Messages in this thread | | | From | Thiago Macieira <> | Subject | Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state | Date | Tue, 31 Aug 2021 15:39:02 -0700 |
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On Tuesday, 31 August 2021 15:15:55 PDT Len Brown wrote: > Indeed, I believe that there is universal agreement that a synchronous > return code > from a system call is a far superior programming model than decoding > the location of a failure in a system call. (no, the IP isn't random -- it > is always the 1st instruction in that thread to touch a TMM register).
That instruction is actually likely going to be a memory load, probably an LDTILECFG. So the developer will see a crashing instruction with a pointer and will spend time trying to figure out why that pointer was wrong, when there was nothing wrong with it.
That's why I suggested (and Chang implemented) a SIGILL for when #NM is received and the arch_prctl() wasn't previously done. The OOM condition, if the extra state is dynamically allocated, was meant to stay a SIGSEGV, but maybe should change to SIGKILL.
On the other hand, if it it's allocated at the syscall, then the kernel can return -ENOMEM for it (which would allow for graceful degradation) or for a later clone() syscall starting a new thread (which I don't expect to ever gracefully degrade).
> decoding the location of the failure in a *signal hander*
That's a separate problem.
We can't be sure that the portion of the userspace doing the alt-stack crash handler is aware of the portion using AMX. There's no way to enforce this. The prctl() is a good indication, but I have no clue how high the correlation will be.
-- Thiago Macieira - thiago.macieira (AT) intel.com Software Architect - Intel DPG Cloud Engineering
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