cifs: fix sb->s_maxbytes so that it casts properly to a signed value

This off-by-one bug causes sendfile() to not work properly. When a task
calls sendfile() on a file on a CIFS filesystem, the syscall returns -1
and sets errno to EOVERFLOW.

do_sendfile uses s_maxbytes to verify the returned offset of the file.
The problem there is that this value is cast to a signed value (loff_t).
When this is done on the s_maxbytes value that cifs uses, it becomes
negative and the comparisons against it fail.

Even though s_maxbytes is an unsigned value, it seems that it's not OK
to set it in such a way that it'll end up negative when it's cast to a
signed value. These casts happen in other codepaths besides sendfile
too, but the VFS is a little hard to follow in this area and I can't
be sure if there are other bugs that this will fix.

It's not clear to me why s_maxbytes isn't just declared as loff_t in the
first place, but either way we still need to fix these values to make
sendfile work properly. This is also an opportunity to replace the magic
bit-shift values here with the standard #defines for this.

This fixes the reproducer program I have that does a sendfile and
will probably also fix the situation where apache is serving from a
CIFS share.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/cifs/connect.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

Fix seems logical, although would like to see the maxbytes field the
correct size.  If it really is a loff_t rather than unsigned why
wasn't sparse warning on the vfs in sendfile when it did this
incorrect cast?

When did this start breaking, am a little surprised that connectathon
(and the usual dbench, fsstress, fsx etc.) didn't break if sendfile
was broken, and I don't think that cifs has changed in this area in a
long time.

Shouldn't this cc stable ... sendfile is important.

On Tue, Jul 21, 2009 at 6:42 PM, Jeff Layton<jlayton@redhat.com> wrote:
> This off-by-one bug causes sendfile() to not work properly. When a task
> calls sendfile() on a file on a CIFS filesystem, the syscall returns -1
> and sets errno to EOVERFLOW.
>
> do_sendfile uses s_maxbytes to verify the returned offset of the file.
> The problem there is that this value is cast to a signed value (loff_t).
> When this is done on the s_maxbytes value that cifs uses, it becomes
> negative and the comparisons against it fail.
>
> Even though s_maxbytes is an unsigned value, it seems that it's not OK
> to set it in such a way that it'll end up negative when it's cast to a
> signed value. These casts happen in other codepaths besides sendfile
> too, but the VFS is a little hard to follow in this area and I can't
> be sure if there are other bugs that this will fix.
>
> It's not clear to me why s_maxbytes isn't just declared as loff_t in the
> first place, but either way we still need to fix these values to make
> sendfile work properly. This is also an opportunity to replace the magic
> bit-shift values here with the standard #defines for this.
>
> This fixes the reproducer program I have that does a sendfile and
> will probably also fix the situation where apache is serving from a
> CIFS share.
>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  fs/cifs/connect.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
> index 3e9936d..82ad2a8 100644
> --- a/fs/cifs/connect.c
> +++ b/fs/cifs/connect.c
> @@ -2446,10 +2446,10 @@ try_mount_again:
>                tcon->local_lease = volume_info->local_lease;
>        }
>        if (pSesInfo) {
> -               if (pSesInfo->capabilities & CAP_LARGE_FILES) {
> -                       sb->s_maxbytes = (u64) 1 << 63;
> -               } else
> -                       sb->s_maxbytes = (u64) 1 << 31; /* 2 GB */
> +               if (pSesInfo->capabilities & CAP_LARGE_FILES)
> +                       sb->s_maxbytes = MAX_LFS_FILESIZE;
> +               else
> +                       sb->s_maxbytes = MAX_NON_LFS;
>        }
>
>        /* BB FIXME fix time_gran to be larger for LANMAN sessions */
> --
> 1.6.0.6
>
>

On Tue, 2009-07-21 at 19:45 -0500, Steve French wrote:
> Fix seems logical, although would like to see the maxbytes field the
> correct size.  If it really is a loff_t rather than unsigned why
> wasn't sparse warning on the vfs in sendfile when it did this
> incorrect cast?
> 

*shrug* -- maybe sparse does throw a warning, I haven't checked. It's
also not necessarily an incorrect cast I guess -- depends on whether
it's just set too large.

I think we should consider changing s_maxbytes to loff_t, but I need to
have a closer look and make sure it wouldn't break anything. There are
also other fs's that probably need similar fixes.

> When did this start breaking, am a little surprised that connectathon
> (and the usual dbench, fsstress, fsx etc.) didn't break if sendfile
> was broken, and I don't think that cifs has changed in this area in a
> long time.
> 

This has been broken for a long, long time (at least a couple of years).
Most of the reports that I have are people complaining that web serving
using apache from CIFS shares doesn't work right. I think apache uses
multiple sendfile calls per file, and bails out when it gets an error on
the first call.

> Shouldn't this cc stable ... sendfile is important.
> 

No objection to -stable if everyone thinks it's important enough.