[PATCH AUTOSEL 6.15 04/21] NFSv4: Always set NLINK even if the server doesn't support it

[Sasha Levin <sashal@kernel.org>]

From: Han Young <hanyang.tony@bytedance.com>

[ Upstream commit 3a3065352f73381d3a1aa0ccab44aec3a5a9b365 ]

fattr4_numlinks is a recommended attribute, so the client should emulate
it even if the server doesn't support it. In decode_attr_nlink function
in nfs4xdr.c, nlink is initialized to 1. However, this default value
isn't set to the inode due to the check in nfs_fhget.

So if the server doesn't support numlinks, inode's nlink will be zero,
the mount will fail with error "Stale file handle". Set the nlink to 1
if the server doesn't support it.

Signed-off-by: Han Young <hanyang.tony@bytedance.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

Based on my analysis of the commit and the kernel source code, here is
my determination: **YES** This commit should be backported to stable
kernel trees. Here's my extensive analysis: ## Critical Bug Fix Analysis
### 1. **Root Cause Understanding** The commit addresses a critical
issue where NFSv4 mounts fail with "Stale file handle" errors when the
server doesn't support the `fattr4_numlinks` (NLINK) attribute. Looking
at the code: - In `/home/sasha/linux/fs/nfs/nfs4xdr.c:3969`, the
`decode_attr_nlink` function initializes `*nlink = 1` as a default -
However, in `/home/sasha/linux/fs/nfs/inode.c:556-559`, the current
logic only sets the inode's nlink if `fattr->valid &
NFS_ATTR_FATTR_NLINK` is true - When the server doesn't support
numlinks, the `fattr->valid` flag isn't set, so the inode's nlink
remains 0 (from initial inode allocation) ### 2. **Impact of Zero
nlink** From `/home/sasha/linux/fs/nfs/dir.c:1578-1582`, I can see the
critical check: ```c if (inode->i_nlink > 0 || (inode->i_nlink == 0 &&
test_bit(NFS_INO_PRESERVE_UNLINKED, &NFS_I(inode)->flags))) return 0;
else return -ESTALE; ``` When `i_nlink == 0` and the
`NFS_INO_PRESERVE_UNLINKED` flag isn't set, NFS returns `-ESTALE`,
causing "Stale file handle" errors and mount failures. ### 3. **Backport
Suitability Criteria** **✅ Fixes important user-affecting bug**: Mount
failures prevent basic filesystem functionality **✅ Small and contained
fix**: The change adds only 2 lines: ```c else if (fattr_supported &
NFS_ATTR_FATTR_NLINK) nfs_set_cache_invalid(inode,
NFS_INO_INVALID_NLINK); +else + set_nlink(inode, 1); ``` **✅ Minimal
regression risk**: Setting nlink=1 as fallback is safe and follows the
NFSv4 spec (fattr4_numlinks is a "recommended" attribute, so emulation
is appropriate) **✅ No architectural changes**: This is a targeted fix
within existing error handling logic **✅ Follows stable tree rules**:
It's an important bugfix with minimal risk ### 4. **Comparison with
Similar Commits** Unlike the provided similar commits that were marked
"NO" (which involved more complex nlink management, cache invalidation
changes, or feature additions), this commit: - Fixes a fundamental
compatibility issue - Prevents complete mount failures - Is minimal and
defensive - Addresses server interoperability problems ### 5. **Server
Compatibility** This fix ensures NFSv4 clients remain functional with
servers that don't fully implement all recommended attributes, which is
critical for real-world deployments where server implementations vary.
The commit directly prevents mount failures that would otherwise require
server-side fixes or workarounds, making it essential for stable kernel
trees where users expect reliable NFS functionality.

 fs/nfs/inode.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 119e447758b99..4695292378bbe 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -557,6 +557,8 @@ nfs_fhget(struct super_block *sb, struct nfs_fh *fh, struct nfs_fattr *fattr)
 			set_nlink(inode, fattr->nlink);
 		else if (fattr_supported & NFS_ATTR_FATTR_NLINK)
 			nfs_set_cache_invalid(inode, NFS_INO_INVALID_NLINK);
+		else
+			set_nlink(inode, 1);
 		if (fattr->valid & NFS_ATTR_FATTR_OWNER)
 			inode->i_uid = fattr->uid;
 		else if (fattr_supported & NFS_ATTR_FATTR_OWNER)
-- 
2.39.5