Hello,
We have some fedora maintainers and users who are eager to have this patch (
https://bugzilla.redhat.com/show_bug.cgi?id=2372973), so we would really
appreciate if it could get review asap. SUSE have been carrying it for
about a year
https://build.opensuse.org/projects/Base:System/packages/grub2/files/grub2-grubenv-in-btrfs-header.patch
thank you so much!
marta
On Fri, Jul 11, 2025 at 6:24 AM Michael Chang via Grub-devel <
grub-devel@gnu.org> wrote:
> On Btrfs, GRUB cannot update the environment block (grubenv) because
> file based writes via blocklists are incompatible with Btrfs COW (Copy
> On Write) design. Although GRUB’s filesystem drivers are read only,
> environment updates rely on raw block access to fixed locations, which
> is not safe on Btrfs due to its dynamic block relocation.
>
> To address this, we introduce support for storing the GRUB environment
> block in a fixed block within Btrfs reserved bootloader area, an unused
> region in the device header intended for bootloader use.
>
> This patch adds fs_envblk helpers to grub-editenv for accessing the
> reserved area directly. Variables that require runtime write access
> during boot, such as next_entry, will be mirrored into this external
> block. Other variables will remain stored in the file based grubenv so
> they can keep per snapshot.
>
> The embedding logic is also updated to mark the reserved area as used to
> avoid conflicts with embedded core images.
>
> This enables support for runtime environment updates on Btrfs root
> volumes, allowing tools like grub-reboot to boot an entry once.
>
> Signed-off-by: Michael Chang <mch...@suse.com>
> ---
> grub-core/fs/btrfs.c | 3 +-
> include/grub/fs.h | 2 +
> util/grub-editenv.c | 337 ++++++++++++++++++++++++++++++++++++++-
> util/grub.d/00_header.in | 20 ++-
> 4 files changed, 357 insertions(+), 5 deletions(-)
>
> diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
> index 7bf8d922f..ceacc1151 100644
> --- a/grub-core/fs/btrfs.c
> +++ b/grub-core/fs/btrfs.c
> @@ -2341,7 +2341,7 @@ struct embed_region {
>
> static const struct {
> struct embed_region available;
> - struct embed_region used[6];
> + struct embed_region used[7];
> } btrfs_head = {
> .available = {0, GRUB_DISK_KiB_TO_SECTORS (1024)}, /* The first 1 MiB.
> */
> .used = {
> @@ -2349,6 +2349,7 @@ static const struct {
> {GRUB_DISK_KiB_TO_SECTORS (64) - 1, 1}, /*
> Overflow guard. */
> {GRUB_DISK_KiB_TO_SECTORS (64), GRUB_DISK_KiB_TO_SECTORS (4)}, /* 4
> KiB superblock. */
> {GRUB_DISK_KiB_TO_SECTORS (68), 1}, /*
> Overflow guard. */
> + {GRUB_DISK_KiB_TO_SECTORS (ENV_BTRFS_OFFSET) - 1, 3}, /*
> Environment Block. */
> {GRUB_DISK_KiB_TO_SECTORS (1024) - 1, 1}, /*
> Overflow guard. */
> {0, 0} /*
> Array terminator. */
> }
> diff --git a/include/grub/fs.h b/include/grub/fs.h
> index df4c93b16..9c8206133 100644
> --- a/include/grub/fs.h
> +++ b/include/grub/fs.h
> @@ -132,4 +132,6 @@ grub_fs_unregister (grub_fs_t fs)
