SchOS/tools/mkschfs.c

// SPDX-License-Identifier: Apache-2.0
//
// mkschfs — build a SchFS image from a host directory tree.
//
// This is the tool that makes the on-disk format real: everything the kernel
// mounts in phase 2 is produced here. It deliberately shares the actual
// filesystem code (btree.c, vol.c, obj.c, objw.c) rather than reimplementing
// the layout, so a disagreement between writer and reader is impossible by
// construction — there is only one implementation, exercised on the host where
// a mistake costs a second instead of a boot.
//
//   mkschfs out.img --sysroot DIR [--size BLOCKS] [--label NAME]
//
// Two behaviours worth knowing about:
//
//   * Identical file contents are stored once and hard-linked. sysroot/bin
//     holds a full copy of the toybox binary per applet, so this is the
//     difference between a 190 MB image and a 6 MB one.
//   * Output is reproducible. Directory entries are sorted, the UUID is
//     derived rather than random, and timestamps come from the source files,
//     so building the same tree twice gives byte-identical images and a diff
//     means something actually changed.

#define _FILE_OFFSET_BITS 64
#define SCHFS_HOSTED 1

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <dirent.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
/* major()/minor() for device nodes. glibc puts them here; mingw has no such
   header and no device numbers at all, because Windows has no mknod. On that
   host the macros are stubbed: a device node cannot exist in the source tree
   being imaged, so the values are never anything but zero, and stubbing is
   honest where pretending to read st_rdev would not be. */
#ifdef __MINGW32__
#define major(x) 0
#define minor(x) 0
/* Symlinks, likewise. Windows has no lstat/readlink and mingw exposes neither,
   so on this host a symlink cannot be encountered while walking the tree --
   lstat degrades to stat, nothing is ever S_ISLNK, and readlink can only be
   reached through a branch that is now unreachable. Verified rather than
   assumed: `find sysroot -type l` returns nothing.

   If symlinks are ever staged (procfs wants /proc/<pid>/exe to be one), this
   host stops being able to build a faithful image and the stub must become a
   real Win32 reparse-point read. Failing loudly then is the point of the
   abort() below. */
#define lstat(p, b)      stat((p), (b))
#define S_ISLNK(m)       0
static long readlink(const char *p, char *b, unsigned long n)
{ (void)p; (void)b; (void)n; return -1; }
#elif defined(__linux__)
/* glibc and musl both put major()/minor() here, and glibc warns and then
   errors if you take them from <sys/types.h> instead. Linux only: macOS and
   the BSDs have no <sys/sysmacros.h> at all and define both in <sys/types.h>,
   which is already included above -- including it unconditionally is a build
   failure on every non-Linux Unix. */
#include <sys/sysmacros.h>
#endif

#include "../kernel/fs/schfs/btree.c"
#include "../kernel/fs/schfs/space.c"
#include "../kernel/fs/schfs/vol.c"
#include "../kernel/fs/schfs/obj.c"
#include "../kernel/fs/schfs/objw.c"

/* ------------------------------------------------------------------ */
/* File-backed block device                                            */
/* ------------------------------------------------------------------ */

struct imgdev {
    FILE             *f;
    struct schfs_vol *vol;
    uint64_t          nblocks;
    long              reads, writes;
};

static struct imgdev IMG;

static int
img_read(void *ctx, uint64_t b, void *buf)
{
    struct imgdev *d = ctx;
    if (b >= d->nblocks)
        return -1;
    if (fseeko(d->f, (off_t)b * SCHFS_BLKSIZE, SEEK_SET) != 0)
        return -1;
    if (fread(buf, 1, SCHFS_BLKSIZE, d->f) != SCHFS_BLKSIZE)
        return -1;
    d->reads++;
    return 0;
}

static int
img_write(void *ctx, uint64_t b, const void *buf)
{
    struct imgdev *d = ctx;
    if (b >= d->nblocks)
        return -1;
    if (fseeko(d->f, (off_t)b * SCHFS_BLKSIZE, SEEK_SET) != 0)
        return -1;
    if (fwrite(buf, 1, SCHFS_BLKSIZE, d->f) != SCHFS_BLKSIZE)
        return -1;
    d->writes++;
    return 0;
}

