[A quick top post about synchronous aborts for aarch64.]


> Synchronous exceptions can occur for a number of possible reasons:
>       • Aborts from the MMU. For example, permission failures or memory areas 
> marked as Access flag fault. 
>       • SP and PC alignment checking.
>       • Unallocated instructions. 
>       • Service Calls (SVCs, SMCs and HVCs).
> Such exceptions may be part of the normal operation of the OS. For example, 
> in Linux, when a task wishes to request allocation of a new memory page, this 
> is handled through the MMU abort mechanism.
> In the ARMv7-A architecture, the prefetch abort, Data Abort and undef 
> exceptions are separate items. In AArch64, all of these events generate a 
> Synchronous abort. The exception handler may then read the syndrome and FAR 
> registers to obtain the necessary information to distinguish between them 
> (described in more detail later.)

Mark Millard
markmi at dsl-only.net

On 2016-Oct-17, at 2:11 PM, Shawn Webb <shawn.webb at hardenedbsd.org> wrote:

Hey All,

I'm submitting this report to this list as directed by Ed Maste.

The Raspberry Pi 3 is an ARMv8 SoC dev board. Within the last week,
FreeBSD added basic initial support for it. We don't currently have a
linker in HEAD for the arm64/aarch64 platform. The
projects/clang390-import branch contains lld, though, which supports

Over the past few days, I've been working on testing clang 3.9.0 and lld
on the RPI3. I've jotted down a few issues in FreeBSD's wiki:



low-level code like bootloaders (boot1.efi and loader.efi) do NOT work
with projects/clang390-import. When I use boot1.efi and loader.efi
compiled with HEAD (clang 3.8), they work.

When the kernel is compiled with clang 3.9.0, it seems to freeze after
being loaded by the clang 3.8 boot1.efi/loader.efi. I'm unsure if it's
actually frozen or if simply nothing is being outputted to the console.
Either way, I don't see console messages and the RPI3 _appears_ frozen.

I'm tracking down a potential issue with jemalloc compiled with clang
3.9.0, but I need to verify that the issue exists in vanilla FreeBSD in
addition to HardenedBSD. Most of my testing has been done with
HardenedBSD to-date.

If I find more issues or find the causes of the ones I've already
documented, I'll be sure to not only update the wiki, but update this
email thread as well.

If there's anything anyone would like me to try, please let me know.


Shawn Webb
Cofounder and Security Engineer

GPG Key ID:          0x6A84658F52456EEE
GPG Key Fingerprint: 2ABA B6BD EF6A F486 BE89  3D9E 6A84 658F 5245 6EEE

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