>
> grub_fs_t EXPORT_FUNC(grub_fs_probe) (grub_device_t device);
>
> +#define ENV_BTRFS_OFFSET (256)
> +
> #endif /* ! GRUB_FS_HEADER */
> diff --git a/util/grub-editenv.c b/util/grub-editenv.c
> index db6f187cc..31103df1d 100644
> --- a/util/grub-editenv.c
> +++ b/util/grub-editenv.c
> @@ -23,8 +23,11 @@
> #include <grub/util/misc.h>
> #include <grub/lib/envblk.h>
> #include <grub/i18n.h>
> -#include <grub/emu/hostfile.h>
> +#include <grub/emu/hostdisk.h>
> #include <grub/util/install.h>
> +#include <grub/emu/getroot.h>
> +#include <grub/fs.h>
> +#include <grub/crypto.h>
>
> #include <stdio.h>
> #include <unistd.h>
> @@ -120,6 +123,142 @@ block, use `rm %s'."),
> NULL, help_filter, NULL
> };
>
> +struct fs_envblk_spec {
> + const char *fs_name;
> + int offset;
> + int size;
> +} fs_envblk_spec[] = {
> + { "btrfs", ENV_BTRFS_OFFSET * 1024, GRUB_DISK_SECTOR_SIZE },
> + { NULL, 0, 0 }
> +};
> +
> +struct fs_envblk {
> + struct fs_envblk_spec *spec;
> + const char *dev;
> +};
> +
> +typedef struct fs_envblk_spec *fs_envblk_spec_t;
> +typedef struct fs_envblk *fs_envblk_t;
> +
> +fs_envblk_t fs_envblk = NULL;
> +
> +static int
> +read_envblk_fs (const char *varname, const char *value, void *hook_data)
> +{
> + grub_envblk_t *p_envblk = (grub_envblk_t *)hook_data;
> +
> + if (!p_envblk || !fs_envblk)
> + return 0;
> +
> + if (strcmp (varname, "env_block") == 0)
> + {
> + int off, sz;
> + char *p;
> +
> + off = strtol (value, &p, 10);
> + if (*p == '+')
> + sz = strtol (p+1, &p, 10);
> + else
> + return 0;
> +
> + if (*p == '\0')
> + {
> + FILE *fp;
> + char *buf;
> +
> + off <<= GRUB_DISK_SECTOR_BITS;
> + sz <<= GRUB_DISK_SECTOR_BITS;
> +
> + fp = grub_util_fopen (fs_envblk->dev, "rb");
> + if (! fp)
> + grub_util_error (_("cannot open `%s': %s"), fs_envblk->dev,
> + strerror (errno));
> +
> +
> + if (fseek (fp, off, SEEK_SET) < 0)
> + grub_util_error (_("cannot seek `%s': %s"), fs_envblk->dev,
> + strerror (errno));
> +
> + buf = xmalloc (sz);
> + if ((fread (buf, 1, sz, fp)) != sz)
> + grub_util_error (_("cannot read `%s': %s"), fs_envblk->dev,
> + strerror (errno));
> +
> + fclose (fp);
> +
> + *p_envblk = grub_envblk_open (buf, sz);
> + }
> + }
> +
> + return 0;
> +}
> +
> +static void
> +create_envblk_fs (void)
> +{
> + FILE *fp;
> + char *buf;
> + const char *device;
> + int offset, size;
> +
> + if (!fs_envblk)
> + return;
> +
> + device = fs_envblk->dev;
> + offset = fs_envblk->spec->offset;
> + size = fs_envblk->spec->size;
> +
> + fp = grub_util_fopen (device, "r+b");
> + if (! fp)
> + grub_util_error (_("cannot open `%s': %s"), device, strerror (errno));
> +
> + buf = xmalloc (size);
> + memcpy (buf, GRUB_ENVBLK_SIGNATURE, sizeof (GRUB_ENVBLK_SIGNATURE) - 1);
> + memset (buf + sizeof (GRUB_ENVBLK_SIGNATURE) - 1, '#', size - sizeof
> (GRUB_ENVBLK_SIGNATURE) + 1);
> +
> + if (fseek (fp, offset, SEEK_SET) < 0)
> + grub_util_error (_("cannot seek `%s': %s"), device, strerror (errno));
