signpost: GPT priority manipulation utility

[iliana]

Description of changes

This introduces signpost, Thar's utility for modifying GPT priority bits on the OS disk.

signpost is a small but important part of the overall Thar update story, and builds on the gptprio patchset to GRUB.

I think I have a small expectation that over the next couple of months, this will morph into a library that happens to have a low-level utility binary. But for now, this is what we will use.

Testing

Initial boot:

localhost login: root
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=1 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=0 tries_left=0 successful=false
Active:  Set A
Next:    Set A
bash-5.0# signpost clear-inactive
bash-5.0# dd if=/dev/vda2 of=/dev/vda6
40960+0 records in
40960+0 records out
20971520 bytes (21 MB, 20 MiB) copied, 0.337728 s, 62.1 MB/s
bash-5.0# dd if=/dev/vda3 of=/dev/vda7 conv=sparse
1843200+0 records in
1843200+0 records out
943718400 bytes (944 MB, 900 MiB) copied, 3.17942 s, 297 MB/s
bash-5.0# dd if=/dev/vda4 of=/dev/vda8
16384+0 records in
16384+0 records out
8388608 bytes (8.4 MB, 8.0 MiB) copied, 0.242723 s, 34.6 MB/s
bash-5.0# signpost upgrade-to-inactive
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=1 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=2 tries_left=1 successful=false
Active:  Set A
Next:    Set B
bash-5.0# reboot

Next boot:

localhost login: root
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=1 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=2 tries_left=0 successful=false
Active:  Set B
Next:    Set A
bash-5.0# signpost mark-successful-boot
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=1 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=2 tries_left=0 successful=true
Active:  Set B
Next:    Set B
bash-5.0# signpost rollback-to-inactive
[   46.962730] random: signpost: uninitialized urandom read (16 bytes read)
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=2 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=1 tries_left=0 successful=true
Active:  Set B
Next:    Set A
bash-5.0# reboot

Next boot:

localhost login: root
bash-5.0# signpost status
OS disk: /dev/vda
Set A:   boot=/dev/vda2 root=/dev/vda3 hash=/dev/vda4 priority=2 tries_left=0 successful=true
Set B:   boot=/dev/vda6 root=/dev/vda7 hash=/dev/vda8 priority=1 tries_left=0 successful=true
Active:  Set A
Next:    Set A

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[jahkeup]

Lot's of cool stuff in here! I have some questions and comments that I had while reading, but I'm not very astute in rust just yet, so I ask partially in ignorance.

[tjkirch]

Nice, there's some very clean and manageable code here. 🍔

[tjkirch]

Random thought: Do you think it'd be hard to extend signpost to handle partition sets being on different disks? It should just be different pointers in grub, and here, things like searching for partitions on the single "os_disk"...

[iliana]

Do you think it'd be hard to extend signpost to handle partition sets being on different disks? It should just be different pointers in grub, and here, things like searching for partitions on the single "os_disk"...

I think it's more difficult than what I'd like to implement right now.