/* ------------------------------------------------------------------ */
/* Allocation                                                           */
/*                                                                      */
/* Straight through to the volume's space manager. An earlier version    */
/* kept a build-time free list of its own, because the bump allocator it */
/* replaced could only ever take back the block it handed out last and   */
/* a build otherwise leaked one root-to-leaf path per insert. Two things */
/* made that unnecessary: the space manager reuses any freed block, and  */
/* the tree now rewrites its own nodes in place within a transaction     */
/* group, so a build barely frees anything in the first place.           */
/* ------------------------------------------------------------------ */

static uint64_t
img_alloc(void *ctx)
{
    return schfs_vol_alloc(((struct imgdev *)ctx)->vol);
}

static uint64_t
img_alloc_run(void *ctx, uint64_t n)
{
    return schfs_vol_alloc_run(((struct imgdev *)ctx)->vol, n);
}

static void
img_free(void *ctx, uint64_t b)
{
    schfs_vol_free(((struct imgdev *)ctx)->vol, b);
}

// Pages for the allocation bitmap. calloc rather than malloc so a bug that
// forgets to initialise one shows up as "everything is free" consistently
// rather than as whatever the heap happened to contain.
static void *
img_palloc(void)
{
    return calloc(1, SCHFS_BLKSIZE);
}

/* ------------------------------------------------------------------ */
/* Content dedup                                                       */
/*                                                                     */
/* sysroot/bin contains one independent ~800 KB copy of toybox per      */
/* applet name. Storing each separately is the difference between an    */
/* image that fits in RAM and one that does not, so identical content   */
/* becomes one object with several names -- a plain hard link, which    */
/* the format already supports through nlink.                          */
/* ------------------------------------------------------------------ */

struct dedup_ent {
    char     *path;
    dev_t     hdev;                    /* host device+inode: when these match, */
    ino_t     hino;                    /* it is literally the same file        */
    off_t     size;
    uint32_t  crc;
    uint64_t  ino;
};

static struct dedup_ent *dedup_tab;
static int               dedup_n, dedup_cap;

/* A file the host already told us is the same file.
 *
 * ports/toybox/Makefile hard-links its 236 applet names to one binary, so this
 * settles all but the first of them without reading anything: the content path
 * below reads the candidate to CRC it and then re-reads both files to confirm,
 * which is half a gigabyte of I/O for an answer fstat already has. */
static uint64_t
dedup_lookup_ino(dev_t hdev, ino_t hino)
{
    if (hino == 0)
        return 0;
    for (int i = 0; i < dedup_n; i++)
        if (dedup_tab[i].hdev == hdev && dedup_tab[i].hino == hino)
            return dedup_tab[i].ino;
    return 0;
}

static int
files_identical(const char *a, const char *b)
{
    FILE *fa = fopen(a, "rb"), *fb = fopen(b, "rb");
    if (!fa || !fb) { if (fa) fclose(fa); if (fb) fclose(fb); return 0; }
    int same = 1;
    for (;;) {
        char ba[8192], bb[8192];
        size_t na = fread(ba, 1, sizeof(ba), fa);
        size_t nb = fread(bb, 1, sizeof(bb), fb);
        if (na != nb || memcmp(ba, bb, na) != 0) { same = 0; break; }
        if (na == 0) break;
    }
    fclose(fa); fclose(fb);
    return same;
}

