Re: fuse readdirplus skip one entry when interrupted by signal

[Marios Titas <redneb@gmx.com>]

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@redhat.com>
>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@gmail.com>
>Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
>Fixes: 0b05b18381ee ("fuse: implement NFS-like readdirplus support")
>Cc: <stable@vger.kernel.org> # 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;