Re: [PATCH v5] eventfs: Remove eventfs_file and just use eventfs_inode

[Heiko Carstens <hca@linux.ibm.com>]

On Fri, Nov 17, 2023 at 03:38:29PM +0100, Heiko Carstens wrote:
> On Fri, Nov 17, 2023 at 03:23:35PM +0100, Heiko Carstens wrote:
> > I think this patch causes from time to time crashes when running ftrace
> > selftests. In particular I guess there is a bug wrt error handling in this
> > function (see below for call trace):
> > 
> > > +static struct dentry *
> > > +create_file_dentry(struct eventfs_inode *ei, struct dentry **e_dentry,
> > > +		   struct dentry *parent, const char *name, umode_t mode, void *data,
> > > +		   const struct file_operations *fops, bool lookup)
> > > +{
> ...
> > Note that the compare and swap instruction within d_invalidate() generates
> > a specification exception because it operates on an invalid address
> > (0xffffffffffffffef), which happens to be -EEXIST. So my assumption is that
> > create_dir_dentry() has incorrect error handling and passes -EEXIST instead
> > of a valid dentry pointer to d_invalidate().
> > 
> > But I leave it up to you to figure this out :)
> 
> Ok, wrong function quoted of course. But the rest of my statement
> should be correct.

So, if it helps (this still happens with Linus' master branch):

create_dir_dentry() is called with a "struct eventfs_inode *ei" (second
parameter), which points to a data structure where "is_freed" is 1. Then it
looks like create_dir() returned "-EEXIST". And looking at the code this
combination then must lead to d_invalidate() incorrectly being called with
"-EEXIST" as dentry pointer.

Now, I have no idea how the code should work, but it is quite obvious that
something is broken :)

Here the dump of the struct eventfs_inode that was passed to
create_file_dentry() when the crash happened:

crash> struct eventfs_inode 00000000eada7680
struct eventfs_inode {
  list = {
    next = 0x10f802da0,
    prev = 0x122
  },
  entries = 0x12c031328 <event_entries>,
  name = 0x12b90bbac <__tpstrtab_xfs_alloc_vextent_exact_bno> "xfs_alloc_vextent_exact_bno",
  children = {
    next = 0xeada76a0,
    prev = 0xeada76a0
  },
  dentry = 0x0,
  d_parent = 0x107c75d40,
  d_children = 0xeada5700,
  entry_attrs = 0x0,
  attr = {
    mode = 0,
    uid = {
      val = 0
    },
    gid = {
      val = 0
    }
  },
  data = 0xeada6660,
  {
    llist = {
      next = 0xeada7668
    },
    rcu = {
      next = 0xeada7668,
      func = 0x12ad2a5b8 <free_rcu_ei>
    }
  },
  is_freed = 1,
  nr_entries = 6
}