2023-09-27 06:21:31

by Nicholas Piggin

Subject: [RFC PATCH] nfs: reduce stack usage of nfs_get_root

Move fsinfo allocation off stack, reducing stack overhead of
nfs_get_root from 304 to 192 bytes.

Signed-off-by: Nicholas Piggin <[email protected]>
---
Hi,

This is motivated by a stack overflow described here:
https://lore.kernel.org/netdev/[email protected]/

NFS is not really a major culprit but it seems not too hard to
shrink the stack a little.

Thanks,
Nick

fs/nfs/getroot.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/nfs/getroot.c b/fs/nfs/getroot.c
index 11ff2b2e060f..6e4188c09639 100644
--- a/fs/nfs/getroot.c
+++ b/fs/nfs/getroot.c
@@ -68,7 +68,7 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
{
struct nfs_fs_context *ctx = nfs_fc2context(fc);
struct nfs_server *server = NFS_SB(s), *clone_server;
- struct nfs_fsinfo fsinfo;
+ struct nfs_fsinfo *fsinfo;
struct dentry *root;
struct inode *inode;
char *name;
@@ -79,19 +79,23 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
if (!name)
goto out;

- /* get the actual root for this mount */
- fsinfo.fattr = nfs_alloc_fattr_with_label(server);
- if (fsinfo.fattr == NULL)
+ fsinfo = kmalloc(sizeof(*fsinfo), GFP_KERNEL);
+ if (!fsinfo)
goto out_name;

- error = server->nfs_client->rpc_ops->getroot(server, ctx->mntfh, &fsinfo);
+ /* get the actual root for this mount */
+ fsinfo->fattr = nfs_alloc_fattr_with_label(server);
+ if (fsinfo->fattr == NULL)
+ goto out_fsinfo;
+
+ error = server->nfs_client->rpc_ops->getroot(server, ctx->mntfh, fsinfo);
if (error < 0) {
dprintk("nfs_get_root: getattr error = %d\n", -error);
nfs_errorf(fc, "NFS: Couldn't getattr on root");
goto out_fattr;
}

- inode = nfs_fhget(s, ctx->mntfh, fsinfo.fattr);
+ inode = nfs_fhget(s, ctx->mntfh, fsinfo->fattr);
if (IS_ERR(inode)) {
dprintk("nfs_get_root: get root inode failed\n");
error = PTR_ERR(inode);
@@ -148,11 +152,13 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
!(kflags_out & SECURITY_LSM_NATIVE_LABELS))
server->caps &= ~NFS_CAP_SECURITY_LABEL;

- nfs_setsecurity(inode, fsinfo.fattr);
+ nfs_setsecurity(inode, fsinfo->fattr);
error = 0;

out_fattr:
- nfs_free_fattr(fsinfo.fattr);
+ nfs_free_fattr(fsinfo->fattr);
+out_fsinfo:
+ kfree(fsinfo);
out_name:
kfree(name);
out:
--
2.40.1

2023-09-27 13:57:43

by Benjamin Coddington

Subject: Re: [RFC PATCH] nfs: reduce stack usage of nfs_get_root

On 26 Sep 2023, at 20:16, Nicholas Piggin wrote:

> Move fsinfo allocation off stack, reducing stack overhead of
> nfs_get_root from 304 to 192 bytes.
>
> Signed-off-by: Nicholas Piggin <[email protected]>
> ---
> Hi,
>
> This is motivated by a stack overflow described here:
> https://lore.kernel.org/netdev/[email protected]/
>
> NFS is not really a major culprit but it seems not too hard to
> shrink the stack a little.
>
> Thanks,
> Nick

Looks good, might be able to get rid of the local "root" var too if the
compiler doesn't optimize it away.

Reviewed-by: Benjamin Coddington <[email protected]>

Ben