Re: [PATCH v1] mount: handle OOM on mnt_warn_timestamp_expiry

[Christian Brauner <brauner@kernel.org>]

On Tue, Jul 30, 2024 at 10:58:13AM GMT, Olaf Hering wrote:
> If no page could be allocated, an error pointer was used as format
> string in pr_warn.
> 
> Rearrange the code to return early in case of OOM. Also add a check
> for the return value of d_path. The API of that function is not
> documented. It currently returns only ERR_PTR values, but may return
> also NULL in the future. Use PTR_ERR_OR_ZERO to cover both cases.
> 
> Fixes: f8b92ba67c5d ("mount: Add mount warning for impending timestamp expiry")
> 
> Signed-off-by: Olaf Hering <olaf@aepfle.de>
> ---
>  fs/namespace.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 328087a4df8a..539d4f203a20 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2922,7 +2922,14 @@ static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *
>  	   (!(sb->s_iflags & SB_I_TS_EXPIRY_WARNED)) &&
>  	   (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
>  		char *buf = (char *)__get_free_page(GFP_KERNEL);
> -		char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
> +		char *mntpath;
> +		
> +		if (!buf)
> +			return;
> +
> +		mntpath = d_path(mountpoint, buf, PAGE_SIZE);
> +		if (PTR_ERR_OR_ZERO(mntpath))

This needs to be IS_ERR_OR_NULL().

> +			goto err;

We should still warn when decoding the mountpoint fails. I'll just amend
your patch to something like:

diff --git a/fs/namespace.c b/fs/namespace.c
index 328087a4df8a..0f2f140aaf05 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2921,16 +2921,21 @@ static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *
        if (!__mnt_is_readonly(mnt) &&
           (!(sb->s_iflags & SB_I_TS_EXPIRY_WARNED)) &&
           (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
-               char *buf = (char *)__get_free_page(GFP_KERNEL);
-               char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
+               char *buf, *mntpath = NULL;
+
+               buf = (char *)__get_free_page(GFP_KERNEL);
+               if (buf)
+                       mntpath = d_path(mountpoint, buf, PAGE_SIZE);
+               if (IS_ERR_OR_NULL(mntpath))
+                       mntpath = "(unknown)";

                pr_warn("%s filesystem being %s at %s supports timestamps until %ptTd (0x%llx)\n",
                        sb->s_type->name,
                        is_mounted(mnt) ? "remounted" : "mounted",
                        mntpath, &sb->s_time_max,
                        (unsigned long long)sb->s_time_max);
-
-               free_page((unsigned long)buf);
+               if (buf)
+                       free_page((unsigned long)buf);
                sb->s_iflags |= SB_I_TS_EXPIRY_WARNED;
        }
 }