> +
> + if (fwrite (buf, 1, size, fp) != size)
> + grub_util_error (_("cannot write to `%s': %s"), device, strerror
> (errno));
> +
> + grub_util_file_sync (fp);
> + free (buf);
> + fclose (fp);
> +}
> +
> +static grub_envblk_t
> +open_envblk_fs (grub_envblk_t envblk)
> +{
> + grub_envblk_t envblk_fs = NULL;
> + char *val;
> + int offset, size;
> +
> + if (!fs_envblk)
> + return NULL;
> +
> + offset = fs_envblk->spec->offset;
> + size = fs_envblk->spec->size;
> +
> + grub_envblk_iterate (envblk, &envblk_fs, read_envblk_fs);
> +
> + if (envblk_fs && grub_envblk_size (envblk_fs) == size)
> + return envblk_fs;
> +
> + create_envblk_fs ();
> +
> + offset = offset >> GRUB_DISK_SECTOR_BITS;
> + size = (size + GRUB_DISK_SECTOR_SIZE - 1) >> GRUB_DISK_SECTOR_BITS;
> +
> + val = xasprintf ("%d+%d", offset, size);
> + if (! grub_envblk_set (envblk, "env_block", val))
> + grub_util_error ("%s", _("environment block too small"));
> + grub_envblk_iterate (envblk, &envblk_fs, read_envblk_fs);
> + free (val);
> +
> + return envblk_fs;
> +}
> +
> static grub_envblk_t
> open_envblk_file (const char *name)
> {
> @@ -182,10 +321,17 @@ static void
> list_variables (const char *name)
> {
> grub_envblk_t envblk;
> + grub_envblk_t envblk_fs = NULL;
>
> envblk = open_envblk_file (name);
> + grub_envblk_iterate (envblk, &envblk_fs, read_envblk_fs);
> grub_envblk_iterate (envblk, NULL, print_var);
> grub_envblk_close (envblk);
> + if (envblk_fs)
> + {
> + grub_envblk_iterate (envblk_fs, NULL, print_var);
> + grub_envblk_close (envblk_fs);
> + }
> }
>
> static void
> @@ -208,6 +354,38 @@ write_envblk (const char *name, grub_envblk_t envblk)
> fclose (fp);
> }
>
> +static void
> +write_envblk_fs (grub_envblk_t envblk)
> +{
> + FILE *fp;
> + const char *device;
> + int offset, size;
> +
> + if (!fs_envblk)
> + return;
> +
> + device = fs_envblk->dev;
> + offset = fs_envblk->spec->offset;
> + size = fs_envblk->spec->size;
> +
> + if (grub_envblk_size (envblk) > size)
> + grub_util_error ("%s", _("environment block too small"));
> +
> + fp = grub_util_fopen (device, "r+b");
> +
> + if (! fp)
> + grub_util_error (_("cannot open `%s': %s"), device, strerror (errno));
> +
> + if (fseek (fp, offset, SEEK_SET) < 0)
> + grub_util_error (_("cannot seek `%s': %s"), device, strerror (errno));
> +
> + if (fwrite (grub_envblk_buffer (envblk), 1, grub_envblk_size (envblk),
> fp) != grub_envblk_size (envblk))
> + grub_util_error (_("cannot write to `%s': %s"), device, strerror
> (errno));
> +
> + grub_util_file_sync (fp);
> + fclose (fp);
> +}
> +
> static void
> set_variables (const char *name, int argc, char *argv[])
> {
> @@ -224,8 +402,27 @@ set_variables (const char *name, int argc, char
> *argv[])
>
> *(p++) = 0;
>
> - if (! grub_envblk_set (envblk, argv[0], p))
> - grub_util_error ("%s", _("environment block too small"));
> + if ((strcmp (argv[0], "next_entry") == 0 ||
