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arm64: fpsimd: Fix bad si_code for undiagnosed SIGFPE
Currently a SIGFPE delivered in response to a floating-point exception trap may have si_code set to 0 on arm64. As reported by Eric, this is a bad idea since this is the value of SI_USER -- yet this signal is definitely not the result of kill(2), tgkill(2) etc. and si_uid and si_pid make limited sense whereas we do want to yield a value for si_addr (which doesn't exist for SI_USER). It's not entirely clear whether the architecure permits a "spurious" fp exception trap where none of the exception flag bits in ESR_ELx is set. (IMHO the architectural intent is to forbid this.) However, it does permit those bits to contain garbage if the TFV bit in ESR_ELx is 0. That case isn't currently handled at all and may result in si_code == 0 or si_code containing a FPE_FLT* constant corresponding to an exception that did not in fact happen. There is nothing sensible we can return for si_code in such cases, but SI_USER is certainly not appropriate and will lead to violation of legitimate userspace assumptions. This patch allocates a new si_code value FPE_UNKNOWN that at least does not conflict with any existing SI_* or FPE_* code, and yields this in si_code for undiagnosable cases. This is probably the best simplicity/incorrectness tradeoff achieveable without relying on implementation-dependent features or adding a lot of code. In any case, there appears to be no perfect solution possible that would justify a lot of effort here. Yielding FPE_UNKNOWN when some well-defined fp exception caused the trap is a violation of POSIX, but this is forced by the architecture. We have no realistic prospect of yielding the correct code in such cases. At present I am not aware of any ARMv8 implementation that supports trapped floating-point exceptions in any case. The new code may be applicable to other architectures for similar reasons. No attempt is made to provide ESR_ELx to userspace in the signal frame, since architectural limitations mean that it is unlikely to provide much diagnostic value, doesn't benefit existing software and would create ABI with no proven purpose. The existing mechanism for passing it also has problems of its own which may result in the wrong value being passed to userspace due to interaction with mm faults. The implied rework does not appear justified. Acked-by:"Eric W. Biederman" <ebiederm@xmission.com> Reported-by: