Re: [PATCH v7 23/28] fanotify: Report fid info for file related file system errors

On Fri 15-10-21 10:56:38, Amir Goldstein wrote:

On Fri, Oct 15, 2021 at 12:39 AM Gabriel Krisman Bertazi
[off-list ref] wrote:

quoted

Plumb the pieces to add a FID report to error records.  Since all error
event memory must be pre-allocated, we pre-allocate the maximum file
handle size possible, such that it should always fit.

For errors that don't expose a file handle report it with an invalid
FID.

Signed-off-by: Gabriel Krisman Bertazi <redacted>

---
Changes since v6:
  - pass fsid from handle_events
Changes since v5:
  - Use preallocated MAX_HANDLE_SZ FH buffer
  - Report superblock errors with a zerolength INVALID FID (jan, amir)
---
 fs/notify/fanotify/fanotify.c | 15 +++++++++++++++
 fs/notify/fanotify/fanotify.h |  8 ++++++++
 2 files changed, 23 insertions(+)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 7032083df62a..8a60c96f5fb2 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c

@@ -611,7 +611,9 @@ static struct fanotify_event *fanotify_alloc_error_event(
 {
        struct fs_error_report *report =
                        fsnotify_data_error_report(data, data_type);
+       struct inode *inode = report->inode;
        struct fanotify_error_event *fee;
+       int fh_len;

        if (WARN_ON(!report))
                return NULL;

@@ -622,6 +624,19 @@ static struct fanotify_event *fanotify_alloc_error_event(

        fee->fae.type = FANOTIFY_EVENT_TYPE_FS_ERROR;
        fee->err_count = 1;
+       fee->fsid = *fsid;
+
+       fh_len = fanotify_encode_fh_len(inode);
+       if (WARN_ON(fh_len > MAX_HANDLE_SZ)) {

WARN_ON_ONCE please and I rather that this sanity check is moved inside
fanotify_encode_fh_len() where it will return 0 for encoding failure.

Yeah, that's better.

quoted

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index 2b032b79d5b0..b58400926f92 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h

@@ -202,6 +202,10 @@ struct fanotify_error_event {
        u32 err_count; /* Suppressed errors count */

        __kernel_fsid_t fsid; /* FSID this error refers to. */
+       /* object_fh must be followed by the inline handle buffer. */
+       struct fanotify_fh object_fh;
+       /* Reserve space in object_fh.buf[] - access with fanotify_fh_buf() */
+       unsigned char _inline_fh_buf[MAX_HANDLE_SZ];
 };

This struct duplicates most of struct fanotify_fid_event.
How about:

#define FANOTIFY_ERROR_FH_LEN \
             (MAX_HANDLE_SZ - FANOTIFY_INLINE_FH_LEN)

struct fanotify_error_event {
         u32 err_count; /* Suppressed errors count */
         struct fanotify_event ffe;
         /* Reserve space in ffe.object_fh.buf[] - access with
fanotify_fh_buf() */
         unsigned char _fh_buf[FANOTIFY_ERROR_FH_LEN];
}

Or leaving out the struct padding and passing
FANOTIFY_ERROR_EVENT_SIZE as mempool object size?

#define FANOTIFY_ERROR_EVENT_SIZE \
            (sizeof(struct fanotify_error_event) + FANOTIFY_ERROR_FH_LEN)

Hrm, I don't like either of these two options too much as IMHO it's rather
hard to understand what's going on. If we want to avoid the duplication,
then I see two relatively clean ways to do it:

a) Remove _inline_fh_buf from fanotify_fid_event (probably just leave a
comment there explaining how space is preallocated) and make sure kmem
cache for fanotify_fid_event has FANOTIFY_INLINE_FH_LEN in each object for
the fh, similarly error event would have MAX_HANDLE_SZ in the object.

b) Define a macro that expands to a struct definition with appropriate
buffer length.

I guess a) seems a bit more obvious to me but I can live with both...

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR