[PATCH] erofs-utils: add --hardlink-dereference option

[Paul M]

Hi Gao,

On 2024/12/11 17:11, Gao Xiang wrote:
>
> Hi Paul,
>
> On 2024/12/11 23:07, Paul Meyer wrote:
> > Add option --hardlink-dereference to dereference hardlinks when
> > creating an image. Instead of reusing the inode, hardlinks are added
> > as separate inodes. This is useful for reproducible builds, when the
> > rootfs is space-optimized using hardlinks on some machines, but not on
> > others.
>
> Thanks for the patch!
>
> Yet I fail to parse the feature, why this feature is useful
> for reproducible builds? IOWs, without this feature (or
> hardlinks are used), what's the exact impact that you don't
> want to?

Sure, here is our full use case: We are building an erofs image with Nix.
Nix stores the rootfs in the nix store (/nix/store). Now there is an option
in nix to enable store optimization via hardlinks. In case optimization is
enabled, files with identical content are turned into hardlinks to save space,
as nix store paths are read-only anyway. If I create a rootfs with
two identical files, those will be hardlinked on systems with optimization
enabled, but have different inodes on systems where optimization is disabled.
When building an erofs, the resulting image will have one inode less on
the system where files are hardlinked.
The goal is to make the image bit-by-bit reproducible on both systems.
By dereferencing hardlinks, we get the exact same image no matter whether
the system uses hardlink optimizations or not.

There is a comparable tar option with the same name.

Thanks,
Paul

>
> Thanks,
> Gao Xiang
>
> >
> > Co-authored-by: Leonard Cohnen <leonard.cohnen at gmail.com>
> > Signed-off-by: Paul Meyer <katexochen0 at gmail.com>
> > ---
> >   include/erofs/config.h | 1 +
> >   lib/inode.c            | 2 +-
> >   mkfs/main.c            | 4 ++++
> >   3 files changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/erofs/config.h b/include/erofs/config.h
> > index cff4cea..8399afb 100644
> > --- a/include/erofs/config.h
> > +++ b/include/erofs/config.h
> > @@ -58,6 +58,7 @@ struct erofs_configure {
> >       bool c_extra_ea_name_prefixes;
> >       bool c_xattr_name_filter;
> >       bool c_ovlfs_strip;
> > +     bool c_hardlink_dereference;
> >
> >   #ifdef HAVE_LIBSELINUX
> >       struct selabel_handle *sehnd;
> > diff --git a/lib/inode.c b/lib/inode.c
> > index 7e5c581..5d181b3 100644
> > --- a/lib/inode.c
> > +++ b/lib/inode.c
> > @@ -1141,7 +1141,7 @@ static struct erofs_inode *erofs_iget_from_srcpath(struct erofs_sb_info *sbi,
> >        * hard-link, just return it. Also don't lookup for directories
> >        * since hard-link directory isn't allowed.
> >        */
> > -     if (!S_ISDIR(st.st_mode)) {
> > +     if (!S_ISDIR(st.st_mode) && (!cfg.c_hardlink_dereference)) {
> >               inode = erofs_iget(st.st_dev, st.st_ino);
> >               if (inode)
> >                       return inode;
> > diff --git a/mkfs/main.c b/mkfs/main.c
> > index d422787..09e39f5 100644
> > --- a/mkfs/main.c
> > +++ b/mkfs/main.c
> > @@ -85,6 +85,7 @@ static struct option long_options[] = {
> >       {"mkfs-time", no_argument, NULL, 525},
> >       {"all-time", no_argument, NULL, 526},
> >       {"sort", required_argument, NULL, 527},
> > +     {"hardlink-dereference", no_argument, NULL, 528},
> >       {0, 0, 0, 0},
> >   };
> >
> > @@ -846,6 +847,9 @@ static int mkfs_parse_options_cfg(int argc, char *argv[])
> >                       if (!strcmp(optarg, "none"))
> >                               erofstar.try_no_reorder = true;
> >                       break;
> > +             case 528:
> > +                     cfg.c_hardlink_dereference = true;
> > +                     break;
> >               case 'V':
> >                       version();
> >                       exit(0);
>