Re: fuse readdirplus skip one entry when interrupted by signal

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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;



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