> + strcmp (argv[0], "health_checker_flag") == 0) && fs_envblk)
> + {
> + grub_envblk_t envblk_fs;
> + envblk_fs = open_envblk_fs (envblk);
> + if (!envblk_fs)
> + grub_util_error ("%s", _("can't open fs environment block"));
> + if (! grub_envblk_set (envblk_fs, argv[0], p))
> + grub_util_error ("%s", _("environment block too small"));
> + write_envblk_fs (envblk_fs);
> + grub_envblk_close (envblk_fs);
> + }
> + else if (strcmp (argv[0], "env_block") == 0)
> + {
> + grub_util_warn ("can't set env_block as it's read-only");
> + }
> + else
> + {
> + if (! grub_envblk_set (envblk, argv[0], p))
> + grub_util_error ("%s", _("environment block too small"));
> + }
>
> argc--;
> argv++;
> @@ -239,20 +436,151 @@ static void
> unset_variables (const char *name, int argc, char *argv[])
> {
> grub_envblk_t envblk;
> + grub_envblk_t envblk_fs;
>
> envblk = open_envblk_file (name);
> +
> + envblk_fs = NULL;
> + if (fs_envblk)
> + envblk_fs = open_envblk_fs (envblk);
> +
> while (argc)
> {
> grub_envblk_delete (envblk, argv[0]);
>
> + if (envblk_fs)
> + grub_envblk_delete (envblk_fs, argv[0]);
> +
> argc--;
> argv++;
> }
>
> write_envblk (name, envblk);
> grub_envblk_close (envblk);
> +
> + if (envblk_fs)
> + {
> + write_envblk_fs (envblk_fs);
> + grub_envblk_close (envblk_fs);
> + }
> +}
> +
> +int have_abstraction = 0;
> +static void
> +probe_abstraction (grub_disk_t disk)
> +{
> + if (disk->partition == NULL)
> + grub_util_info ("no partition map found for %s", disk->name);
> +
> + if (disk->dev->id == GRUB_DISK_DEVICE_DISKFILTER_ID ||
> + disk->dev->id == GRUB_DISK_DEVICE_CRYPTODISK_ID)
> + {
> + have_abstraction = 1;
> + }
> }
>
> +static fs_envblk_t
> +probe_fs_envblk (fs_envblk_spec_t spec)
> +{
> + char **grub_devices;
> + char **curdev, **curdrive;
> + size_t ndev = 0;
> + char **grub_drives;
> + grub_device_t grub_dev = NULL;
> + grub_fs_t grub_fs;
> + const char *fs_envblk_device;
> +
> +#ifdef __s390x__
> + return NULL;
> +#endif
> +
> + grub_util_biosdisk_init (DEFAULT_DEVICE_MAP);
> + grub_init_all ();
> + grub_gcry_init_all ();
> +
> + grub_lvm_fini ();
> + grub_mdraid09_fini ();
> + grub_mdraid1x_fini ();
> + grub_diskfilter_fini ();
> + grub_diskfilter_init ();
> + grub_mdraid09_init ();
> + grub_mdraid1x_init ();
> + grub_lvm_init ();
> +
> + grub_devices = grub_guess_root_devices (DEFAULT_DIRECTORY);
> +
> + if (!grub_devices || !grub_devices[0])
> + grub_util_error (_("cannot find a device for %s (is /dev mounted?)"),
> DEFAULT_DIRECTORY);
> +
> + fs_envblk_device = grub_devices[0];
> +
> + for (curdev = grub_devices; *curdev; curdev++)
> + {
> + grub_util_pull_device (*curdev);
> + ndev++;
> + }
> +
> + grub_drives = xcalloc ((ndev + 1), sizeof (grub_drives[0]));
> +
> + for (curdev = grub_devices, curdrive = grub_drives; *curdev; curdev++,
> + curdrive++)
> + {
> + *curdrive = grub_util_get_grub_dev (*curdev);
> + if (! *curdrive)
> + grub_util_error (_("cannot find a GRUB drive for %s. Check your
