[PATCH 8/9] smb: client: fix domainauto remount by syncing domainname from session

[rajasimandalos@gmail.com]

From: Rajasi Mandal <rajasimandal@microsoft.com>

When domainauto is used at mount time, ctx->domainname stays NULL
while the server negotiates the actual domain into ses->domainName.
cifs_show_options() reads ses->domainName and outputs domain=X in
/proc/mounts.  On remount, libmount re-feeds domain=X, but the
verify check compares against old ctx->domainname (NULL) and rejects
the remount with "can not change domainname during remount".

Steps to reproduce:

  1. Mount with domainauto (no explicit domain= option):
       mount -t cifs //server/share /mnt \
         -o username=u,password=p,domainauto
     At this point ctx->domainname is NULL, but the server populates
     ses->domainName (e.g. "CORP") during session setup.

  2. Bare remount (or any remount):
       mount -o remount /mnt
     libmount reads /proc/mounts, sees domain=CORP (output by
     cifs_show_options() from ses->domainName), and feeds it back
     to the kernel.  The kernel parses domain=CORP into
     new_ctx->domainname="CORP", but old ctx->domainname is still
     NULL.  smb3_verify_reconfigure_ctx() sees the mismatch and
     rejects the remount.

This affects any server that populates ses->domainName during session
setup (Active Directory domain controllers, Azure Files) when the
admin uses domainauto instead of an explicit domain= option.

Fix this by syncing ctx->domainname from ses->domainName in
smb3_sync_ctx_from_negotiated() before the verify check runs.

Signed-off-by: Rajasi Mandal <rajasimandal@microsoft.com>
---
 fs/smb/client/fs_context.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 2be72733ef2e..d804e5da578e 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -1232,6 +1232,9 @@ static void smb3_sync_ctx_from_negotiated(struct cifs_sb_info *cifs_sb)
 	 */
 	ctx->ops = server->ops;
 	ctx->vals = server->vals;
+	/* /proc/mounts shows domain= from ses->domainName */
+	if (tcon->ses->domainName && !ctx->domainname)
+		ctx->domainname = kstrdup(tcon->ses->domainName, GFP_KERNEL);
 }
 
 /*
-- 
2.43.0