Question About Access Fault Handling in the Kernel

[Vincent Chen]

Hi all,
I have a question regarding the current handling flow for access
faults in the kernel.

In the current implementation, the kernel consistently reports tall
access faults using a SIGSEGV with error code SEGV_ACCERR, where
SEGV_ACCERR signifies "Invalid permissions for mapped object"
according to the POSIX specification.

However, in the RISC-V privilege specification, it states that
"Implementations may raise access-fault exceptions instead of
address-misaligned exceptions for some misaligned accesses, indicating
the instruction should not be emulated by a trap handler". In other
words, a load/store access fault may be caused by a misaligned AMO
instruction. In such cases, it seems more appropriate to report the
error using SIGBUS with the error code BUS_ADRALN, which indicates
"Invalid address alignment."

As I understand it, most access faults are typically caused by
violations of either PMA or PMP settings. If these settings are
considered a form of permission for a mapped object, then the current
implementation using SIGSEGV with SEGV_ACCERR makes sense and helps
simplify the fault-handling logic.

I’d like to confirm whether this interpretation reflects the reasoning
behind the current implementation. Any clarification would be
appreciated. Thanks!