SchOS/tools/schfs_fsck.c

// SPDX-License-Identifier: Apache-2.0
//
// schfs_fsck — inspect, verify and extract a SchFS image.
//
// "fsck" is a slight misnomer: copy-on-write means there is nothing to repair.
// A crash leaves either the old tree or the new one, so a volume is either
// consistent or it has no valid uberblock at all. What this does is *check* --
// and, just as usefully, read an image back out so its contents can be compared
// against whatever produced it.
//
//   schfs_fsck img --info                 uberblock ring and volume summary
//   schfs_fsck img --check                full structural verification
//   schfs_fsck img --ls PATH              list a directory
//   schfs_fsck img --stat PATH            one object's metadata
//   schfs_fsck img --cat PATH             a file's contents to stdout
//   schfs_fsck img --extract DIR          the whole tree onto the host
//   schfs_fsck img --tree                 every path in the volume
//
// --extract is what makes the round-trip test possible, and the reason this
// reads the image through the same code the kernel will use: if that code is
// wrong, the extracted tree differs from the source tree and diff says so.

#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 <errno.h>
#include <sys/stat.h>
#include <sys/types.h>

/* --extract on Windows.
 *
 * mingw has no POSIX mkdir(path, mode), no symlink() and no link(), because
 * Windows has no equivalent that can be created without either a privilege or
 * a reparse point. Everything else here -- --info, --check, --ls, --stat,
 * --cat, --tree -- is pure reading and works unchanged; it was only --extract
 * that would not compile, which took the whole tool with it and with it
 * `make schfs-check`.
 *
 * Directories map cleanly: _mkdir takes no mode, and Windows has no POSIX
 * permission bits to apply anyway. Symlinks and hard links do not map, so they
 * fail loudly and say why. Silently writing a regular file in their place would
 * produce an extracted tree that differs from the image while reporting
 * success, which is worse than not extracting at all -- the entire point of
 * --extract is comparing the result against what produced it.
 */
#ifdef _WIN32
#include <direct.h>
#define mkdir(path, mode)  _mkdir(path)

static int symlink(const char *target, const char *path)
{
    (void)target;
    fprintf(stderr, "schfs_fsck: %s: cannot create a symlink on Windows\n", path);
    errno = ENOSYS;
    return -1;
}

static int link(const char *from, const char *to)
{
    (void)from;
    fprintf(stderr, "schfs_fsck: %s: cannot create a hard link on Windows\n", to);
    errno = ENOSYS;
    return -1;
}
#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"
// Included for schfs_index_reload, which schfs_vol_open calls at mount. This
// tool never writes, so the rest of attr.c is dead weight here -- but linking
// against a filesystem whose mount path is missing a function is worse.
#include "../kernel/fs/schfs/attr.c"

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

struct imgdev {
    FILE    *f;
    uint64_t nblocks;
};

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;
    return 0;
}

static int
img_write(void *ctx, uint64_t b, const void *buf)
{
    (void)ctx; (void)b; (void)buf;
    return -1;                       /* this tool never writes */
}

static uint64_t img_alloc(void *ctx) { (void)ctx; return 0; }
static void     img_nofree(void *ctx, uint64_t b) { (void)ctx; (void)b; }

// Pages for the allocation map --check derives. Nothing is written back; this
// is a check on the map, not a use of it.
static void *img_palloc(void) { return calloc(1, SCHFS_BLKSIZE); }
static void  img_pfree(void *p) { free(p); }

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

// Read-only, so only the first of these is ever touched — but a volume owns all
// three for its whole life, and handing it two null pointers would turn a
// mistake into a crash rather than a compile error.
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 const char *
typename(uint16_t t)
{
    switch (t) {
    case SCHFS_T_FILE:    return "file";
    case SCHFS_T_DIR:     return "dir";
    case SCHFS_T_DEV:     return "dev";
    case SCHFS_T_SYMLINK: return "symlink";
    default:              return "?";
    }
}

