UEFI booting survey

Warner Losh imp at bsdimp.com
Wed Dec 20 03:26:36 UTC 2017

On Dec 19, 2017 4:26 PM, "Mark Millard" <markmi at dsl-only.net> wrote:

On 2017-Dec-19, at 1:49 PM, Warner Losh <imp at bsdimp.com> wrote:

> On Dec 19, 2017 10:58 AM, "Mark Millard" <markmi at dsl-only.net> wrote:
>> On 2017-Dec-18, at 2:37 PM, Warner Losh <imp at bsdimp.com> wrote:
>> > . . .
>> >
>> > Or the following pseudo-code with all the weird special cases removed
for clarity
>> >
>> > load loader.efi from ESP
>> > if BootXXXX uefi variable holds a second path, use that for root/kernel
>> > otherwise if an override variable holds a kernel/root path, use that
>> > otherwise scan for a usable ZFS pool, use that if it exists
>> > otherwise use the same partition loader.efi was booted from for
root/kernel if it's usable
>> > otherwise use the first UFS partition on the ESP that's usable.
>> >
>> > A partition is usable if /boot/loader.rc exists on that path.
>> What will be the role of /etc/fstab in establishing
>> were the kernel is loaded from? Where world is
>> loaded from? Where/how does use of /etc/fstab for
>> specifying the root file system mount fit in the
>> above pseudo-code?
> Same as today: it is what the boot loader passes to the kernel as the
Unix name of /. I have no plans to change that. It's of almost no use to
the boot loader, since it can't know what BIOS device da3 is, for example,
if that's in fstab. Or even more complex examples like /dev/mirror/primary.
Efibootmgr can take Unix devices and paths and turn them into UEFI paths so
we know what devices to use for what. In the absence of those, or an
equivalent fallback, we are quite limited in what we can do since we don't
have the context needed to translate.
> Warner


It sounds different then the results I get with ubldr.bin
on a rpi2 V1.1 . With the usdcard having a UFS / with
basically only:

/etc/fstab (redirecting to a USB SSD stick)
/boot/* (with /boot/kernel/ empty and /boot/dtb/ empty)

the result is that all 3 of the following come from the
USB SSD stick based on the "/" line from the /etc/fstab
from the usdcard:

/ (mounted root file system)

In other words: it appears that for ubldr.bin on
a rpi2 V1.1 /etc/fstab is read and used before
finding the kernel that is to be loaded. (It or
another /etc/fstab may be read again later.)

It is read. It is literally only used to set the root for userland. That is
all. Nothing else.

/usr/src/stand/common/boot.c does show an explicit
attempt to find a /etc/fstab:

# grep -r /etc/fstab /usr/src/stand/
. . .
/usr/src/stand/common/boot.c: * Try to find the /etc/fstab file on the
filesystem (rootdev),
/usr/src/stand/common/boot.c:    sprintf(lbuf, "%s/etc/fstab", rootdev);
. . .

That is from getrootmount(char *rootdev):

getrootmount(char *rootdev)
    char        lbuf[128], *cp, *ep, *dev, *fstyp, *options;
    int         fd, error;

    if (getenv("vfs.root.mountfrom") != NULL)

So if you set vfs.root.mountfrom in /boot/loader.conf, we don't read
rootdev:/etc/fstab for the value to set vfs.root.mountfrom to.

    error = 1;
    sprintf(lbuf, "%s/etc/fstab", rootdev);
    if ((fd = open(lbuf, O_RDONLY)) < 0)
        goto notfound;
. . .

Supporting detail for the example rpi2
boot context:

With /mnt being the / from the usdcard:

# find /mnt -print | more

# more /mnt/etc/fstab
/dev/da0p1      /               ufs     rw,noatime      1 1
/dev/da0p2      none            swap    sw              0 0

May be this is somehow special to the rpi2 or to
ubldr.bin operation. (I've never managed to identify
accidents from deliberately working status in this

So you load /boot/loader and the kernel from the sdcard.  /etc/fstab on the
card points to the usb drive for /. This is all standard loader behavior.
It won't change. In your case, you aren't even using UEFI, so it doubly
won't change. UEFI has more direct ways of doing this, but this setup would
work there because it is explicit. It doesn't depend on the current
boot1.efi behavior...


>> (For my particular interest the context uses UFS, not
>> ZFS.)
>> > What is being deleted is one final step: "otherwise use the first UFS
partition on any drive in a random order that's usable." which used to be
at the end of the boot1.efi psuedo code. It's my belief that no such
installations actually use this due to the random factor today (plug in a
new USB drive and it might take over). If my belief is wrong, it's my
belief that efibootmgr will solve it, and failing that, the fallback
mechanism (for platforms that use u-boot + EFI where UEFI variables don't
work) will allow the two or three people that are doing this today.

Mark Millard
markmi at dsl-only.net

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