// Size and CRC agreeing is not proof, so a candidate is confirmed byte by byte.
// Silently hard-linking two files that merely collide would corrupt the image
// in a way nothing downstream could detect.
static uint64_t
dedup_lookup(const char *path, off_t size, uint32_t crc)
{
    for (int i = 0; i < dedup_n; i++)
        if (dedup_tab[i].size == size && dedup_tab[i].crc == crc &&
            files_identical(dedup_tab[i].path, path))
            return dedup_tab[i].ino;
    return 0;
}

static void
dedup_remember(const char *path, const struct stat *st,
               off_t size, uint32_t crc, uint64_t ino)
{
    dev_t dedup_hdev = st->st_nlink > 1 ? st->st_dev : 0;
    ino_t dedup_hino = st->st_nlink > 1 ? st->st_ino : 0;

    if (dedup_n == dedup_cap) {
        dedup_cap = dedup_cap ? dedup_cap * 2 : 256;
        dedup_tab = realloc(dedup_tab, (size_t)dedup_cap * sizeof(*dedup_tab));
        if (!dedup_tab) { perror("realloc"); exit(1); }
    }
    dedup_tab[dedup_n].path = strdup(path);
    dedup_tab[dedup_n].hdev = dedup_hdev;
    dedup_tab[dedup_n].hino = dedup_hino;
    dedup_tab[dedup_n].size = size;
    dedup_tab[dedup_n].crc  = crc;
    dedup_tab[dedup_n].ino  = ino;
    dedup_n++;
}

/* ------------------------------------------------------------------ */

static struct schfs_vol VOL;
static char             DATABLK[SCHFS_BLKSIZE];
static char             CURBLK[SCHFS_BLKSIZE];

// The volume's three scratch blocks. Statics rather than locals because the
// same code runs in the kernel, where a 4096-byte local is the whole stack —
// keeping the shapes identical is what makes the host the place bugs are found.
static char             CTX_A[SCHFS_BLKSIZE];
static char             CTX_B[SCHFS_BLKSIZE];
static char             CTX_C[SCHFS_BLKSIZE];
static struct schfs_ctx CTX = { CTX_A, CTX_B, CTX_C };

static long nfiles, ndirs, nlinks, nsymlinks, ndevs;
static long long nbytes, ndeduped;

static void
die(const char *what, int r)
{
    fprintf(stderr, "mkschfs: %s failed (%d)%s\n", what, r,
            r == -3 ? " -- image is full, try a larger --size" : "");
    exit(1);
}

static void
inode_init(struct schfs_inode *in, const struct stat *st, uint16_t type,
           uint64_t parent)
{
    memset(in, 0, sizeof(*in));
    in->type   = type;
    in->parent = parent;
    in->nlink  = 1;
    in->mode   = st ? (uint32_t)(st->st_mode & 07777)
                    : (type == SCHFS_T_DIR ? 0755u : 0644u);
    in->uid    = 0;
    in->gid    = 0;
    if (st) {
        // All three come from mtime, deliberately.
        //
        // Copying the host's atime would make the output depend on when the
        // source files were last *read* -- and mkschfs reads every one of them,
        // so building the same tree twice produced two different images. ctime
        // is similar: it tracks host inode changes that mean nothing inside the
        // image. mtime is the only one that describes the content, which is the
        // only thing being copied.
        in->atime = (uint64_t)st->st_mtime;
        in->mtime = (uint64_t)st->st_mtime;
        in->ctime = (uint64_t)st->st_mtime;
    }
}

static void
bump_nlink(uint64_t ino)
{
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    int r = schfs_inode_get(&VOL, &c, ino, &in);
    if (r != 0)
        die("re-reading an inode for a hard link", r);
    in.nlink++;
    // Under v1 this was the bug that shipped broken images: nlink was bumped
    // but the inode's checksum was not recomputed, so every deduped file failed
    // its own integrity check on first open. Here the checksum belongs to the
    // tree node and is recomputed by the insert, so it cannot get out of step.
    r = schfs_inode_put(&VOL, ino, &in);
    if (r != 0)
        die("writing back a hard link count", r);
}

static uint64_t
add_file(uint64_t parent, const char *host, const char *name,
         const struct stat *st)
{
    // A hard link is the same inode by definition, so ask the host before
    // reading anything.
    if (st->st_nlink > 1) {
        uint64_t hit = dedup_lookup_ino(st->st_dev, st->st_ino);
        if (hit) {
            int r = schfs_dir_add(&VOL, parent, name, (unsigned)strlen(name),
                                  hit, SCHFS_T_FILE);
            if (r != 0)
                die("linking a deduplicated file", r);
            bump_nlink(hit);
            nlinks++;
            ndeduped += st->st_size;
            return hit;
        }
    }

