Re: [PATCH v4 3/3] fs: let filldir_t return bool instead of an error code

[Darrick J. Wong]

On Wed, Jan 23, 2019 at 04:07:59PM +0100, Jann Horn wrote:
> On Mon, Jan 21, 2019 at 11:24 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > On Mon, Jan 21, 2019 at 04:49:45PM +0100, Jann Horn wrote:
> > > On Sun, Jan 20, 2019 at 11:41 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > > > On Fri, Jan 18, 2019 at 05:14:40PM +0100, Jann Horn wrote:
> > > > > As Al Viro pointed out, many filldir_t functions return error codes, but
> > > > > all callers of filldir_t functions just check whether the return value is
> > > > > non-zero (to determine whether to continue reading the directory); more
> > > > > precise errors have to be signalled via struct dir_context.
> > > > > Change all filldir_t functions to return bool instead of int.
> > > > >
> > > > > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > > > > ---
> > > > >  arch/alpha/kernel/osf_sys.c | 12 +++----
> > > > >  fs/afs/dir.c                | 30 +++++++++--------
> > > > >  fs/ecryptfs/file.c          | 13 ++++----
> > > > >  fs/exportfs/expfs.c         |  8 ++---
> > > > >  fs/fat/dir.c                |  8 ++---
> > > > >  fs/gfs2/export.c            |  6 ++--
> > > > >  fs/nfsd/nfs4recover.c       |  8 ++---
> > > > >  fs/nfsd/vfs.c               |  6 ++--
> > > > >  fs/ocfs2/dir.c              | 10 +++---
> > > > >  fs/ocfs2/journal.c          | 14 ++++----
> > > > >  fs/overlayfs/readdir.c      | 24 +++++++-------
> > > > >  fs/readdir.c                | 64 ++++++++++++++++++-------------------
> > > > >  fs/reiserfs/xattr.c         | 20 ++++++------
> > > > >  fs/xfs/scrub/dir.c          |  8 ++---
> > > > >  fs/xfs/scrub/parent.c       |  4 +--
> > > > >  include/linux/fs.h          | 10 +++---
> > > > >  16 files changed, 125 insertions(+), 120 deletions(-)
> > > > >
> > > > > diff --git a/arch/alpha/kernel/osf_sys.c b/arch/alpha/kernel/osf_sys.c
> > > > > index db1c2144d477..14e5ae0dac50 100644
> > > > > --- a/arch/alpha/kernel/osf_sys.c
> > > > > +++ b/arch/alpha/kernel/osf_sys.c
> > > > > @@ -108,7 +108,7 @@ struct osf_dirent_callback {
> > > > >       int error;
> > > > >  };
> > > > >
> > > > > -static int
> > > > > +static bool
> > > > >  osf_filldir(struct dir_context *ctx, const char *name, int namlen,
> > > > >           loff_t offset, u64 ino, unsigned int d_type)
> > > > >  {
> > > > > @@ -120,14 +120,14 @@ osf_filldir(struct dir_context *ctx, const char *name, int namlen,
> > > > >
> > > > >       buf->error = check_dirent_name(name, namlen);
> > > > >       if (unlikely(buf->error))
> > > > > -             return -EFSCORRUPTED;
> > > > > +             return false;
> > > > >       buf->error = -EINVAL;   /* only used if we fail */
> > > > >       if (reclen > buf->count)
> > > > > -             return -EINVAL;
> > > > > +             return false;
> > > >
> > > > Oh, it's because the error being returned is being squashed by
> > > > dir_emit():
> > >
> > > Yeah.
> > >
> > > > >  struct dir_context {
> > > > > @@ -3469,17 +3471,17 @@ static inline bool dir_emit(struct dir_context *ctx,
> > > > >                           const char *name, int namelen,
> > > > >                           u64 ino, unsigned type)
> > > > >  {
> > > > > -     return ctx->actor(ctx, name, namelen, ctx->pos, ino, type) == 0;
> > > > > +     return ctx->actor(ctx, name, namelen, ctx->pos, ino, type);
> > > > >  }
> > > >
> > > > /me wonders if it would be cleaner to do:
> > > >
> > > > static inline bool dir_emit(...)
> > > > {
> > > >         buf->error = ctx->actor(....)
> > > >         if (buf->error)
> > > >                 return false;
> > > >         return true;
> > > > }
> > > >
> > > > And clean up all filldir actors just to return the error state
> > > > rather than have to jump through hoops to stash the error state in
> > > > the context buffer and return the error state?
> > >
> > > One negative thing about that, IMO, is that it mixes up the request
> > > for termination of the loop and the presence of an error.
> >
> > Doesn't the code already do that, only worse?
> 
> The current code does that, yes. But with this patch, I think that's
> not really the case anymore?
> 
> > > > That then allows callers who want/need the full error info can
> > > > continue to call ctx->actor directly,
> > >
> > > "continue to call ctx->actor directly"? I don't remember any code that
> > > calls ctx->actor directly.
> >
> > ovl_fill_real().
> 
> Ah, right.
> 
> 
> > And the XFS directory scrubber could probably make better use of the
> > error return from ctx->actor when validating the directory contents
> > rather than just calling dir_emit() and aborting the scan at the
> > first error encountered. We eventually want to know exactly what
> > error was encountered here to determine if it is safe to continue,
> > not just a "stop processing" flag. e.g. a bad name length will need
> > to stop traversal because we can't trust the underlying structure,
> > but an invalid file type isn't a structural flaw that prevents us
> > from continuing to traverse and check the rest of the directory....
> 
> Sorry, maybe I'm a bit dense right now, I don't get your point. Are
> you talking about filesystem errors detected in the actor? If so,
> doesn't it make *more* sense for non-fatal errors to put a note that
> an error happened into the xchk_dir_ctx (if that information should be
> kept around), then return a value that says "please continue"?

As I understand the scrub code, we /do/ stash the error state elsewhere
and set the xchk_dir_actor return value as appropriate to continue or
stop the directory iteration.  Granted, it's not very nuanced since
anything out of order sets the CORRUPT flag and aborts the iteration,
but in principle xchk_dir_actor could set the scrub warning flag and
return 0 if it wanted to.

(So maybe I'm dense too, but I don't know what Dave is talking
about...?)

--D

> Or are you talking about filesystem errors detected in the readdir
> implementation? In that case, you're AFAICS going to need special-case
> logic gated on ctx->actor==xchk_dir_actor anyway if you want the
> scrubber to continue while readdir() stops.
> 
> (But as I've said, I don't really care about this patch. If Al takes
> patches 1 and 2 from this series, I'm happy; this patch is just in
> response to <https://lore.kernel.org/lkml/20180731165112.GJ30522@xxxxxxxxxxxxxxxxxx/>.)