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Re: [GIT PULL 0/1] EFI mixed mode fix for v4.18

* Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:

> On 11 July 2018 at 12:13, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> >
> >> The following changes since commit 1e4b044d22517cae7047c99038abb444423243ca:
> >>
> >>   Linux 4.18-rc4 (2018-07-08 16:34:02 -0700)
> >>
> >> are available in the Git repository at:
> >>
> >>   git://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git tags/efi-urgent
> >>
> >> for you to fetch changes up to d7f2e972e702d329fe11d6956df99dfc31211c25:
> >>
> >>   efi/x86: remove pointless call to PciIo->Attributes() (2018-07-11 10:52:46 +0200)
> >>
> >> ----------------------------------------------------------------
> >> A single fix for the x86 PCI I/O protocol handling code that got
> >> broken for mixed mode (64-bit Linux/x86 on 32-bit UEFI) after a
> >> fix was applied in -rc2 to fix it for ordinary 64-bit Linux/x86.
> >
> > Just curious, because it's unclear from the changelog, what was the symptom, a
> > boot hang, instant reboot, or some other misbehavior?
> Hans reported that his mixed mode tablet would not boot at all any
> more, but enter a reboot loop without any logs printed by the kernel.
> > Also, what's the scope of
> > the fix: were all 64-bit on 32-bit UEFI mixed-mode bootups affected, or only a
> > certain subset?
> >
> Any mixed mode system with PCI is likely to be affected. I have added
> a QEMU mixed mode config to my boot test environment to catch errors
> like this one.

Ok, I've added this information to the commit - will be useful to backporters,
to judge the severity of the bug fixed.

> The unfortunate thing here is that this uncovered a fundamental issue with mixed 
> mode, i.e., that any UEFI protocol prototype involving 64-bit by-value 
> parameters needs to be special cased in the stub code, which is rather tedious. 
> There is one other call that is potentially affected, a file open call in the 
> initrd handling code, but that specific occurrence happens to work unmodified. 
> This patch removes the other one. Going forward, we will have to carefully 
> review UEFI protocol invocations for mixed mode compatibility.

Yeah. Is there any, more systematic way to detect such problems perhaps at an 
earlier stage, other than careful review which will often fail to find such bugs? 
Also, testing is good, but could we perhaps do something on a deeper level - 
automate the casting, generate a warning on suspicious patterns, etc. etc?