    // CRC the content: it is both the dedup key and a cheap way to avoid
    // reading candidates that cannot possibly match.
    FILE *f = fopen(host, "rb");
    if (!f) { perror(host); exit(1); }
    uint32_t crc = 0;
    for (;;) {
        char buf[8192];
        size_t n = fread(buf, 1, sizeof(buf), f);
        if (n == 0) break;
        crc = schfs_crc32c(crc, buf, (unsigned)n);
    }

    uint64_t existing = dedup_lookup(host, st->st_size, crc);
    if (existing) {
        fclose(f);
        int r = schfs_dir_add(&VOL, parent, name, (unsigned)strlen(name),
                              existing, SCHFS_T_FILE);
        if (r != 0)
            die("linking a deduplicated file", r);
        bump_nlink(existing);
        nlinks++;
        ndeduped += st->st_size;
        return existing;
    }

    // The final size is known before a byte is written, so the block count goes
    // into the inode on its first and only write, and the whole run of data
    // blocks is claimed up front. That is what collapses a multi-megabyte file
    // into a single extent record instead of one per 4 KB block.
    uint64_t need = ((uint64_t)st->st_size + SCHFS_BLKSIZE - 1) / SCHFS_BLKSIZE;

    uint64_t ino = schfs_alloc_objectid(&VOL);
    struct schfs_inode in;
    inode_init(&in, st, SCHFS_T_FILE, parent);
    in.blocks = need;
    int r = schfs_inode_put(&VOL, ino, &in);
    if (r != 0) die("writing a file inode", r);

    r = schfs_file_reserve(&VOL, ino, need);
    if (r != 0) die("reserving data blocks", r);

    rewind(f);
    for (;;) {
        // Large chunks on purpose: each append call re-reads and rewrites the
        // inode, and every tree write copies its path, so appending a big file
        // 8 KB at a time spends more space on metadata churn than on data.
        static char buf[256 * 1024];
        size_t n = fread(buf, 1, sizeof(buf), f);
        if (n == 0) break;
        r = schfs_file_append(&VOL, ino, buf, (unsigned)n, DATABLK);
        if (r != 0) die("writing file data", r);
        nbytes += (long long)n;
    }
    fclose(f);

    r = schfs_dir_add(&VOL, parent, name, (unsigned)strlen(name),
                      ino, SCHFS_T_FILE);
    if (r != 0) die("adding a directory entry", r);

    dedup_remember(host, st, st->st_size, crc, ino);
    nfiles++;
    return ino;
}

static void
add_symlink(uint64_t parent, const char *host, const char *name,
            const struct stat *st)
{
    char target[4096];
    ssize_t n = readlink(host, target, sizeof(target) - 1);
    if (n < 0) { perror(host); exit(1); }
    target[n] = '\0';

    uint64_t need = ((uint64_t)n + SCHFS_BLKSIZE - 1) / SCHFS_BLKSIZE;
    uint64_t ino = schfs_alloc_objectid(&VOL);
    struct schfs_inode in;
    inode_init(&in, st, SCHFS_T_SYMLINK, parent);
    in.blocks = need;
    int r = schfs_inode_put(&VOL, ino, &in);
    if (r != 0) die("writing a symlink inode", r);

