[PATCH v3] nfs: keep server info for remounts

[Martin Kaiser <martin@kaiser.cx>]

With newer kernels that use fs_context for nfs mounts, remounts fail with
-EINVAL.

$ mount -t nfs -o nolock 10.0.0.1:/tmp/test /mnt/test/
$ mount -t nfs -o remount /mnt/test/
mount: mounting 10.0.0.1:/tmp/test on /mnt/test failed: Invalid argument

For remounts, the nfs server address and port are populated by
nfs_init_fs_context and later overwritten with 0x00 bytes by
nfs23_parse_monolithic. The remount then fails as the server address is
invalid.

Fix this by not overwriting nfs server info in nfs23_parse_monolithic if
we're doing a remount.

Fixes: f2aedb713c28 ("NFS: Add fs_context support.")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
---
 v3:
 - rebased against linux-next from 12th April 2024

 v2:
 - rebased against linux-next from 26th February 2024

Dear all,
I'm resending this patch again. The problem that I'm trying to fix is still
present in linux-next. Thanks in advance for any reviews and comments.

I guess that we're taking this path for remounts

do_remount
    fs_context_for_reconfigure
        alloc_fs_context
            init_fs_context == nfs_init_fs_context
               fc->root is set for remounts
               ctx->nfs_server is populated
    parse_monolithic_mount_data
        nfs_fs_context_parse_monolithic
            nfs23_parse_monolithic
               ctx->nfs_server is overwritten with data from mount request

An alternative to checking for !is_remount_fc(fc) would be to check
if (ctx->nfs_server.addrlen == 0)

fs/nfs/fs_context.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
index d0a0956f8a13..cac1157be2c2 100644
--- a/fs/nfs/fs_context.c
+++ b/fs/nfs/fs_context.c
@@ -1112,9 +1112,12 @@ static int nfs23_parse_monolithic(struct fs_context *fc,
 		ctx->acdirmax	= data->acdirmax;
 		ctx->need_mount	= false;
 
-		memcpy(sap, &data->addr, sizeof(data->addr));
-		ctx->nfs_server.addrlen = sizeof(data->addr);
-		ctx->nfs_server.port = ntohs(data->addr.sin_port);
+		if (!is_remount_fc(fc)) {
+			memcpy(sap, &data->addr, sizeof(data->addr));
+			ctx->nfs_server.addrlen = sizeof(data->addr);
+			ctx->nfs_server.port = ntohs(data->addr.sin_port);
+		}
+
 		if (sap->ss_family != AF_INET ||
 		    !nfs_verify_server_address(sap))
 			goto out_no_address;
-- 
2.39.2