linux-kernel - Re: [PATCH v6 5/9] ovl: Ensure that all layers have the same encoding

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Message-ID: <CAOQ4uxhjPAO4S4O0ZuOqgFzw9pNSpM0BztRZNpPxoPNfCe473w@mail.gmail.com>
Date: Wed, 27 Aug 2025 11:17:20 +0200
From: Amir Goldstein <amir73il@...il.com>
To: André Almeida <andrealmeid@...lia.com>
Cc: Gabriel Krisman Bertazi <gabriel@...sman.be>, Miklos Szeredi <miklos@...redi.hu>, Theodore Tso <tytso@....edu>, 
	linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, kernel-dev@...lia.com
Subject: Re: [PATCH v6 5/9] ovl: Ensure that all layers have the same encoding

On Tue, Aug 26, 2025 at 10:12 PM André Almeida <andrealmeid@...lia.com> wrote:
>
>
>
> Em 25/08/2025 12:32, Amir Goldstein escreveu:
> > On Mon, Aug 25, 2025 at 1:17 PM Gabriel Krisman Bertazi
> > <gabriel@...sman.be> wrote:
> >>
> >> André Almeida <andrealmeid@...lia.com> writes:
> >>
> >>> When merging layers from different filesystems with casefold enabled,
> >>> all layers should use the same encoding version and have the same flags
> >>> to avoid any kind of incompatibility issues.
> >>>
> >>> Also, set the encoding and the encoding flags for the ovl super block as
> >>> the same as used by the first valid layer.
> >>>
> >>> Reviewed-by: Amir Goldstein <amir73il@...il.com>
> >>> Signed-off-by: André Almeida <andrealmeid@...lia.com>
> >>> ---
> >>>   fs/overlayfs/super.c | 25 +++++++++++++++++++++++++
> >>>   1 file changed, 25 insertions(+)
> >>>
> >>> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> >>> index df85a76597e910d00323018f1d2cd720c5db921d..b1dbd3c79961094d00c7f99cc622e515d544d22f 100644
> >>> --- a/fs/overlayfs/super.c
> >>> +++ b/fs/overlayfs/super.c
> >>> @@ -991,6 +991,18 @@ static int ovl_get_data_fsid(struct ovl_fs *ofs)
> >>>        return ofs->numfs;
> >>>   }
> >>>
> >>> +/*
> >>> + * Set the ovl sb encoding as the same one used by the first layer
> >>> + */
> >>> +static void ovl_set_encoding(struct super_block *sb, struct super_block *fs_sb)
> >>> +{
> >>> +#if IS_ENABLED(CONFIG_UNICODE)
> >>> +     if (sb_has_encoding(fs_sb)) {
> >>> +             sb->s_encoding = fs_sb->s_encoding;
> >>> +             sb->s_encoding_flags = fs_sb->s_encoding_flags;
> >>> +     }
> >>> +#endif
> >>> +}
> >>>
> >>>   static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
> >>>                          struct ovl_fs_context *ctx, struct ovl_layer *layers)
> >>> @@ -1024,6 +1036,9 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
> >>>        if (ovl_upper_mnt(ofs)) {
> >>>                ofs->fs[0].sb = ovl_upper_mnt(ofs)->mnt_sb;
> >>>                ofs->fs[0].is_lower = false;
> >>> +
> >>> +             if (ofs->casefold)
> >>> +                     ovl_set_encoding(sb, ofs->fs[0].sb);
> >>>        }
> >>>
> >>>        nr_merged_lower = ctx->nr - ctx->nr_data;
> >>> @@ -1083,6 +1098,16 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
> >>>                l->name = NULL;
> >>>                ofs->numlayer++;
> >>>                ofs->fs[fsid].is_lower = true;
> >>> +
> >>> +             if (ofs->casefold) {
> >>> +                     if (!ovl_upper_mnt(ofs) && !sb_has_encoding(sb))
> >>> +                             ovl_set_encoding(sb, ofs->fs[fsid].sb);
> >>> +
> >>> +                     if (!sb_has_encoding(sb) || !sb_same_encoding(sb, mnt->mnt_sb)) {
> >>
> >> Minor nit, but isn't the sb_has_encoding()  check redundant here?  sb_same_encoding
> >> will check the sb->encoding matches the mnt_sb already.
> >
> > Maybe we did something wrong but the intention was:
> > If all layers root are casefold disabled (or not supported) then
> > a mix of layers with fs of different encoding (and fs with no encoding support)
> > is allowed because we take care that all directories are always
> > casefold disabled.
> >
>
> We are going to reach this code only if ofs->casefold is true, so that
> means that ovl_dentry_casefolded() was true, and that means that
> sb_has_encoding(dentry->d_sb) is also true... so I think that Gabriel is
> right, if we reach this part of the code, that means that casefold is
> enabled and being used by at least one layer, so we can call
> sb_same_encoding() to check if they are consistent for all layers.
>
> For the case that we don't care about the layers having different
> encoding, the code will already skip this because of if (ofs->casefold)

Doh! yeh that was silly. I removed that now.

Thanks,
Amir.