/* ------------------------------------------------------------------ */
/* --info                                                              */
/* ------------------------------------------------------------------ */

static int
do_info(void)
{
    printf("uberblock ring (%d slots from block %d)\n",
           SCHFS_UBER_COUNT, SCHFS_UBER_START);

    int valid = 0, best = -1;
    uint64_t bestgen = 0;
    for (int i = 0; i < SCHFS_UBER_COUNT; i++) {
        struct schfs_uberblock *u = (struct schfs_uberblock *)CTX_A;
        if (img_read(&IMG, SCHFS_UBER_START + i, u) != 0)
            continue;
        if (u->magic != SCHFS_MAGIC)
            continue;
        uint32_t stored = u->crc;
        u->crc = 0;
        uint32_t calc = schfs_crc32c(0, u, sizeof(*u));
        u->crc = stored;
        if (calc != stored) {
            printf("  slot %3d: magic ok, CHECKSUM BAD (generation %llu)\n",
                   i, (unsigned long long)u->generation);
            continue;
        }
        valid++;
        if (best < 0 || u->generation > bestgen) {
            bestgen = u->generation;
            best = i;
        }
    }
    printf("  %d slot%s valid; newest is slot %d at generation %llu\n\n",
           valid, valid == 1 ? "" : "s", best, (unsigned long long)bestgen);

    printf("volume\n");
    printf("  label          %s\n", VOL.label);
    printf("  uuid           ");
    for (int i = 0; i < 16; i++)
        printf("%02x%s", VOL.uuid[i], (i == 3 || i == 5 || i == 7 || i == 9) ? "-" : "");
    printf("\n");
    printf("  block size     %d bytes\n", SCHFS_BLKSIZE);
    printf("  total blocks   %llu (%llu KB)\n",
           (unsigned long long)VOL.total_blocks,
           (unsigned long long)VOL.total_blocks * SCHFS_BLKSIZE / 1024);
    printf("  free blocks    %llu (%llu KB)\n",
           (unsigned long long)VOL.free_blocks,
           (unsigned long long)VOL.free_blocks * SCHFS_BLKSIZE / 1024);
    printf("  tree root      block %llu, generation %llu\n",
           (unsigned long long)VOL.tree.root,
           (unsigned long long)VOL.tree.generation);
    printf("  next object id %llu\n", (unsigned long long)VOL.next_objectid);
    printf("  root object id %llu\n", (unsigned long long)VOL.root_objectid);
    return 0;
}

/* ------------------------------------------------------------------ */
/* --check                                                             */
/* ------------------------------------------------------------------ */

struct tally {
    long inodes, dirents, extents, csums, other;
    long long bytes;
    int  bad;
};

static int
count_rec(const struct schfs_key *k, const void *val, unsigned len, void *arg)
{
    struct tally *t = arg;
    (void)val;
    switch (k->type) {
    case SCHFS_KEY_INODE:
        t->inodes++;
        if (len != sizeof(struct schfs_inode)) {
            fprintf(stderr, "  object %llu: inode record is %u bytes, expected %zu\n",
                    (unsigned long long)k->objectid, len,
                    sizeof(struct schfs_inode));
            t->bad++;
        } else {
            t->bytes += ((const struct schfs_inode *)val)->size;
        }
        break;
    case SCHFS_KEY_DIRENT: t->dirents++; break;
    case SCHFS_KEY_EXTENT:
        t->extents++;
        if (len != sizeof(struct schfs_extent)) {
            fprintf(stderr, "  object %llu: extent record is %u bytes\n",
                    (unsigned long long)k->objectid, len);
            t->bad++;
        }
        break;
    case SCHFS_KEY_CSUM:   t->csums++; break;
    default:               t->other++; break;
    }
    return 0;
}

/* Verify every data block against its stored checksum, by reading every file
   in full. This is the part a metadata-only check would miss entirely. */
static long files_checked, blocks_checked;

