stand: userboot: improve userdisk error handling

[novel]

Summary

Currently, userdisk_init() iterates through disks and
checks whether DIOCGSECTORSIZE and DIOCGMEDIASIZE ioctls
are available for the device. If either ioctl fails, userdisk_init()
returns ENXIO with ud_info only partially initialized.
devinit() records the failure, but loader_main() ignores
devinit()'s return value, so later code may access an
uninitialized entry.

In certain cases this code can still boot from a disk even if
userdisk_init() returns ENXIO, for example, when the first of
two disks is valid and the second errors out.

To address this issue while not breaking the current behavior, do
the following:

• Zero-initialize the userdisk_info array and use a zero media size to mark disks that do not support the ioctls mentioned above or report an empty medium.
• Update userdisk_print() not to print unusable disks.
• Update userdisk_open() to return EIO for unusable disks.
• Update extract_currdev() to pick the first usable disk instead of the first disk. Introduce userboot_disk_firstunit() for that.

Additionally, reset counters on ud_info allocation failure to prevent
accessing unallocated data.

[jhb]

My only other thought in addition to the memset() suggestion is maybe treating mediasize == 0 as ud_usable == false vs adding a new field.

stand/userboot/userboot/userboot_disk.c

88–89

Can we use calloc() here, or if not, memset the array to zero after the allocation? That will simplify initialization a bit I think (and ensure the values are always deterministic).

96

With the memset suggestion above you can trim some of the assignments, and possibly refactor as:

if (CALLBACK(...) == 0) {
    ud_info[i].mediasize = mediasize;
    ud_info[i[.sectorsize = sectorsize;
    ud_info[i].ud_usable = true;
    userdisk_usable++;
}

[novel]

• Replace malloc() with calloc() and simplify code a bit.
• Revert adding of the new 'ud_usable' field to the 'userdisk_info' struct, consider disk usable when 'mediasize != 0'.