bhyve: error out if fwcfg user file isn't read completely

[corvink]

Summary

At the moment, fwcfg reads the file once at startup and passes these
data to the guest. Therefore, we should always read the whole file.
Otherwise we should error out.

Additionally, GCC12 complains that the comparison whether
fwcfg_file->size is lower than 0 is always false due to the limited
range of data type.

[rew]

usr.sbin/bhyve/qemu_fwcfg.c

602–604

Why is a partial read of the file not an error?

[rew]

usr.sbin/bhyve/qemu_fwcfg.c

602–604

to expand the previous question..

should we ensure that all bytes are read or error out?

and why not use the same pattern that you used in pci_passthru.c using mmap()?

[corvink]

• error out on partial reads

[corvink]

usr.sbin/bhyve/qemu_fwcfg.c

602–604

You're right. Partial reads don't make sense.

What's the advantage of using mmap + memcpy instead of just read?

[rew]

usr.sbin/bhyve/qemu_fwcfg.c

596–597

I don't think this is correct since a signed type is being returned into an unsigned variable.

granted - if there is a read error, the check below will still do the right thing unless the file size happens to be 4G.

602–604

What's the advantage of using mmap + memcpy instead of just read?

I'm not aware of an advantage in using one or over the other in this scenario.

[corvink]

• save read output in a signed variable