Disclaimer: I've never compiled sel4 nor executed sel4 and, at this point, am just a follower of this mailing list (with tentative plans to use sel4 "someday")
Regarding the issue that was fixed here, my first thought was "why did the compiler not complain?". My next thought was "maybe sel4 doesn't enable all warnings in their builds or they ignore the warnings!" But surely a kernel that focuses on security would enable all compiler warnings and fix them immediately as they are introduced.
Turns out it doesn't matter what sel4 builds are doing, the gcc compiler has a bug that prevents warnings for several of these common "might be used uninitialized" cases. See here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501
It also appears this bug will not be fixed! An alternative for some projects, though maybe not sel4, is to *also* compile their code with clang, which does catch these uninitialized cases. If sel4 can use clang to catch these types of mistakes earlier, in a compiler warning, it might save a lengthy validation procedure to catch the error (if it gets caught at all).
Just thought folks might be interested in this gcc bug -- I certainly wasn't aware of it.
Andy
-----Original Message-----
From: Devel On Behalf Of Matthew.Brecknell@data61.csiro.au
Sent: Thursday, November 8, 2018 7:22 PM
To: devel@sel4.systems
Subject: Re: [seL4] seL4 10.1.0 and camkes-3.6.0
Hi,
Now that the fix for this issue has made it through our integration pipeline, I can explain what happened, since some of you might be interested. The fix is here:
https://github.com/seL4/seL4/commit/d12bb374ab47d2b13f438969d2eb5dde1021af84
At line 56, the target variable was originally declared uninitialised.
If the loop at lines 57 to 64 executes at least once, then the variable is initialised at line 58. But if the loop does not execute, then the variable would remain uninitialised when it is used at line 65. This would be the case if infer_cpu_gic_id was ever called with nirqs <= 0.
This is *not* a problem in the released kernel, becuase infer_cpu_gic_id is only ever called once in dist_init, with 32 <= nirqs <= 1024.
However, the binary translation validation test that failed does not do the inter-procedural analysis that would be necessary to realise this.
When looking at infer_cpu_gic_id, the analysis allows nirqs to take any value. It considers that the loop might not execute, and therefore target might be uninitialised when it is used at line 65. The dataflow model that is produced by the analysis cannot model such undefined behaviour, and so the test fails. With the fix, the variable is never uninitialised, so the analysis does not run into this problem.
In summary, the fix you see here keeps our proofs happy, but it does not increase the safety of the kernel. This function, as it is actually used, was already safe.
In terms of our development process, this kind of failure in the binary translation validation is quite rare and difficult to predict. And since our proofs take quite a long time to run, we don't always insist that our kernel developers run the proofs for changes in boot code, which is currently unverified except for binary translation validation. We accept that these failures will occasionally happen, and we deal with them when they do. The functions dist_init and infer_cpu_gic_id are boot-only code, so this was one of those rare failures.
Of course, we should insist on running all the tests for versions we intend to formally release, and we will for future releases.
Best regards,
Matt
________________________________________
From: Brecknell, Matthew (Data61, Kensington NSW)
Sent: Thursday, 8 November 2018 11:25 AM
To: devel@sel4.systems
Cc: Mcleod, Kent (Data61, Kensington NSW)
Subject: Re: seL4 10.1.0 and camkes-3.6.0
Hi,
Due to an oversight on my part, seL4 10.1.0 was released without passing the full verification test suite.
We have no reason to believe that there is any problem in the verified parts of this seL4 version.
The test that failed is part of the binary translation validation, which produces a dataflow graph from the Isabelle/HOL model of the C code, and then attempts to automatically prove a refinement relation between the C and that dataflow graph. The failure occurred in boot code, which in any case is currently not verified. The failure does not mean that there is any problem with the C code. Most likely, it just means that some new code uses some C idiom which is currently not handled by the automatic translation process.
All other components of the verification test suite pass, including security proofs and functional correctness proofs for the Isabelle/HOL model of the C code.
Nevertheless, we intend to rectify this failure as soon as possible, and we'll make further announcements when we understand what happened.
I want to reiterate that this was entirely my fault, and was not at all Kent's fault.
Best regards,
Matt
________________________________________
From: Devel on behalf of Kent.Mcleod@data61.csiro.au
Sent: Thursday, 8 November 2018 8:44 AM
To: devel@sel4.systems
Subject: [seL4] seL4 10.1.0 and camkes-3.6.0
See an online copy of the release notes at:
https://docs.sel4.systems/sel4_release/seL4_10.1.0
https://docs.sel4.systems/camkes_release/Camkes_3.6.0
10.1.0 2018-11-07: SOURCE COMPATIBLE
## Changes
* structures in the boot info are not declared 'packed'
- these were previously packed (in the GCC attribute sense)
- some field lengths are tweaked to avoid padding
- this is a source-compatible change
* ARM platforms can now set the trigger of an IRQ Handler capability
- seL4_IRQControl_GetTrigger allows users to obtain an IRQ Handler capability
and set the trigger (edge or level) in the interrupt controller.
* Initial support for NVIDIA Jetson TX2 (ARMv8a, Cortex A57)
* AARCH64 support added for raspberry pi 3 platform.
* Code generation now use jinja2 instead of tempita.
* AARCH32 HYP support added for running multiple ARM VMs
* AARCH32 HYP VCPU registers updated.
* A new invocation for setting TLSBase on all platforms.
- seL4_TCB_SetTLSBase
* Kbuild/Kconfig/Makefile build system removed.
----
camkes-3.6.0 2018-11-07
Using seL4 version 10.1.0
## Changes
* AARCH64 is now supported.
* CakeML components are now supported.
* Added `query` type to Camkes ADL to allow for querying plugins for component configuration values.
* Components can now make dtb queries to parse device information from dts files.
* Component definitions for serial and timer added on exynos5422, exynos5410, pc99.
* Preliminary support for Isabelle verification of generated capDL.
- See cdl-refine-tests/README for more information
* Simplify and refactor the alignment and section linking policy for generated Camkes binaries.
* Dataports are now required to declare their size in the ADL.
* Templates now use seL4_IRQHandler instead of seL4_IRQControl, which is consistent with the seL4 API.
- This change is BREAKING.
* Remove Kbuild based build system.
* Remove caches that optimised the Kbuild build system, which are not required with the new Cmake build system.
* Added virtqueue infrastructure to libsel4camkes, which allows virtio style queues between components.
## Upgrade Notes
* Any dataport definitions that did not specify a size must be updated to use a size.
* Any template that used seL4_IRQControl must be updated to use seL4_IRQHandler.
* Projects must now use the new Cmake based build system.
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