * will allow use to filter by `FAN_FS_ERROR` down the line.
+	 */
+    fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, O_RDONLY);
+    if (fd < 0) {
+        perror("fanotify_init");
+        exit(EXIT_FAILURE);
+    }
+
+	/*
+	 * Add a 'fanotify' mark to monitor all files from the filesystem pointed by
+	 * 'fd'. Note that this general monitoring via 'FAN_MARK_FILESYSTEM' is the
+	 * thing that makes this program to need root permission as documented in
+	 * the manpage: "Use of this flag requires the CAP_SYS_ADMIN capability".
+	 */
+    if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_FILESYSTEM,
+                      FAN_FS_ERROR, AT_FDCWD, argv[1]) < 0) {
+        perror("fanotify_mark");
+        exit(EXIT_FAILURE);
+    }
+
+    printf("Listening for filesystem errors on '%s'\n", argv[1]);
+
+    // Event Loop
+    while ((len = read(fd, buf, sizeof(buf))) > 0) {
+        metadata = (struct fanotify_event_metadata *)buf;
+
+        while (FAN_EVENT_OK(metadata, len)) {
+			/*
+			 * We only care about filesystem errors, which is the only thing we
+			 * passed to 'fanotify_mark'. That being said, just to be sure,
+			 * let's filter this out.
+			 */
+            if (metadata->mask & FAN_FS_ERROR) {
+                // fancy...
+                error_info = (struct fanotify_event_info_error *)
+                             ((char *)metadata + metadata->event_len - sizeof(*error_info));
+
+				if (error_info->error == ESHUTDOWN) {
+					printf("Shutdown detected!\n");
+					goto done;
+				} else {
+					printf("Filesystem error detected with code: %d\n", error_info->error);
+				}
+            }
+
+            metadata = FAN_EVENT_NEXT(metadata, len);
+        }
+    }
+
+ done:
+    close(fd);
+    return 0;
+}
-- 
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