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Thu, 14 Aug 2025 01:00:25 +0000 Message-ID: <084b7506-028a-4213-8523-57610b498837@intel.com> Date: Wed, 13 Aug 2025 18:00:22 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/2] x86/fpu: Update the debug flow for x86_task_fpu() Content-Language: en-US To: Oleg Nesterov CC: "Lai, Yi" , Dave Hansen , , Borislav Petkov , "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar , Dave Hansen , Sean Christopherson , "Peter Zijlstra" , Vignesh Balasubramanian , Rick Edgecombe , "Chang S . 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But this doesn't really matter. > Thanks for checking. > First of all, I think that in the long term kthreads and PF_USER_WORKERs > should run without "struct fpu" attached to task_struct, so x86_task_fpu() > should return NULL regardless of CONFIG_X86_DEBUG_FPU in this case. Having x86_task_fpu() to be consistent would definitely reduce the number of corner cases we have to deal with :) "struct fpu" seems to hold some metadata such as avx512_timestamp as well as some permissions. Hopefully we never need any of that for PF_USER_WORKERs. Is the eventual goal of running without "struct fpu" to save memory or having a cleaner implementation? > So. The problem is that do_coredump() paths or ptrace can abuse > PF_USER_WORKER's FPU state for no reason. > Agreed, this requires immediate solving. > To simplify, lets only discuss REGSET64_FP for now. As for xfpregs_get(), > everything looks simple, but needs some preparatory patches. membuf_write() > and copy_xstate_to_uabi_buf() should use &init_fpstate instead of > x86_task_fpu(target)->fpstate when target->flags & PF_USER_WORKER. This > matches the reality. > Yeah, using init_fpstate seems reasonable. I don't know what userspace would do with that information though. But, not returning anything would likely break some debugging flows. > But what about xfpregs_set() ? Can it simply return, say, -EPERM ? > This would modify legacy behavior, but I don't think changing the register values would have any effect either, right? Another option is to silently ignore the set(). Maybe we can go with your approach unless someone complains. > What do you think? > My knowledge about the FPU handling is fairly limited. So I don't have a strong opinion on this. Just curiosity and questions! :) Sohil