// Chain of object ids from the root down to whatever is being checked.
//
// A directory that is its own ancestor makes any recursive walk run until the
// stack runs out, and a segfault is a terrible way to be told the filesystem
// has a loop. Directories cannot be hard-linked, so a repeat in this chain is
// always a fault -- and the first version of mkschfs produced exactly one, by
// handing a subdirectory the root's object id.
#define MAX_DEPTH 64
static uint64_t chain[MAX_DEPTH];
static int      chain_n;

static int
chain_push(uint64_t ino, const char *path)
{
    for (int i = 0; i < chain_n; i++)
        if (chain[i] == ino) {
            fprintf(stderr, "  %s: directory loop -- object %llu is its own "
                    "ancestor\n", path, (unsigned long long)ino);
            return -1;
        }
    if (chain_n >= MAX_DEPTH) {
        fprintf(stderr, "  %s: directory nesting deeper than %d\n",
                path, MAX_DEPTH);
        return -1;
    }
    chain[chain_n++] = ino;
    return 0;
}

static void chain_pop(void) { if (chain_n) chain_n--; }

static int check_dir(uint64_t ino, const char *path);

static int
check_entry(uint64_t ino, uint16_t type, const char *name, unsigned namelen,
            void *arg)
{
    const char *parent = arg;
    char path[8192];
    snprintf(path, sizeof(path), "%s%s%.*s",
             parent, strcmp(parent, "/") == 0 ? "" : "/", (int)namelen, name);

    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, ino, &in) != 0) {
        fprintf(stderr, "  %s: entry points at object %llu, which has no inode\n",
                path, (unsigned long long)ino);
        return -1;
    }
    if (in.type != type)
        fprintf(stderr, "  %s: dirent says %s, inode says %s\n",
                path, typename(type), typename(in.type));

    if (in.type == SCHFS_T_DIR)
        return check_dir(ino, path);

    uint64_t off = 0;
    while (off < in.size) {
        // A full block is requested every time, even at the tail. Clamping
        // here instead would mean nothing ever tests that schfs_file_read
        // stops at end-of-file -- and the kernel's read(2) will depend on
        // exactly that, since a real program just asks for a bufferful.
        char buf[SCHFS_BLKSIZE];
        unsigned want = sizeof(buf);
        long got = schfs_file_read(&VOL, &c, ino, off, buf, want, DATABLK);
        if (got <= 0) {
            fprintf(stderr, "  %s: read failed at offset %llu (%ld)\n",
                    path, (unsigned long long)off, got);
            return -1;
        }
        off += (uint64_t)got;
        blocks_checked++;
    }
    files_checked++;
    return 0;
}

static int
check_dir(uint64_t ino, const char *path)
{
    if (chain_push(ino, path) != 0)
        return -1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    int r = schfs_dir_iterate(&VOL, &c, ino, check_entry, (void *)path);
    chain_pop();
    return r;
}

static int
do_check(void)
{
    printf("checking %s\n", VOL.label);

    if (schfs_btree_check(&VOL.tree) != 0) {
        fprintf(stderr, "  FAIL: tree structure is invalid\n");
        return 1;
    }
    printf("  [ok] tree structure, checksums and key ordering\n");

    struct tally t;
    memset(&t, 0, sizeof(t));
    if (schfs_btree_iterate(&VOL.tree, count_rec, &t) != 0) {
        fprintf(stderr, "  FAIL: could not walk every record\n");
        return 1;
    }
    if (t.bad) {
        fprintf(stderr, "  FAIL: %d malformed records\n", t.bad);
        return 1;
    }
    printf("  [ok] %ld inodes, %ld dirent items, %ld extents, %ld data checksums\n",
           t.inodes, t.dirents, t.extents, t.csums);

    if (check_dir(VOL.root_objectid, "/") != 0) {
        fprintf(stderr, "  FAIL: walking the directory tree found a problem\n");
        return 1;
    }
    printf("  [ok] %ld files read end to end, every data block matched its checksum\n",
           files_checked);

    // Rebuild the allocation map the way a mount would, and hold it against the
    // count the writer left in the uberblock. The map is derived rather than
    // stored, so nothing else would ever notice if the scan and the writer
    // disagreed — right up until a mounted volume handed out a block that was
    // still in use.
    if (schfs_space_init(&VOL.space, VOL.total_blocks, img_palloc, img_pfree) != 0) {
        fprintf(stderr, "  FAIL: could not build an allocation map\n");
        return 1;
    }
    if (schfs_space_scan(&VOL) != 0) {
        fprintf(stderr, "  FAIL: could not walk the tree to derive free space\n");
        return 1;
    }
    if (VOL.space.free_blocks != VOL.free_blocks) {
        fprintf(stderr, "  FAIL: uberblock says %llu blocks free, walking the "
                "tree says %llu\n",
                (unsigned long long)VOL.free_blocks,
                (unsigned long long)VOL.space.free_blocks);
        return 1;
    }
    printf("  [ok] free space derived from the tree matches the uberblock "
           "(%llu of %llu blocks in use)\n",
           (unsigned long long)(VOL.total_blocks - VOL.space.free_blocks),
           (unsigned long long)VOL.total_blocks);
    return 0;
}

/* ------------------------------------------------------------------ */
/* --ls / --stat / --cat / --tree                                      */
/* ------------------------------------------------------------------ */

static int
print_entry(uint64_t ino, uint16_t type, const char *name, unsigned namelen,
            void *arg)
{
    (void)arg;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, ino, &in) != 0) {
        printf("  ?          %.*s\n", (int)namelen, name);
        return 0;
    }
    printf("  %-7s %6llu %4o %2u links  %.*s\n",
           typename(type), (unsigned long long)in.size, in.mode & 07777,
           in.nlink, (int)namelen, name);
    return 0;
}

static uint64_t
resolve(const char *path)
{
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    uint64_t ino = schfs_path_lookup(&VOL, &c, path);
    if (ino == 0)
        fprintf(stderr, "schfs_fsck: %s: no such file or directory\n", path);
    return ino;
}

static int
do_ls(const char *path)
{
    uint64_t ino = resolve(path);
    if (!ino)
        return 1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    printf("%s\n", path);
    return schfs_dir_iterate(&VOL, &c, ino, print_entry, NULL) != 0;
}

static int
do_stat(const char *path)
{
    uint64_t ino = resolve(path);
    if (!ino)
        return 1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, ino, &in) != 0) {
        fprintf(stderr, "schfs_fsck: %s: inode unreadable\n", path);
        return 1;
    }
    printf("path:  %s\n", path);
    printf("objectid: %llu\n", (unsigned long long)ino);
    printf("type:  %s\n", typename(in.type));
    printf("size:  %llu\n", (unsigned long long)in.size);
    printf("blocks: %llu\n", (unsigned long long)in.blocks);
    printf("mode:  %04o\n", in.mode & 07777);
    printf("nlink: %u\n", in.nlink);
    printf("uid:   %u\n", in.uid);
    printf("gid:   %u\n", in.gid);
    printf("mtime: %llu\n", (unsigned long long)in.mtime);
    printf("parent: %llu\n", (unsigned long long)in.parent);
    if (in.type == SCHFS_T_DEV)
        printf("rdev:  %u:%u\n", in.rdev_major, in.rdev_minor);
    return 0;
}

static int
do_cat(const char *path)
{
    uint64_t ino = resolve(path);
    if (!ino)
        return 1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, ino, &in) != 0)
        return 1;

    uint64_t off = 0;
    while (off < in.size) {
        // A full block is requested every time, even at the tail. Clamping
        // here instead would mean nothing ever tests that schfs_file_read
        // stops at end-of-file -- and the kernel's read(2) will depend on
        // exactly that, since a real program just asks for a bufferful.
        char buf[SCHFS_BLKSIZE];
        unsigned want = sizeof(buf);
        long got = schfs_file_read(&VOL, &c, ino, off, buf, want, DATABLK);
        if (got <= 0) {
            fprintf(stderr, "schfs_fsck: %s: read error at %llu\n",
                    path, (unsigned long long)off);
            return 1;
        }
        fwrite(buf, 1, (size_t)got, stdout);
        off += (uint64_t)got;
    }
    return 0;
}

static int tree_walk(uint64_t ino, const char *path);

static int
tree_entry(uint64_t ino, uint16_t type, const char *name, unsigned namelen,
           void *arg)
{
    const char *parent = arg;
    char path[8192];
    snprintf(path, sizeof(path), "%s%s%.*s",
             parent, strcmp(parent, "/") == 0 ? "" : "/", (int)namelen, name);
    printf("%-8s %s\n", typename(type), path);
    if (type == SCHFS_T_DIR)
        return tree_walk(ino, path);
    return 0;
}

static int
tree_walk(uint64_t ino, const char *path)
{
    if (chain_push(ino, path) != 0)
        return -1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    int r = schfs_dir_iterate(&VOL, &c, ino, tree_entry, (void *)path);
    chain_pop();
    return r;
}

/* ------------------------------------------------------------------ */
/* --extract                                                           */
/* ------------------------------------------------------------------ */

// Objects already written out, so a hard link on the image becomes a hard link
// on the host rather than a second copy. Without this the round-trip test's
// link-count check could not distinguish dedup from duplication.
struct seen_ent { uint64_t ino; char *path; };
static struct seen_ent *seen;
static int seen_n, seen_cap;

static const char *
seen_lookup(uint64_t ino)
{
    for (int i = 0; i < seen_n; i++)
        if (seen[i].ino == ino)
            return seen[i].path;
    return NULL;
}

static void
seen_add(uint64_t ino, const char *path)
{
    if (seen_n == seen_cap) {
        seen_cap = seen_cap ? seen_cap * 2 : 128;
        seen = realloc(seen, (size_t)seen_cap * sizeof(*seen));
        if (!seen) { perror("realloc"); exit(1); }
    }
    seen[seen_n].ino  = ino;
    seen[seen_n].path = strdup(path);
    seen_n++;
}

static long xfiles, xdirs, xlinks, xsymlinks;

static int extract_dir(uint64_t ino, const char *hostpath);

static int
extract_entry(uint64_t ino, uint16_t type, const char *name, unsigned namelen,
              void *arg)
{
    const char *parent = arg;
    char host[8192];
    (void)type;                      /* the inode is authoritative, not the dirent */
    snprintf(host, sizeof(host), "%s/%.*s", parent, (int)namelen, name);

    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    struct schfs_inode in;
    if (schfs_inode_get(&VOL, &c, ino, &in) != 0) {
        fprintf(stderr, "schfs_fsck: %s: inode %llu unreadable\n",
                host, (unsigned long long)ino);
        return -1;
    }

    if (in.type == SCHFS_T_DIR) {
        if (mkdir(host, in.mode & 07777) != 0 && errno != EEXIST) {
            perror(host);
            return -1;
        }
        xdirs++;
        return extract_dir(ino, host);
    }

    if (in.type == SCHFS_T_SYMLINK) {
        char target[4096];
        if (in.size >= sizeof(target)) {
            fprintf(stderr, "schfs_fsck: %s: symlink target too long\n", host);
            return -1;
        }
        long got = schfs_file_read(&VOL, &c, ino, 0, target,
                                   (unsigned)in.size, DATABLK);
        if (got < 0 || (uint64_t)got != in.size) {
            fprintf(stderr, "schfs_fsck: %s: could not read symlink target\n", host);
            return -1;
        }
        target[in.size] = '\0';
        unlink(host);
        if (symlink(target, host) != 0) { perror(host); return -1; }
        xsymlinks++;
        return 0;
    }

    if (in.type == SCHFS_T_DEV) {
        // Creating a real device node needs privileges the build does not have,
        // so record it as an empty placeholder. The round-trip tree contains no
        // device nodes; --check is what validates them.
        FILE *f = fopen(host, "wb");
        if (!f) { perror(host); return -1; }
        fclose(f);
        return 0;
    }