> device.map"),
> + *curdev);
> + }
> + *curdrive = 0;
> +
> + grub_dev = grub_device_open (grub_drives[0]);
> + if (! grub_dev)
> + grub_util_error ("%s", grub_errmsg);
> +
> + grub_fs = grub_fs_probe (grub_dev);
> + if (! grub_fs)
> + grub_util_error ("%s", grub_errmsg);
> +
> + if (grub_dev->disk)
> + {
> + probe_abstraction (grub_dev->disk);
> + }
> + for (curdrive = grub_drives + 1; *curdrive; curdrive++)
> + {
> + grub_device_t dev = grub_device_open (*curdrive);
> + if (!dev)
> + continue;
> + if (dev->disk)
> + probe_abstraction (dev->disk);
> + grub_device_close (dev);
> + }
> +
> + free (grub_drives);
> + grub_device_close (grub_dev);
> + grub_gcry_fini_all ();
> + grub_fini_all ();
> + grub_util_biosdisk_fini ();
> +
> + fs_envblk_spec_t p;
> +
> + for (p = spec; p->fs_name; p++)
> + {
> + if (strcmp (grub_fs->name, p->fs_name) == 0 && !have_abstraction)
> + {
> + if (p->offset % GRUB_DISK_SECTOR_SIZE == 0 &&
> + p->size % GRUB_DISK_SECTOR_SIZE == 0)
> + {
> + fs_envblk = xmalloc (sizeof (fs_envblk_t));
> + fs_envblk->spec = p;
> + fs_envblk->dev = strdup(fs_envblk_device);
> + return fs_envblk;
> + }
> + }
> + }
> +
> + return NULL;
> +}
> +
> +
> int
> main (int argc, char *argv[])
> {
> @@ -284,6 +612,9 @@ main (int argc, char *argv[])
> command = argv[curindex++];
> }
>
> + if (strcmp (filename, DEFAULT_ENVBLK_PATH) == 0)
> + fs_envblk = probe_fs_envblk (fs_envblk_spec);
> +
> if (strcmp (command, "create") == 0)
> grub_util_create_envblk_file (filename);
> else if (strcmp (command, "list") == 0)
> diff --git a/util/grub.d/00_header.in b/util/grub.d/00_header.in
> index f86b69bad..0d3928df6 100644
> --- a/util/grub.d/00_header.in
> +++ b/util/grub.d/00_header.in
> @@ -46,6 +46,13 @@ cat << EOF
> if [ -s \$prefix/grubenv ]; then
> load_env
> fi
> +
> +if [ "\${env_block}" ] ; then
> + set env_block="(\${root})\${env_block}"
> + export env_block
> + load_env -f "\${env_block}"
> +fi
> +
> EOF
> if [ "x$GRUB_BUTTON_CMOS_ADDRESS" != "x" ]; then
> cat <<EOF
> @@ -55,6 +62,9 @@ elif [ "\${next_entry}" ] ; then
> set default="\${next_entry}"
> set next_entry=
> save_env next_entry
> + if [ "\${env_block}" ] ; then
> + save_env -f "\${env_block}" next_entry
> + fi
> set boot_once=true
> else
> set default="${GRUB_DEFAULT}"
> @@ -66,6 +76,9 @@ if [ "\${next_entry}" ] ; then
> set default="\${next_entry}"
> set next_entry=
> save_env next_entry
> + if [ "\${env_block}" ] ; then
> + save_env -f "\${env_block}" next_entry
> + fi
> set boot_once=true
> else
> set default="${GRUB_DEFAULT}"
> @@ -93,7 +106,12 @@ fi
> function savedefault {
> if [ -z "\${boot_once}" ]; then
> saved_entry="\${chosen}"
> - save_env saved_entry
> + if [ "\${env_block}" ] ; then
> + save_env -f "\${env_block}" saved_entry
> + else
> + save_env saved_entry
> + fi
> +
> fi
> }
>
> --
> 2.50.0
>
>
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel
>
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