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Give userspace a raw disk: /dev/hda and /dev/hdb

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Nothing outside the kernel could touch a disk. sys_mount does not even
look at the device it is handed -- it mounts the one SchFS volume found
at boot and ignores `source` -- so a program had no way to read a
partition table, format a volume or write a boot sector. An installer is
a program that needs all three, so this is the first piece of it.

A character device in devsw rather than a separate block layer, because
that is where the offset already lives: fileread/filewrite pass f->off
and advance it by what comes back, which is everything a seekable device
needs. The minor is the IDE device number, the same numbering iderw()
uses; two schemes for the same disks would be a bug waiting to happen.

Writes bypass every filesystem above them, which is inherent to a raw
device and why an installer must target a disk it has not mounted.

The nodes are created unconditionally, like every other entry in /dev.
That matters more than it looks: with no node, `echo x > /dev/hdb`
silently creates an ordinary file of that name and appears to work. It
did, and it agreed with itself on read-back; only the host's copy of the
disk showed nothing had been written.

st_rdev was never filled, so every node in /dev reported "0, 0" under
ls -l. Harmless while nothing read it, wrong as soon as something has to
tell two disks apart. Filled with the full Linux dev_t encoding rather
than (major << 8 | minor), which only works while both stay small.

Verified against the host as an independent oracle, both directions: the
guest wrote to /dev/hdb and the host saw those bytes at offset 0; the
host wrote at offset 4096 and the guest read them back with dd skip=.
ls -l now shows 1,1 for console, 2,0 for fb0 and 8,1 for hdb.

Known limit, commented in disk.c: devsw passes the offset as a 32-bit
uint, so a raw disk is addressable to 4 GB. That must be widened before
anyone installs onto a real disk.

bootcheck 108 [ok] with -Fb -Tty -Usb -Sock -Evdev -Ps.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-09 22:19:40 +02:00
commit 66152ad2f2
7 changed files with 148 additions and 1 deletions
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17
kernel/sys/sys_fs.c
View file

@ -47,6 +47,17 @@ struct linux_stat {
#define LINUX_S_IFREG 0100000
#define LINUX_S_IFLNK 0120000
// Linux's dev_t: 12 bits of major and 8 of minor packed into the low 32, with
// the overflow of each above that. Spelled out in full rather than as
// (major << 8 | minor), because that is how the other side decodes it and a
// shortcut here would only work while both numbers stayed small.
static uint64
linux_makedev(uint32 major, uint32 minor)
{
return ((uint64)(major & 0xfffu) << 8) | (uint64)(minor & 0xffu)
| ((uint64)(major & ~0xfffu) << 32) | ((uint64)(minor & ~0xffu) << 12);
}
static void
stati_linux(struct inode *ip, struct linux_stat *st)
{
@ -74,6 +85,12 @@ stati_linux(struct inode *ip, struct linux_stat *st)
}
st->st_mode = type_bits | perm_bits;
// Which device this node actually is. Left at zero until now, so every
// entry in /dev printed as "0, 0" under ls -l -- harmless while nothing
// read it, and wrong the moment something has to tell two disks apart.
if (ip->type == T_DEV)
st->st_rdev = linux_makedev(ip->major, ip->minor);
st->st_atime = ip->atime; st->st_mtime = ip->mtime; st->st_ctime = ip->ctime;
}