Re: F38 proposal: Unified Kernel Support Phase 1 (System-Wide Change proposal)

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On Thu, Dec 22, 2022 at 04:24:11PM +0100, Lennart Poettering wrote:
> On Do, 22.12.22 14:49, Daniel P. Berrangé (berrange@xxxxxxxxxx) wrote:
>
> > When you say it dooesn't get the offsets right, can you elaborate ?
>
> dracut uses fixed offsets for the sections to be placed in memory
> in. The values are simply hardcoded, literally specified address
> offsets, that worked for the original authors. This typically works –
> as long as your sections are not much larger than they were for the
> people wo came up with these offsets initially. But as it turns out
> this doesn't work for some cases. In such cases the sections will be
> loaded into memory overlapped and bad things happen.
>
> ukify hence calculates the offsets manually (by adding up the section
> sizes so that this cannot happen.

The issue was detected in CI [1]. Some code changes made the .text
section bigger, causing other sections to overlap, causing an actual
failure during boot. But it seems that the problem is more widespread and
we were just being lucky ;(  We're figuring out the details,

See the attached program:
$ dracut --uefi /tmp/initrd 6.0.13-300.fc37.x86_64
$ python info.py /tmp/initrd
...
#   4 .rela         000010c8  000000000001f000  000000000001f000  00017f40  2**2
  start=126976 end=131272
#   5 .osrel        000002df  0000000000020000  0000000000020000  00019140  2**2
  start=131072 end=131807
  vma overlap with previous section: 200 bytes
...

I plan to return to this after the holidays.

Zbyszek

[1] https://github.com/systemd/systemd/pull/23706#issuecomment-1354729112
'''\
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00013aa0  0000000000005000  0000000000005000  00000370  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .reloc        0000000a  0000000000019000  0000000000019000  00013f70  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .data         000051a8  000000000001a000  000000000001a000  00014170  2**4
                  CONTENTS, ALLOC, LOAD, DATA
  3 .dynamic      00000100  0000000000020000  0000000000020000  00019370  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  4 .osrel        0000029c  0000000000020000  0000000000020000  00019570  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  5 .rela         000014e8  0000000000021000  0000000000021000  00019970  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  6 .dynsym       00000018  0000000000023000  0000000000023000  0001af70  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  7 .sbat         000000d5  0000000000025980  0000000000025980  0001b170  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  8 .sdmagic      00000027  0000000000025a60  0000000000025a60  0001b370  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  9 .cmdline      00000032  0000000000030000  0000000000030000  0001b570  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
 10 .linux        00c285e8  0000000002000000  0000000002000000  0001b770  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
 11 .initrd       038a76ee  0000000003000000  0000000003000000  00c43d70  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
'''

import subprocess
import sys
dump = subprocess.check_output(['objdump', '-h', sys.argv[1]], text=True)

prev = None

print(dump)

for line in dump.splitlines()[5::2]:
    print(f'# {line}')
    idx, name, size, vma, lma, file_off, align = line.split()

    idx = int(idx)
    size = int(size, 16)
    vma = int(vma, 16)
    lma = int(lma, 16)
    file_off = int(file_off, 16)
    align = eval(align)

    print(f'  start={vma} end={vma + size}')

    if prev:
        gap = file_off - prev[5] - prev[2]
        if gap < 0:
            print(f'  file offset overlap with previous section: {-gap} bytes')

        gap = vma - prev[3] - prev[2]
        if gap < 0:
            print(f'  vma overlap with previous section: {-gap} bytes')

    prev = (idx, name, size, vma, lma, file_off, align)
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