    // The target is stored as the object's data, exactly like a file's
    // contents, so nothing special is needed to read it back.
    r = schfs_file_reserve(&VOL, ino, need);
    if (r != 0) die("reserving a symlink block", r);
    r = schfs_file_append(&VOL, ino, target, (unsigned)n, DATABLK);
    if (r != 0) die("writing a symlink target", r);

    r = schfs_dir_add(&VOL, parent, name, (unsigned)strlen(name),
                      ino, SCHFS_T_SYMLINK);
    if (r != 0) die("adding a symlink entry", r);
    nsymlinks++;
}

static void
add_dev(uint64_t parent, const char *name, const struct stat *st)
{
    uint64_t ino = schfs_alloc_objectid(&VOL);
    struct schfs_inode in;
    inode_init(&in, st, SCHFS_T_DEV, parent);
    in.rdev_major = (uint16_t)major(st->st_rdev);
    in.rdev_minor = (uint16_t)minor(st->st_rdev);
    int r = schfs_inode_put(&VOL, ino, &in);
    if (r != 0) die("writing a device inode", r);

    r = schfs_dir_add(&VOL, parent, name, (unsigned)strlen(name),
                      ino, SCHFS_T_DEV);
    if (r != 0) die("adding a device entry", r);
    ndevs++;
}

/* Sorted directory listing, so images are reproducible. */
struct namelist { char **v; int n, cap; };

static void
nl_add(struct namelist *l, const char *s)
{
    if (l->n == l->cap) {
        l->cap = l->cap ? l->cap * 2 : 64;
        l->v = realloc(l->v, (size_t)l->cap * sizeof(*l->v));
        if (!l->v) { perror("realloc"); exit(1); }
    }
    l->v[l->n++] = strdup(s);
}

static int
nl_cmp(const void *a, const void *b)
{
    return strcmp(*(char *const *)a, *(char *const *)b);
}

static void add_tree(uint64_t dirino, const char *hostdir);

// Paths kept out of the image, sysroot-relative.
//
// Same rule and same spelling as mkfs.c, because the two tools build the same
// tree and a path excluded from one must be excluded from the other. This is a
// licensing mechanism before it is a size one: the ramdisk image is linked into
// the kernel binary, so anything staged in sysroot/ when it is built becomes
// part of a distributable artifact.
#define MAXEXCLUDE 32
static const char *excludes[MAXEXCLUDE];
static int         nexcludes;
// strlen(sysroot) + 1, so a host path can be turned back into the
// sysroot-relative one the exclude patterns are written against.
static size_t      sysroot_prefix;

// Matches exactly, or as a directory prefix so that "usr/share/misc" also drops
// "usr/share/misc/big.dat".
static int
is_excluded(const char *relpath)
{
    for (int i = 0; i < nexcludes; i++) {
        size_t n = strlen(excludes[i]);
        if (strncmp(relpath, excludes[i], n) == 0 &&
            (relpath[n] == '\0' || relpath[n] == '/'))
            return 1;
    }
    return 0;
}

// Returns 1 if the entry was a directory, so the caller can total up the link
// count without walking the directory a second time.
static int
add_entry(uint64_t dirino, const char *hostdir, const char *name)
{
    char path[8192];
    snprintf(path, sizeof(path), "%s/%s", hostdir, name);

    // Checked before the entry is dispatched, so an excluded directory is never
    // opened and never gets an inode -- an empty directory left behind would
    // still describe what was removed.
    if (nexcludes && strlen(path) > sysroot_prefix &&
        is_excluded(path + sysroot_prefix)) {
        fprintf(stderr, "mkschfs: excluding %s\n", path + sysroot_prefix);
        return 0;
    }

    struct stat st;
    // lstat, not stat: a symlink must be stored as a symlink rather than
    // silently turned into a second copy of whatever it points at.
    if (lstat(path, &st) != 0) { perror(path); exit(1); }

    if (strlen(name) > SCHFS_NAME_MAX) {
        fprintf(stderr, "mkschfs: name too long (max %d): %s\n",
                SCHFS_NAME_MAX, path);
        exit(1);
    }

