[Bug 1459872] Re: using progress mod w/ net files crashes

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** Branch linked: lp:ubuntu/wily-proposed/grub2

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https://bugs.launchpad.net/bugs/1459872

Title:
  using progress mod w/ net files crashes

Status in grub2 package in Ubuntu:
  Fix Committed
Status in grub2 source package in Trusty:
  New
Status in grub2 source package in Vivid:
  New
Status in grub2 source package in Wily:
  Fix Committed

Bug description:
  [Impact]
  If the progress module is loaded, GRUB will crash when loading files over the network. A common scenario where this occurs is when building a GRUB image for EFI netbooting w/ all modules included. MAAS does this when generating it's grubnet EFI binary. MAAS currently works around this by explicitly blacklisting the progress module - but only on arm64 (we'd only observed this on arm64 at the time, and didn't yet know the root cause).

  While the easy workaround is to just omit the progress module, it
  isn't at all obvious from the crash that progress.mod is the cause.

  [Test Case]
  The simple case is to create a grub image with *just* the progress module and netboot it. For arm64:

  $ grub-mkimage -v -o grubaa64.efi -O arm64-efi -d
  /home/dannf/x/usr/lib/grub/arm64-efi progress

  Booting the resulting image:

  [1] PXE
  [2] ubuntu
  [3] Shell
  [4] Boot Manager
  Start: 1
  ..

  Synchronous Exception at 0x0000000FF7C3DDC8

  Synchronous Exception at 0x0000000FF7C3DDC8

  Synchronous Exception at 0x0000000FF7C3DDC8

  Synchronous Exception at 0x0000000FF7C3DDC8
  [...]

  [Regression Risk]
  The upstream fix for this is a pretty surgical fix to check for NULL before dereferencing a pointer.

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