Hello,
I've created some patches for grub that adds zstdio support to boot
kernels compressed with zstd. I just sent them upstream a few minutes
ago[1].
I've attached a patch which I've used to successfully build Debian
packages that, when installed on the Xen guest, will boot standard
Trixie kernels.
I've also placed some very unofficial pre-built packages to tide other
people in the same boat over while this gets sorted out:
https://staff.deltatee.com/~logang/grub/
Thanks,
Logan
[1] https://lists.gnu.org/archive/html/grub-devel/2025-09/msg00227.html
Description: Implement zstdio decompression
Support zstdio compression for loading files compressed with zstd.
As of Trixie, debian kernels packages are compressed with zstd. These
kernels cannot be booted with grub-xen seeing they must be decompressed
before being booted. Adding zstdio supported fixes this issue.
Author: Logan Gunthorpe <[email protected]>
Origin: (upstream|backport|vendor|other), (<patch-url>|commit:<commit-id>)
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1102066
Last-Update: 2025-09-23
--- grub2-2.12.orig/Makefile.util.def
+++ grub2-2.12/Makefile.util.def
@@ -160,6 +160,7 @@ library = {
common = grub-core/io/gzio.c;
common = grub-core/io/xzio.c;
common = grub-core/io/lzopio.c;
+ common = grub-core/io/zstdio.c;
common = grub-core/kern/ia64/dl_helper.c;
common = grub-core/kern/arm/dl_helper.c;
common = grub-core/kern/arm64/dl_helper.c;
--- grub2-2.12.orig/grub-core/Makefile.core.def
+++ grub2-2.12/grub-core/Makefile.core.def
@@ -2467,6 +2467,13 @@ module = {
};
module = {
+ name = zstdio;
+ common = io/zstdio.c;
+ cppflags = '-I$(srcdir)/lib/posix_wrap -I$(srcdir)/lib/zstd';
+ cflags='-Wno-unreachable-code';
+};
+
+module = {
name = testload;
common = commands/testload.c;
};
--- /dev/null
+++ grub2-2.12/grub-core/io/zstdio.c
@@ -0,0 +1,237 @@
+/* zstdio.c - decompression support for zstd */
+/*
+ * GRUB -- GRand Unified Bootloader
+ * Copyright (C) 2010 Free Software Foundation, Inc.
+ *
+ * GRUB is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * GRUB is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GRUB. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/err.h>
+#include <grub/mm.h>
+#include <grub/misc.h>
+#include <grub/file.h>
+#include <grub/fs.h>
+#include <grub/dl.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+#include "zstd.h"
+
+#define STREAM_HEADER_SIZE 16
+
+struct grub_zstdio
+{
+ grub_file_t file;
+ ZSTD_DCtx *dctx;
+ grub_size_t insize;
+ grub_size_t outsize;
+
+ ZSTD_outBuffer output;
+ ZSTD_inBuffer input;
+
+ grub_off_t saved_offset;
+ grub_uint8_t bufs[];
+};
+
+typedef struct grub_zstdio *grub_zstdio_t;
+static struct grub_fs grub_zstdio_fs;
+
+static int
+test_header (grub_file_t file)
+{
+ grub_zstdio_t zstdio = file->data;
+ size_t zret;
+
+ zstdio->input.pos = 0;
+ zstdio->output.pos = 0;
+ zstdio->output.size = zstdio->outsize;
+ zstdio->input.size = grub_file_read (zstdio->file, zstdio->bufs,
+ STREAM_HEADER_SIZE);
+ if (zstdio->input.size != STREAM_HEADER_SIZE)
+ return 0;
+
+ zret = ZSTD_decompressStream(zstdio->dctx, &zstdio->output, &zstdio->input);
+ if (ZSTD_isError(zret))
+ return 0;
+
+ return 1;
+}
+
+static grub_file_t
+grub_zstdio_open (grub_file_t io, enum grub_file_type type)
+{
+ grub_file_t file;
+ grub_zstdio_t zstdio;
+
+ if (type & GRUB_FILE_TYPE_NO_DECOMPRESS)
+ return io;
+
+ file = (grub_file_t) grub_zalloc (sizeof (*file));
+ if (!file)
+ return 0;
+
+ zstdio = grub_zalloc (sizeof (*zstdio) + ZSTD_DStreamInSize() +
+ ZSTD_DStreamOutSize());
+ if (!zstdio)
+ {
+ grub_free (file);
+ return 0;
+ }
+
+ zstdio->file = io;
+ zstdio->insize = ZSTD_DStreamInSize();
+ zstdio->outsize = ZSTD_DStreamOutSize();
