Re: [PATCH] libfs: fix infinite directory reads for offset dir

[yangerkun <yangerkun@xxxxxxxxxxxxxxx>]

Hi!

在 2024/7/31 21:10, Filipe Manana 写道:
> On Wed, Jul 31, 2024 at 1:51 PM yangerkun <yangerkun@xxxxxxxxxxxxxxx> wrote:
> > Hi!
> >
> > 在 2024/7/31 19:51, Jan Kara 写道:
> > > On Wed 31-07-24 12:38:35, yangerkun wrote:
> > > > After we switch tmpfs dir operations from simple_dir_operations to
> > > > simple_offset_dir_operations, every rename happened will fill new dentry
> > > > to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
> > > > key starting with octx->newx_offset, and then set newx_offset equals to
> > > > free key + 1. This will lead to infinite readdir combine with rename
> > > > happened at the same time, which fail generic/736 in xfstests(detail show
> > > > as below).
> > > >
> > > > 1. create 5000 files(1 2 3...) under one dir
> > > > 2. call readdir(man 3 readdir) once, and get one entry
> > > > 3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
> > > > 4. loop 2~3, until readdir return nothing or we loop too many
> > > >      times(tmpfs break test with the second condition)
> > > >
> > > > We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
> > > > directory reads") to fix it, record the last_index when we open dir, and
> > > > do not emit the entry which index >= last_index. The file->private_data
> > > > now used in offset dir can use directly to do this, and we also update
> > > > the last_index when we llseek the dir file.
> > >
> > > The patch looks good! Just I'm not sure about the llseek part. As far as I
> > > understand it was added due to this sentence in the standard:
> > >
> > > "If a file is removed from or added to the directory after the most recent
> > > call to opendir() or rewinddir(), whether a subsequent call to readdir()
> > > returns an entry for that file is unspecified."
> > >
> > > So if the offset used in offset_dir_llseek() is 0, then we should update
> > > last_index. But otherwise I'd leave it alone because IMHO it would do more
> > > harm than good.
> >
> > IIUC, what you means is that we should only reset the private_data to
> > new last_index when we call rewinddir(which will call lseek to set
> > offset of dir file to 0)?
> >
> > Yeah, I prefer the logic you describle! Besides, we may also change
> > btrfs that do the same(e60aa5da14d0 ("btrfs: refresh dir last index
> > during a rewinddir(3) call")). Filipe, how do you think?
>
> What problem does it solve?
> The standard doesn't forbid it, and I can't see anything wrong with it.


Yeah, I didn't find any obvious bugs for this behavior. It's a choose
does this seems better, and I think it's also ok to keep btrfs not
change now. Thanks for your reply!

> > Thanks,
> > Erkun.
> >
> > >                                                                Honza
> > >
> > > > Fixes: a2e459555c5f ("shmem: stable directory offsets")
> > > > Signed-off-by: yangerkun <yangerkun@xxxxxxxxxx>
> > > > ---
> > > >    fs/libfs.c | 34 +++++++++++++++++++++++-----------
> > > >    1 file changed, 23 insertions(+), 11 deletions(-)
> > > >
> > > > diff --git a/fs/libfs.c b/fs/libfs.c
> > > > index 8aa34870449f..38b306738c00 100644
> > > > --- a/fs/libfs.c
> > > > +++ b/fs/libfs.c
> > > > @@ -450,6 +450,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
> > > >       mtree_destroy(&octx->mt);
> > > >    }
> > > >
> > > > +static int offset_dir_open(struct inode *inode, struct file *file)
> > > > +{
> > > > +    struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > > +
> > > > +    file->private_data = (void *)ctx->next_offset;
> > > > +    return 0;
> > > > +}
> > > > +
> > > >    /**
> > > >     * offset_dir_llseek - Advance the read position of a directory descriptor
> > > >     * @file: an open directory whose position is to be updated
> > > > @@ -463,6 +471,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
> > > >     */
> > > >    static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > > >    {
> > > > +    struct inode *inode = file->f_inode;
> > > > +    struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > > +
> > > >       switch (whence) {
> > > >       case SEEK_CUR:
> > > >               offset += file->f_pos;
> > > > @@ -476,7 +487,7 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > > >       }
> > > >
> > > >       /* In this case, ->private_data is protected by f_pos_lock */
> > > > -    file->private_data = NULL;
> > > > +    file->private_data = (void *)ctx->next_offset;
> > > >       return vfs_setpos(file, offset, LONG_MAX);
> > > >    }
> > > >
> > > > @@ -507,7 +518,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
> > > >                         inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> > > >    }
> > > >
> > > > -static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> > > > +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> > > >    {
> > > >       struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> > > >       struct dentry *dentry;
> > > > @@ -515,17 +526,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> > > >       while (true) {
> > > >               dentry = offset_find_next(octx, ctx->pos);
> > > >               if (!dentry)
> > > > -                    return ERR_PTR(-ENOENT);
> > > > +                    return;
> > > > +
> > > > +            if (dentry2offset(dentry) >= last_index) {
> > > > +                    dput(dentry);
> > > > +                    return;
> > > > +            }
> > > >
> > > >               if (!offset_dir_emit(ctx, dentry)) {
> > > >                       dput(dentry);
> > > > -                    break;
> > > > +                    return;
> > > >               }
> > > >
> > > >               ctx->pos = dentry2offset(dentry) + 1;
> > > >               dput(dentry);
> > > >       }
> > > > -    return NULL;
> > > >    }
> > > >
> > > >    /**
> > > > @@ -552,22 +567,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> > > >    static int offset_readdir(struct file *file, struct dir_context *ctx)
> > > >    {
> > > >       struct dentry *dir = file->f_path.dentry;
> > > > +    long last_index = (long)file->private_data;
> > > >
> > > >       lockdep_assert_held(&d_inode(dir)->i_rwsem);
> > > >
> > > >       if (!dir_emit_dots(file, ctx))
> > > >               return 0;
> > > >
> > > > -    /* In this case, ->private_data is protected by f_pos_lock */
> > > > -    if (ctx->pos == DIR_OFFSET_MIN)
> > > > -            file->private_data = NULL;
> > > > -    else if (file->private_data == ERR_PTR(-ENOENT))
> > > > -            return 0;
> > > > -    file->private_data = offset_iterate_dir(d_inode(dir), ctx);
> > > > +    offset_iterate_dir(d_inode(dir), ctx, last_index);
> > > >       return 0;
> > > >    }
> > > >
> > > >    const struct file_operations simple_offset_dir_operations = {
> > > > +    .open           = offset_dir_open,
> > > >       .llseek         = offset_dir_llseek,
> > > >       .iterate_shared = offset_readdir,
> > > >       .read           = generic_read_dir,
> > > > --
> > > > 2.39.2