Re: Accessing ESR from signal handlers on AArch64

> On Feb 27, 2026, at 10:00 AM, Adrian Chadd <[email protected]> wrote:
> 
> (fwiw, i recommend "silence" as a response being "noone's doing it,
> please go do it and ask questions" :-) )

Noted, thank you! I've opened a pull request on GitHub with the described
changes: https://github.com/freebsd/freebsd-src/pull/2053. If it would be
more conducive to review/discussion to have that in Phabricator rather than
GitHub, I can close that pull request and resubmit to Phabricator, though
it seems Andrew Turner was already auto-pinged for review on GitHub.

> -a
> 
> On Wed, 25 Feb 2026 at 12:09, Alex Arslan <[email protected]> wrote:
>> 
>> Hello,
>> 
>> Currently there's no way to access the exception syndrome register (ESR)
>> on AArch64 from a signal handler. Nearly 10 years ago, this was "expected
>> to be fixed soon"[1]; is that still on anybody's radar? In 2019, a patch
>> was submitted by a contributor to expose ESR via `mcontext_t`[2], but that
>> has had no comments to date. I reached out to the author and it sounds
>> like she isn't planning to push on that revision. I would be happy to help
>> make this happen, but I would need some handholding, and I don't want to
>> duplicate effort if somebody else is already planning to take it on.
>> 
>> From what I understand, the way to go about this would be to:
>> 
>> - Update `struct __mcontext` to include a new `__register_t` field `mc_esr`
>> - Reduce the size of `mc_spare` by 1 to account for the addition
>> - Put `mc_esr` between `mc_ptr` and `mc_spare` to avoid affecting the ABI
>> - Define a new bitmask `_MC_ESR_VALID` to set for `mc_flags`, analogous to
>>  the existing `_MC_FP_VALID` for `mc_fpregs`
>> - In `sendsig`, populate the `struct trapframe` field `tf_esr` with the
>>  register value using `READ_SPECIALREG(esr_el1)`
>> - In `get_mcontext`, set `mc_esr` to `tf_esr` and update `mc_flags`
>> - In `set_mcontext`, set `tf_esr` to `mc_esr` if indicated by `mc_flags`
>> 
>> Is that correct? Are there any steps or changes there that are missing?
>> 
>> I'm not clear on whether it's appropriate to do so in the FreeBSD source
>> tree or if it should first go into upstream LLVM, but the compiler-rt
>> sanitizers could be updated to grab `mc_esr` via `ucontext_t` on FreeBSD
>> so that write vs. read faults may be differentiated.
>> 
>> The Julia programming language performs a similar query to the sanitizers,
>> checking ESR on AArch64 to determine whether a `SIGSEGV` occurred due to
>> a bad write and generating an appropriate runtime exception. On FreeBSD
>> AArch64, we can't determine this, so we end up in the state of "well, there
>> was a segfault and we haven't otherwise handled it, so RIP your process."
>> (Addressing this is my motivation for pursuing the change described here.)
>> 
>> Thanks!
>> 
>> [1]: https://reviews.llvm.org/D32172
>> [2]: https://reviews.freebsd.org/D20838