+ zstdio->input.src = zstdio->bufs;
+ zstdio->output.dst = &zstdio->bufs[zstdio->insize];
+
+ file->device = io->device;
+ file->data = zstdio;
+ file->fs = &grub_zstdio_fs;
+ file->size = GRUB_FILE_SIZE_UNKNOWN;
+ file->not_easily_seekable = 1;
+
+ if (grub_file_tell (zstdio->file) != 0)
+ grub_file_seek (zstdio->file, 0);
+
+ zstdio->dctx = ZSTD_createDCtx();
+ if (!zstdio->dctx)
+ {
+ grub_free (file);
+ grub_free (zstdio);
+ return 0;
+ }
+
+ if (!test_header (file))
+ {
+ grub_errno = GRUB_ERR_NONE;
+ grub_file_seek (io, 0);
+ ZSTD_freeDCtx(zstdio->dctx);
+ grub_free (zstdio);
+ grub_free (file);
+
+ return io;
+ }
+
+ return file;
+}
+
+static grub_ssize_t
+grub_zstdio_read (grub_file_t file, char *buf, grub_size_t len)
+{
+ grub_zstdio_t zstdio = file->data;
+ grub_ssize_t ret = 0;
+ grub_ssize_t readret;
+ grub_off_t current_offset;
+ grub_size_t zret;
+
+ /* If seek backward need to reset decoder and start from beginning of file. */
+ if (file->offset < zstdio->saved_offset)
+ {
+ ZSTD_initDStream(zstdio->dctx);
+ zstdio->input.pos = 0;
+ zstdio->input.size = 0;
+ zstdio->output.pos = 0;
+ zstdio->saved_offset = 0;
+ grub_file_seek (zstdio->file, 0);
+ }
+
+ current_offset = zstdio->saved_offset;
+
+ while (len > 0)
+ {
+ zstdio->output.size = file->offset + ret + len - current_offset;
+ if (zstdio->output.size > zstdio->outsize)
+ zstdio->output.size = zstdio->outsize;
+ if (zstdio->input.pos == zstdio->input.size)
+ {
+ readret = grub_file_read (zstdio->file, zstdio->bufs,
+ zstdio->insize);
+ if (readret < 0)
+ return -1;
+ zstdio->input.size = readret;
+ zstdio->input.pos = 0;
+ }
+
+ zret = ZSTD_decompressStream(zstdio->dctx, &zstdio->output,
+ &zstdio->input);
+ if (ZSTD_isError(zret))
+ {
+ grub_error (GRUB_ERR_BAD_COMPRESSED_DATA,
+ N_("zstd file corrupted or unsupported block options"));
+ return -1;
+ }
+
+ grub_off_t new_offset = current_offset + zstdio->output.pos;
+
+ if (file->offset <= new_offset)
+ /* Store first chunk of data in buffer. */
+ {
+ grub_size_t delta = new_offset - (file->offset + ret);
+ grub_memmove (buf, (grub_uint8_t *)zstdio->output.dst +
+ (zstdio->output.pos - delta),
+ delta);
+ len -= delta;
+ buf += delta;
+ ret += delta;
+ }
+ current_offset = new_offset;
+
+ zstdio->output.pos = 0;
+
+ if (zstdio->input.pos == 0 && zstdio->output.pos == 0)
+ break;
+ }
+
+ if (ret >= 0)
+ zstdio->saved_offset = file->offset + ret;
+
+ return ret;
+}
+
+/* Release everything, including the underlying file object. */
+static grub_err_t
+grub_zstdio_close (grub_file_t file)
+{
+ grub_zstdio_t zstdio = file->data;
+
+ ZSTD_freeDCtx(zstdio->dctx);
+
+ grub_file_close (zstdio->file);
+ grub_free (zstdio);
+
+ /* Device must not be closed twice. */
+ file->device = 0;
+ file->name = 0;
+ return grub_errno;
+}
+
+static struct grub_fs grub_zstdio_fs = {
+ .name = "zstdio",
+ .fs_dir = 0,
+ .fs_open = 0,
+ .fs_read = grub_zstdio_read,
+ .fs_close = grub_zstdio_close,
+ .fs_label = 0,
+ .next = 0
+};
+
+GRUB_MOD_INIT (zstdio)
+{
+ grub_file_filter_register (GRUB_FILE_FILTER_ZSTDIO, grub_zstdio_open);
+}
+
+GRUB_MOD_FINI (zstdio)
+{
+ grub_file_filter_unregister (GRUB_FILE_FILTER_ZSTDIO);
+}
--- grub2-2.12.orig/include/grub/file.h
+++ grub2-2.12/include/grub/file.h
@@ -187,9 +187,10 @@ typedef enum grub_file_filter_id
GRUB_FILE_FILTER_GZIO,
GRUB_FILE_FILTER_XZIO,
GRUB_FILE_FILTER_LZOPIO,
+ GRUB_FILE_FILTER_ZSTDIO,
GRUB_FILE_FILTER_MAX,
GRUB_FILE_FILTER_COMPRESSION_FIRST = GRUB_FILE_FILTER_GZIO,
- GRUB_FILE_FILTER_COMPRESSION_LAST = GRUB_FILE_FILTER_LZOPIO,
+ GRUB_FILE_FILTER_COMPRESSION_LAST = GRUB_FILE_FILTER_ZSTDIO,
} grub_file_filter_id_t;
typedef grub_file_t (*grub_file_filter_t) (grub_file_t in, enum grub_file_type type);
