Re: fuse readdirplus skip one entry when interrupted by signal

[Miklos Szeredi]

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.

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;