    // A second name for an object already written: make it a real hard link so
    // the extracted tree has the same link structure as the image.
    const char *first = seen_lookup(ino);
    if (first) {
        unlink(host);
        if (link(first, host) != 0) { perror(host); return -1; }
        xlinks++;
        return 0;
    }

    FILE *f = fopen(host, "wb");
    if (!f) { perror(host); return -1; }
    uint64_t off = 0;
    while (off < in.size) {
        // A full block is requested every time, even at the tail. Clamping
        // here instead would mean nothing ever tests that schfs_file_read
        // stops at end-of-file -- and the kernel's read(2) will depend on
        // exactly that, since a real program just asks for a bufferful.
        char buf[SCHFS_BLKSIZE];
        unsigned want = sizeof(buf);
        long got = schfs_file_read(&VOL, &c, ino, off, buf, want, DATABLK);
        if (got <= 0) {
            fprintf(stderr, "schfs_fsck: %s: read error at %llu\n",
                    host, (unsigned long long)off);
            fclose(f);
            return -1;
        }
        fwrite(buf, 1, (size_t)got, f);
        off += (uint64_t)got;
    }
    fclose(f);
    if (chmod(host, in.mode & 07777) != 0)
        perror(host);
    seen_add(ino, host);
    xfiles++;
    return 0;
}

static int
extract_dir(uint64_t ino, const char *hostpath)
{
    if (chain_push(ino, hostpath) != 0)
        return -1;
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);
    int r = schfs_dir_iterate(&VOL, &c, ino, extract_entry, (void *)hostpath);
    chain_pop();
    return r;
}

static int
do_extract(const char *dir)
{
    if (mkdir(dir, 0755) != 0 && errno != EEXIST) {
        perror(dir);
        return 1;
    }
    if (extract_dir(VOL.root_objectid, dir) != 0)
        return 1;
    printf("extracted %ld dirs, %ld files, %ld hard links, %ld symlinks into %s\n",
           xdirs, xfiles, xlinks, xsymlinks, dir);
    return 0;
}

/* --------------------------------------------------------------------- */
/* --snapshots                                                            */
/*                                                                        */
/* A snapshot is a retained tree root, stored as an ordinary record under  */
/* SCHFS_SNAP_OBJECTID. Listing them is a range scan over that objectid.   */
/* --------------------------------------------------------------------- */

static int
do_snapshots(void)
{
    struct schfs_cursor c;
    schfs_cursor_init(&c, &VOL.tree, CURBLK);

    struct schfs_key k;
    schfs_mkkey(&k, SCHFS_SNAP_OBJECTID, SCHFS_KEY_SNAP, 0);

    int r = schfs_cursor_seek(&c, &k);
    if (r == -2) {
        fprintf(stderr, "schfs_fsck: tree is unreadable\n");
        return 1;
    }

    int n = 0;
    while (r == 0) {
        const struct schfs_key *got = schfs_cursor_key(&c);
        if (!got || got->objectid != SCHFS_SNAP_OBJECTID ||
            got->type != SCHFS_KEY_SNAP)
            break;

        unsigned len;
        const struct schfs_snap *sn =
            (const struct schfs_snap *)schfs_cursor_val(&c, &len);
        if (sn && len == sizeof(*sn)) {
            if (n == 0)
                printf("%-6s  %-24s  %-12s  %s\n",
                       "id", "name", "root", "created at");
            // The name is bounded but not guaranteed terminated by a bad
            // image, so it is printed with an explicit precision.
            printf("%-6llu  %-24.*s  %-12llu  generation %llu\n",
                   (unsigned long long)got->offset,
                   SCHFS_SNAP_NAME_MAX, sn->name,
                   (unsigned long long)sn->root,
                   (unsigned long long)sn->created);
            n++;
        }
        r = schfs_cursor_next(&c);
    }

