[PATCH v2] cifs: remove unnecessary copies of tcon->crfid.fid

[PATCH v2] cifs: remove unnecessary copies of tcon->crfid.fid

[Muhammad Usama Anjum]

pfid is being set to tcon->crfid.fid and they are copied in each other
multiple times. Remove the memcopy between same pointers - memory
locations.

Addresses-Coverity: ("Overlapped copy")
Fixes: 9e81e8ff74b9 ("cifs: return cached_fid from open_shroot")
Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
---
Changes in V2:
	refcount increment is necessary. Don't remove it.
	Add and improve comments.

fs/cifs/smb2ops.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index caa5432a5ed1..797a20714ca1 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -848,11 +848,9 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
 		};
 
 		/*
-		 * caller expects this func to set pfid to a valid
-		 * cached root, so we copy the existing one and get a
-		 * reference.
+		 * caller expects this func to set the fid in crfid to valid
+		 * cached root, so increment the refcount.
 		 */
-		memcpy(pfid, tcon->crfid.fid, sizeof(*pfid));
 		kref_get(&tcon->crfid.refcount);
 
 		mutex_unlock(&tcon->crfid.fid_mutex);
@@ -885,7 +883,6 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
 	oparms.fid->mid = le64_to_cpu(o_rsp->sync_hdr.MessageId);
 #endif /* CIFS_DEBUG2 */
 
-	memcpy(tcon->crfid.fid, pfid, sizeof(struct cifs_fid));
 	tcon->crfid.tcon = tcon;
 	tcon->crfid.is_valid = true;
 	tcon->crfid.dentry = dentry;
@@ -894,6 +891,10 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
 
 	/* BB TBD check to see if oplock level check can be removed below */
 	if (o_rsp->OplockLevel == SMB2_OPLOCK_LEVEL_LEASE) {
+		/*
+		 * caller expects this func to set the fid in crfid to valid
+		 * cached root, so increment the refcount.
+		 */
 		kref_get(&tcon->crfid.refcount);
 		tcon->crfid.has_lease = true;
 		smb2_parse_contexts(server, o_rsp,
-- 
2.25.1

Re: [PATCH v2] cifs: remove unnecessary copies of tcon->crfid.fid

[Aurélien Aptel]

Hi,

This is better I think.

Muhammad Usama Anjum <musamaanjum@gmail.com> writes:
> @@ -894,6 +891,10 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
>  
>  	/* BB TBD check to see if oplock level check can be removed below */
>  	if (o_rsp->OplockLevel == SMB2_OPLOCK_LEVEL_LEASE) {
> +		/*
> +		 * caller expects this func to set the fid in crfid to valid
> +		 * cached root, so increment the refcount.
> +		 */

This comment is misleading. crfid variable doesn't exist anymore, and
the kref_get() here is because of this commit:

    commit 2f94a3125b87
    Author: Ronnie Sahlberg <lsahlber@redhat.com>
    Date:   Thu Mar 28 11:20:02 2019 +1000
    
        cifs: fix kref underflow in close_shroot()

        [...]
-->     This extra get() is only used to hold the structure until we get a lease
-->     break from the server at which point we will kref_put() it during lease
-->     processing.
        [...]



When we queue a lease break, we usually get() the cifsFileInfo, but if
that cifsFileInfo is backed by a cached_fid, the cached_fid isn't
bumped. That commit was probably a work around for that.

@Ronnie :

struct cached_fid is starting to look very much like struct
cifsFileInfo. I wonder why we couldn't use it, along with
find_writable_file()/find_readable_file() to handle the caching.

Alternatively, make cifsFileInfo use cached_fid (perhaps renaming it in
the process, I don't know)

Because I suspect a lot more issues will come up regarding cached_fid
refcount and cifsFileInfo refcount going out of sync otherwise.

Cheers,
-- 
Aurélien Aptel / SUSE Labs Samba Team
GPG: 1839 CB5F 9F5B FB9B AA97  8C99 03C8 A49B 521B D5D3
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg, DE
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah HRB 247165 (AG München)

Re: [PATCH v2] cifs: remove unnecessary copies of tcon->crfid.fid

[Steve French]

I changed the comment to

+               /*
+                * See commit 2f94a3125b87. Increment the refcount when we
+                * get a lease for root, release it if lease break occurs
+                */

and added Aurelien's Reviewed-by.  Let me know if you see any
additional problems.

On Sat, Apr 17, 2021 at 5:54 AM Aurélien Aptel <aaptel@suse.com> wrote:
>
> Hi,
>
> This is better I think.
>
> Muhammad Usama Anjum <musamaanjum@gmail.com> writes:
> > @@ -894,6 +891,10 @@ int open_cached_dir(unsigned int xid, struct cifs_tcon *tcon,
> >
> >       /* BB TBD check to see if oplock level check can be removed below */
> >       if (o_rsp->OplockLevel == SMB2_OPLOCK_LEVEL_LEASE) {
> > +             /*
> > +              * caller expects this func to set the fid in crfid to valid
> > +              * cached root, so increment the refcount.
> > +              */
>
> This comment is misleading. crfid variable doesn't exist anymore, and
> the kref_get() here is because of this commit:
>
>     commit 2f94a3125b87
>     Author: Ronnie Sahlberg <lsahlber@redhat.com>
>     Date:   Thu Mar 28 11:20:02 2019 +1000
>
>         cifs: fix kref underflow in close_shroot()
>
>         [...]
> -->     This extra get() is only used to hold the structure until we get a lease
> -->     break from the server at which point we will kref_put() it during lease
> -->     processing.
>         [...]
>
>
>
> When we queue a lease break, we usually get() the cifsFileInfo, but if
> that cifsFileInfo is backed by a cached_fid, the cached_fid isn't
> bumped. That commit was probably a work around for that.
>
> @Ronnie :
>
> struct cached_fid is starting to look very much like struct
> cifsFileInfo. I wonder why we couldn't use it, along with
> find_writable_file()/find_readable_file() to handle the caching.
>
> Alternatively, make cifsFileInfo use cached_fid (perhaps renaming it in
> the process, I don't know)
>
> Because I suspect a lot more issues will come up regarding cached_fid
> refcount and cifsFileInfo refcount going out of sync otherwise.
>
> Cheers,
> --
> Aurélien Aptel / SUSE Labs Samba Team
> GPG: 1839 CB5F 9F5B FB9B AA97  8C99 03C8 A49B 521B D5D3
> SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg, DE
> GF: Felix Imendörffer, Mary Higgins, Sri Rasiah HRB 247165 (AG München)
>


-- 
Thanks,

Steve

Re: [PATCH v2] cifs: remove unnecessary copies of tcon->crfid.fid

[Muhammad Usama Anjum]

On Mon, 2021-04-19 at 18:39 -0500, Steve French wrote:
> I changed the comment to
> 
> +               /*
> +                * See commit 2f94a3125b87. Increment the refcount when we
> +                * get a lease for root, release it if lease break occurs
> +                */
> 
> and added Aurelien's Reviewed-by.  Let me know if you see any
> additional problems.
> 
Thank you so much!