    if (S_ISDIR(st.st_mode)) {
        uint64_t ino = schfs_alloc_objectid(&VOL);
        struct schfs_inode in;
        inode_init(&in, &st, SCHFS_T_DIR, dirino);
        in.nlink = 2;                /* itself and its own "." */
        int r = schfs_inode_put(&VOL, ino, &in);
        if (r != 0) die("writing a directory inode", r);
        r = schfs_dir_add(&VOL, dirino, name, (unsigned)strlen(name),
                          ino, SCHFS_T_DIR);
        if (r != 0) die("adding a subdirectory entry", r);
        ndirs++;
        add_tree(ino, path);
        return 1;
    }
    if (S_ISLNK(st.st_mode))
        add_symlink(dirino, path, name, &st);
    else if (S_ISCHR(st.st_mode) || S_ISBLK(st.st_mode))
        add_dev(dirino, name, &st);
    else if (S_ISREG(st.st_mode))
        add_file(dirino, path, name, &st);
    else
        fprintf(stderr, "mkschfs: skipping %s (unsupported file type)\n", path);
    return 0;
}

static void
add_tree(uint64_t dirino, const char *hostdir)
{
    DIR *d = opendir(hostdir);
    if (!d) { perror(hostdir); exit(1); }

    struct namelist l = { 0, 0, 0 };
    struct dirent *e;
    while ((e = readdir(d)) != NULL) {
        if (strcmp(e->d_name, ".") == 0 || strcmp(e->d_name, "..") == 0)
            continue;
        nl_add(&l, e->d_name);
    }
    closedir(d);

    qsort(l.v, l.n, sizeof(*l.v), nl_cmp);
    long subdirs = 0;
    for (int i = 0; i < l.n; i++) {
        subdirs += add_entry(dirino, hostdir, l.v[i]);
        free(l.v[i]);
    }
    free(l.v);

    // A directory's link count is 2 (itself, and its own ".") plus one per
    // subdirectory, because each subdirectory's ".." points back here. ".." is
    // synthetic in SchFS, but the count still has to match what POSIX tools
    // expect, or "find -type d" prunes real directories and du walks the tree
    // wrongly. Written last because the count is not known until the children
    // have been added.
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, dirino, &in) != 0)
        die("re-reading a directory inode", -2);
    in.nlink = (uint32_t)(2 + subdirs);
    if (schfs_inode_put(&VOL, dirino, &in) != 0)
        die("writing a directory link count", -2);
}

/* ------------------------------------------------------------------ */

int
main(int argc, char **argv)
{
    const char *out = NULL, *sysroot = NULL, *label = "schos";
    uint64_t nblocks = 6000;

    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "--sysroot") == 0 && i + 1 < argc)
            sysroot = argv[++i];
        else if (strcmp(argv[i], "--size") == 0 && i + 1 < argc)
            nblocks = strtoull(argv[++i], NULL, 0);
        else if (strcmp(argv[i], "--label") == 0 && i + 1 < argc)
            label = argv[++i];
        else if (strcmp(argv[i], "--exclude") == 0 && i + 1 < argc) {
            if (nexcludes >= MAXEXCLUDE) {
                fprintf(stderr, "mkschfs: too many --exclude paths (max %d)\n",
                        MAXEXCLUDE);
                return 1;
            }
            excludes[nexcludes++] = argv[++i];
        }
        else if (strcmp(argv[i], "--hash-bits") == 0 && i + 1 < argc) {
            // Testing only: shrink the name hash so collisions are the norm
            // rather than a once-in-a-lifetime event. schfs_fsck takes the
            // same flag and must be given the same value to read the image.
            int bits = atoi(argv[++i]);
            schfs_hash_mask = bits >= 64 ? ~0ULL : ((1ULL << bits) - 1);
        }
        else if (argv[i][0] == '-') {
            fprintf(stderr, "mkschfs: unknown option %s\n", argv[i]);
            return 1;
        } else if (!out)
            out = argv[i];
        else {
            fprintf(stderr, "mkschfs: too many arguments\n");
            return 1;
        }
    }

