[Bug 2047450] Re: tail emits no output for sysfs files when using large page kernels

[Andreas Hasenack]

The migration-reference/0 runs passed, to the dep8 results are still not
good. I retriggered the failing arm64 tests one more time.

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Title:
  tail emits no output for sysfs files when using large page kernels

Status in coreutils package in Ubuntu:
  Fix Released
Status in coreutils source package in Jammy:
  Fix Committed
Status in coreutils source package in Lunar:
  Won't Fix
Status in coreutils source package in Mantic:
  Fix Committed
Status in coreutils source package in Noble:
  Fix Released

Bug description:
  [Impact]
  Ubuntu provides 64K page size kernels for ppc64el (always) and arm64 (optional -64k flavors). When booted on 64K kernels, tail emits no output when tailing a sysfs file. The difference in behavior can be a source for bugs in scripts that use tail, and general user confusion.

  [Test Plan]
  The upstream fix includes a test case that tails the /sys/kernel/profiling file, if it exists. That case would fail with an unfixed coreutils package as shown below:

  = When booted on a 4K kernel =
  ubuntu at gunyolk:~$ tail /sys/kernel/profiling 
  0

  = When booted on a 64K kernel =
  ubuntu at gunyolk:~$ tail /sys/kernel/profiling 
  ubuntu at gunyolk:~$ 

  Since the upstream test cases are executed at build time, the existing
  tests and this new test will be used  to regression test behavior.
  This should cover both 4K (!ppc64el) and 64K (ppc64el) cases. We
  should also do a manual verification on arm64 w/ the 64K kernel since
  that case is not covered by our builders.

  [Where Problems Could Occur]
  The biggest risk for a regression I see is due to the side-effect of the fix now allocating a dynamic buffer instead of the stack. An error in logic there could cause a crash or a memory leak in scenarios undetected during testing. I used valgrind when developing the fix to derisk the memory leak scenario.

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