[PATCH] ovl: warn if trusted xattr creation fails

[PATCH] ovl: warn if trusted xattr creation fails

[Alois Wohlschlager]

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When mounting overlayfs in an unprivileged user namespace, trusted xattr
creation will fail. This will lead to failures in some file operations,
e.g. in the following situation:

  mkdir lower upper work merged
  mkdir lower/directory
  mount -toverlay -olowerdir=lower,upperdir=upper,workdir=work none merged
  rmdir merged/directory
  mkdir merged/directory

The last mkdir will fail:

  mkdir: cannot create directory 'merged/directory': Input/output error

The cause for these failures is currently extremely non-obvious and hard
to debug. Hence, warn the user and suggest using the userxattr mount
option, if it is not already supplied and xattr creation fails during
the self-check.

Signed-off-by: Alois Wohlschlager <alois1@gmx-topmail.de>
---
 fs/overlayfs/super.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7bb0a47cb615..11123fe967e0 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1427,6 +1427,8 @@ static int ovl_make_workdir(struct super_block *sb,
struct ovl_fs *ofs,
 			ofs->config.xino = OVL_XINO_OFF;
 			pr_warn("upper fs does not support xattr,
falling back to xino=off.\n");
 		}
+		if (!ofs->config.userxattr)
+			pr_warn("trusted xattr creation not
supported, some file operations may fail. Try mounting with userxattr next
time.\n");
 		err = 0;
 	} else {
 		ovl_do_removexattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE);
--
2.35.1


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Re: [PATCH] ovl: warn if trusted xattr creation fails

[Miklos Szeredi]

On Thu, Feb 03, 2022 at 12:02:46PM +0100, Alois Wohlschlager wrote:
> When mounting overlayfs in an unprivileged user namespace, trusted xattr
> creation will fail. This will lead to failures in some file operations,
> e.g. in the following situation:
> 
>   mkdir lower upper work merged
>   mkdir lower/directory
>   mount -toverlay -olowerdir=lower,upperdir=upper,workdir=work none merged
>   rmdir merged/directory
>   mkdir merged/directory
> 
> The last mkdir will fail:
> 
>   mkdir: cannot create directory 'merged/directory': Input/output error
> 
> The cause for these failures is currently extremely non-obvious and hard
> to debug. Hence, warn the user and suggest using the userxattr mount
> option, if it is not already supplied and xattr creation fails during
> the self-check.

Thanks for the patch.

How about the following (untested) variant?

Thanks,
Miklos


diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7bb0a47cb615..955aeefc3b29 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1413,11 +1413,12 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 	 */
 	err = ovl_do_setxattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE, "0", 1);
 	if (err) {
+		pr_warn("failed to set xattr on upper\n");
 		ofs->noxattr = true;
 		if (ofs->config.index || ofs->config.metacopy) {
 			ofs->config.index = false;
 			ofs->config.metacopy = false;
-			pr_warn("upper fs does not support xattr, falling back to index=off,metacopy=off.\n");
+			pr_warn("...falling back to index=off,metacopy=off.\n");
 		}
 		/*
 		 * xattr support is required for persistent st_ino.
@@ -1425,8 +1426,10 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 		 */
 		if (ofs->config.xino == OVL_XINO_AUTO) {
 			ofs->config.xino = OVL_XINO_OFF;
-			pr_warn("upper fs does not support xattr, falling back to xino=off.\n");
+			pr_warn("...falling back to xino=off.\n");
 		}
+		if (err == -EPERM && !ofs->config.userxattr)
+			pr_info("try mounting with 'userxattr' option\n");
 		err = 0;
 	} else {
 		ovl_do_removexattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE);

Re: [PATCH] ovl: warn if trusted xattr creation fails

[Alois Wohlschlager]

Am Montag, 21. Februar 2022, 12:11:47 CET schrieb Miklos Szeredi:
>
> Thanks for the patch.
>
> How about the following (untested) variant?
>
> Thanks,
> Miklos
>
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 7bb0a47cb615..955aeefc3b29 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1413,11 +1413,12 @@ static int ovl_make_workdir(struct super_block *sb,
> struct ovl_fs *ofs, */
>  	err = ovl_do_setxattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE, "0", 1);
>  	if (err) {
> +		pr_warn("failed to set xattr on upper\n");
>  		ofs->noxattr = true;
>  		if (ofs->config.index || ofs->config.metacopy) {
>  			ofs->config.index = false;
>  			ofs->config.metacopy = false;
> -			pr_warn("upper fs does not support xattr, falling back to
> index=off,metacopy=off.\n"); +			pr_warn("...falling back to
> index=off,metacopy=off.\n");
>  		}
>  		/*
>  		 * xattr support is required for persistent st_ino.
> @@ -1425,8 +1426,10 @@ static int ovl_make_workdir(struct super_block *sb,
> struct ovl_fs *ofs, */
>  		if (ofs->config.xino == OVL_XINO_AUTO) {
>  			ofs->config.xino = OVL_XINO_OFF;
> -			pr_warn("upper fs does not support xattr, falling back to xino=off.\n");
> +			pr_warn("...falling back to xino=off.\n");
>  		}
> +		if (err == -EPERM && !ofs->config.userxattr)
> +			pr_info("try mounting with 'userxattr' option\n");
>  		err = 0;
>  	} else {
>  		ovl_do_removexattr(ofs, ofs->workdir, OVL_XATTR_OPAQUE);

Seems sensible to me, since it doesn't duplicate information in case index, metacopy or xino are attempted to be used.

Alois