[File] Rework detection of Linux swap files

Sat Dec 21 16:29:35 UTC 2019

Committed, thanks!

christos

> On Dec 21, 2019, at 10:55 AM, Christoph Biedl <astron.com.bwoj at manchmal.in-ulm.de> wrote:
> 
> Hello,
> 
> Currently, file(1) identifies Linux swap files as "Linux/i386",
> "Linux/ia64", or "Linux/ppc". While not completely wrong, it is
> misleading for other architectures since the first is as well reported
> for e.g. "amd64", "armhf", or "powerpc" (aka "ppc32").
> 
> Reason for this: The implementation actually probes the page size of the
> swap file. Which happens to match the description - but it does so for
> other architectures as well. For example, even ppc64 swap files with an
> according page size configuration (PPC_4K_PAGES) will be reported as
> "i386".
> 
> So let's fix this. And while on it, provide full information for all
> page sizes, support all combinations of page size and hibernate file
> type, and mark the old and long-gone format that way.
> 
> Besides the Linux kernel, the util-linux sources provided some
> information for suspend file magic and page sizes - I never encountered
> page sizes of 8k or 32k but it's better to be in sync.
> 
> The rules are limited to swap format v1. Given the fact this has been
> around for more then twenty years, it's sane to assume there will not be
> a v2 format any time soon.
> 
> Gotchas:
> 
> * While the offset of the magic is relative (at the end of the first
>  page size), the other information is absolute at 0x400. The kernel
>  sources mention this and suggest a change in 2.5 :-)  [1]
> 
> * Version number, page size and bad pages count are stored in the host's
>  native endianess. So this patch also fixes a bug here for swap files
>  created on BE machines.
> 
> Tests were done with a bunch of swap files, taken from production and
> hand-crafted[2], on both big- end little-endian machines:
> 
> < swap-04k.be: Linux/i386 swap file (new style), version 16777216 (4K pages), size -16580608 pages, LABEL=swap-04k, UUID=1f9c3545-ba55-4671-9265-313852bd3ebd
>> swap-04k.be: Linux swap file, 4k page size, big endian, version 1, size 1023 pages, 0 bad pages, LABEL=swap-04k, UUID=1f9c3545-ba55-4671-9265-313852bd3ebd
> 
> < swap-04k.le: Linux/i386 swap file (new style), version 1 (4K pages), size 1023 pages, LABEL=swap-04k, UUID=1411ded3-5511-48b9-aa3a-ce9ddd980ca1
>> swap-04k.le: Linux swap file, 4k page size, little endian, version 1, size 1023 pages, 0 bad pages, LABEL=swap-04k, UUID=1411ded3-5511-48b9-aa3a-ce9ddd980ca1
> 
> < swap-04k-hibernate: Linux/i386 swap file (new style) with SWSUSP1 image
>> swap-04k-hibernate: Linux swap file, 4k page size, little endian, version 1, size 1023 pages, 0 bad pages, no label, UUID=a182dcb0-545a-4390-a8e3-f99dd70fc3dd, with SWSUSP1 image
> 
> < swap-08k.be: data
>> swap-08k.be: Linux swap file, 8k page size, big endian, version 1, size 511 pages, 0 bad pages, LABEL=swap-08k, UUID=5c1ca23d-eade-406e-b374-8e45dfedfa6b
> 
> < swap-08k.le: data
>> swap-08k.le: Linux swap file, 8k page size, little endian, version 1, size 511 pages, 0 bad pages, LABEL=swap-08k, UUID=908bb488-c659-4ce6-8c40-ceaa2db60353
> 
> < swap-16k.be: Linux/ia64 swap file
>> swap-16k.be: Linux swap file, 16k page size, big endian, version 1, size 255 pages, 0 bad pages, LABEL=swap-16k, UUID=e77317df-19d1-4d1f-9180-833e97ee18d5
> 
> < swap-16k.le: Linux/ia64 swap file
>> swap-16k.le: Linux swap file, 16k page size, little endian, version 1, size 255 pages, 0 bad pages, LABEL=swap-16k, UUID=ef09b166-f92f-4ab0-970d-5b1e319e3d30
> 
> < swap-32k.be: data
>> swap-32k.be: Linux swap file, 32k page size, big endian, version 1, size 127 pages, 0 bad pages, LABEL=swap-32k, UUID=03a96585-4d2e-4349-b7c6-2ebafc40610d
> 
> < swap-32k.le: data
>> swap-32k.le: Linux swap file, 32k page size, little endian, version 1, size 127 pages, 0 bad pages, LABEL=swap-32k, UUID=f72f0246-9344-40c6-ade8-a249ceec73bf
> 
> < swap-64k.be: Linux/ppc swap file version 16777216, size 1056964608 pages, LABEL=swap-64k, UUID=187cabb6-6478-493c-9f02-e5858b38c3c6
>> swap-64k.be: Linux swap file, 64k page size, big endian, version 1, size 63 pages, 0 bad pages, LABEL=swap-64k, UUID=187cabb6-6478-493c-9f02-e5858b38c3c6
> 
> < swap-64k.le: Linux/ppc swap file version 1, size 63 pages, LABEL=swap-64k, UUID=63caa820-b8f1-4e6b-abe5-a5902dc20424
>> swap-64k.le: Linux swap file, 64k page size, little endian, version 1, size 63 pages, 0 bad pages, LABEL=swap-64k, UUID=63caa820-b8f1-4e6b-abe5-a5902dc20424
> 
> < swap-i386-suspend: Linux/i386 swap file (new style) with SWSUSP1 image
>> swap-i386-suspend: Linux swap file, 4k page size, little endian, version 1, size 262143 pages, 0 bad pages, no label, UUID=3feffb71-ec0b-4723-9e2c-d6c872144d15, with SWSUSP1 image
> 
> < swap-ppc64: Linux/i386 swap file (new style), version 16777216 (4K pages), size -16318464 pages, LABEL=swap-ppc64, UUID=5b94af95-2c8e-451c-9bb3-a0e890184540
>> swap-ppc64: Linux swap file, 4k page size, big endian, version 1, size 2047 pages, 0 bad pages, LABEL=swap-ppc64, UUID=5b94af95-2c8e-451c-9bb3-a0e890184540
> 
> 
> Cheers,
> 
>    Christoph
> 
> [1] Kernel 2.6 was release in December 2003.
> [2] "mkswap --pagesize" allows creation of swap files with page sizes differing from the system value.
> <0001-Rework-detection-of-Linux-swap-files.patch>--
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