    if (!out || !sysroot) {
        fprintf(stderr,
            "usage: mkschfs out.img --sysroot DIR [--size BLOCKS] [--label NAME]\n"
            "                       [--exclude PATH]...\n"
            "  BLOCKS are %d-byte SchFS blocks (default 6000 = %d MB)\n"
            "  --exclude omits a sysroot-relative path and its subtree\n",
            SCHFS_BLKSIZE, (int)(6000 * SCHFS_BLKSIZE / (1024 * 1024)));
        return 1;
    }
    if (nblocks <= SCHFS_FIRST_BLOCK) {
        fprintf(stderr, "mkschfs: --size must exceed %d blocks (boot block "
                "plus the uberblock ring)\n", SCHFS_FIRST_BLOCK);
        return 1;
    }

    FILE *f = fopen(out, "w+b");
    if (!f) { perror(out); return 1; }
    // Pre-extend so every block is readable. The bump allocator hands out
    // blocks in order, but the tree reads a block back the moment it is
    // written, and a short file would fail that read rather than the write.
    if (ftruncate(fileno(f), (off_t)nblocks * SCHFS_BLKSIZE) != 0) {
        perror("ftruncate");
        return 1;
    }

    IMG.f = f;
    IMG.vol = &VOL;
    IMG.nblocks = nblocks;

    struct schfs_bdev bd;
    bd.ctx    = &IMG;
    bd.bread  = img_read;
    bd.bwrite = img_write;
    bd.balloc = img_alloc;
    bd.bfree  = img_free;
    bd.balloc_run = img_alloc_run;

    int r = schfs_vol_format(&VOL, &bd, nblocks, label, &CTX, img_palloc);
    if (r != 0) die("formatting", r);

    // The root directory is an object like any other; nothing points at it
    // except the uberblock's root_objectid.
    struct schfs_inode root;
    struct stat rst;
    int have_rst = (lstat(sysroot, &rst) == 0);
    inode_init(&root, have_rst ? &rst : NULL, SCHFS_T_DIR, 0);
    root.mode  = 0755;
    root.nlink = 2;
    r = schfs_inode_put(&VOL, VOL.root_objectid, &root);
    if (r != 0) die("writing the root inode", r);
    ndirs++;

    // add_entry() builds host paths as "<sysroot>/<name>", so this is where the
    // sysroot-relative part of such a path begins.
    sysroot_prefix = strlen(sysroot) + 1;

    add_tree(VOL.root_objectid, sysroot);

    // Publish. Everything above this line is invisible to a reader; if the tool
    // died half way through, the image would simply have no valid uberblock and
    // be rejected as "not a SchFS volume" rather than mounted half-built.
    r = schfs_vol_commit(&VOL);
    if (r != 0) die("committing the uberblock", r);

    if (fflush(f) != 0 || fclose(f) != 0) { perror(out); return 1; }

    uint64_t used = nblocks - VOL.free_blocks - SCHFS_FIRST_BLOCK;
    printf("mkschfs: %s\n", out);
    printf("  %ld dirs, %ld files, %ld hard links, %ld symlinks, %ld devices\n",
           ndirs, nfiles, nlinks, nsymlinks, ndevs);
    printf("  %lld bytes of data written, %lld bytes saved by dedup\n",
           nbytes, ndeduped);
    printf("  %llu of %llu blocks used (%llu KB of %llu KB), generation %llu\n",
           (unsigned long long)used, (unsigned long long)nblocks,
           (unsigned long long)used * SCHFS_BLKSIZE / 1024,
           (unsigned long long)nblocks * SCHFS_BLKSIZE / 1024,
           (unsigned long long)VOL.generation);
    return 0;
}