    if (n == 0)
        printf("no snapshots\n");
    return 0;
}


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

int
main(int argc, char **argv)
{
    if (argc < 3) {
        fprintf(stderr,
            "usage: schfs_fsck img.schfs <action>\n"
            "  --info            uberblock ring and volume summary\n"
            "  --check           full structural and data verification\n"
            "  --ls PATH         list a directory\n"
            "  --stat PATH       one object's metadata\n"
            "  --cat PATH        a file's contents\n"
            "  --tree            every path in the volume\n"
            "  --extract DIR     write the whole tree onto the host\n"
            "  --snapshots       list the retained tree roots\n");
        return 2;
    }

    const char *imgpath = argv[1];
    const char *action  = argv[2];
    const char *arg     = argc > 3 ? argv[3] : NULL;

    // Must match whatever mkschfs used, or every directory lookup misses.
    for (int i = 3; i + 1 < argc; i++)
        if (strcmp(argv[i], "--hash-bits") == 0) {
            int bits = atoi(argv[i + 1]);
            schfs_hash_mask = bits >= 64 ? ~0ULL : ((1ULL << bits) - 1);
            if (arg == argv[i])
                arg = NULL;
        }

    FILE *f = fopen(imgpath, "rb");
    if (!f) { perror(imgpath); return 1; }
    if (fseeko(f, 0, SEEK_END) != 0) { perror(imgpath); return 1; }
    off_t sz = ftello(f);

    IMG.f = f;
    IMG.nblocks = (uint64_t)sz / SCHFS_BLKSIZE;

    struct schfs_bdev bd;
    memset(&bd, 0, sizeof(bd));      /* optional hooks must read as absent */
    bd.ctx    = &IMG;
    bd.bread  = img_read;
    bd.bwrite = img_write;
    bd.balloc = img_alloc;
    bd.bfree  = img_nofree;
    bd.balloc_run = NULL;            /* read-only: nothing is ever allocated */

    // No page allocator: this tool never writes, and a volume without an
    // allocation map cannot allocate even by accident.
    int r = schfs_vol_open(&VOL, &bd, &CTX, NULL, NULL);
    if (r == -1) {
        fprintf(stderr, "schfs_fsck: %s: no valid uberblock -- not a SchFS "
                "volume, or never successfully written\n", imgpath);
        return 1;
    }
    if (r == -3) {
        fprintf(stderr, "schfs_fsck: %s: written by a newer SchFS than this "
                "tool understands (this tool: version %d)\n",
                imgpath, SCHFS_VERSION);
        fprintf(stderr, "schfs_fsck: refusing -- older uberblocks may be "
                "readable, but they are a stale filesystem\n");
        return 1;
    }
    if (r != 0) {
        fprintf(stderr, "schfs_fsck: %s: newest uberblock names an unreadable "
                "tree root\n", imgpath);
        return 1;
    }

    if (strcmp(action, "--info") == 0)
        return do_info();
    if (strcmp(action, "--check") == 0)
        return do_check();
    if (strcmp(action, "--tree") == 0)
        return tree_walk(VOL.root_objectid, "");
    if (strcmp(action, "--ls") == 0 && arg)
        return do_ls(arg);
    if (strcmp(action, "--stat") == 0 && arg)
        return do_stat(arg);
    if (strcmp(action, "--cat") == 0 && arg)
        return do_cat(arg);
    if (strcmp(action, "--extract") == 0 && arg)
        return do_extract(arg);
    if (strcmp(action, "--snapshots") == 0)
        return do_snapshots();

    fprintf(stderr, "schfs_fsck: unknown or incomplete action '%s'\n", action);
    return 2;
}