Re: fuse readdirplus skip one entry when interrupted by signal

[Marios Titas]

On Wed, Oct 25, 2017 at 11:38:09AM +0200, Miklos Szeredi wrote:
> On Tue, Oct 24, 2017 at 08:10:49PM +0200, Jakob Unterwurzacher wrote:
> > A user running a Haskell program [1] noticed a problem with fuse's
> > readdirplus: when it is interrupted by a signal, it skips one
> > directory entry.
> >
> > The problem is most apparent with Haskell as it uses
> > SIGVTALRM to interrupt it's own green threads.
> >
> > A minimal reproducer in C, "ls-count.c", is available [2]. The problem
> > has been reproduced against libfuse's "passthrough_fh.c", but also against
> > gocryptfs, which uses go-fuse instead of libfuse. This suggest
> > that the bug is in kernel-space, which also the opinion of libfuse
> > upstream [3].
> >
> > What "ls-count.c" does is that it loops over readdir while sending itself
> > SIGVTALRM. When the count of directory entries changes, it exits:
> >
> > 	$ ./ls-count b
> > 	ls-count: counts do not match: 2 vs 1
> >
> > strace against ls-count shows that we get one entry, when we should get
> > two ("." and ".."):
> >
> > 	getdents(3, /* 1 entries */, 32768)     = 24
> > 	--- SIGVTALRM ---
> > 	rt_sigreturn({mask=[]})                 = 24
> > 	getdents(3, /* 0 entries */, 32768)     = 0
> >
> > The debug output from go-fuse [4] shows what seems to be happening:
> >
> > 	Dispatch 548: READDIRPLUS, NodeId: 1. data: {Fh 3 off 0 sz 4096}
> > 	Serialize 548: READDIRPLUS code: OK value:  320 bytes data
> > 	Dispatch 549: READDIRPLUS, NodeId: 1. data: {Fh 3 off 2 sz 4096}
> > 	Serialize 549: READDIRPLUS code: OK value:
> >
> > The kernel starts reading the directory from "off 0", where it is
> > interrupted, and only returns one entry to userspace. Then it continues
> > reading at "off 2". Offset 1 is skipped.
> >
> > I can reliably reproduce this within 1 second against kernel 4.12.5.
>
> Thanks for the report.  The patch below should fix it.

Hi Miklos,

Original reporter here ("Haskell user"). I tested your patch and 
everything works as expected now. Thanks for the prompt response!

- Marios


> Thanks,
> Miklos
> ---
>
> From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Subject: fuse: fix READDIRPLUS skipping an entry
>
> A user running a Haskell program noticed a problem with fuse's readdirplus:
> when it is interrupted by a signal, it skips one directory entry.
>
> The reason is that fuse erronously updates ctx->pos after a failed
> dir_emit().
>
> The issue originates from the patch adding readdirplus support.
>
> Reported-by: Jakob Unterwurzacher <jakobunt@xxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Fixes: 0b05b18381ee ("fuse: implement NFS-like readdirplus support")
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.9
> ---
> fs/fuse/dir.c |    3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -1308,7 +1308,8 @@ static int parse_dirplusfile(char *buf,
> 			*/
> 			over = !dir_emit(ctx, dirent->name, dirent->namelen,
> 				       dirent->ino, dirent->type);
> -			ctx->pos = dirent->off;
> +			if (!over)
> +				ctx->pos = dirent->off;
> 		}
>
> 		buf += reclen;