Messages in this thread Patch in this message | | | Subject | Re: [BUG Report] undefined reference to `convert_to_fxsr' | From | Miaohe Lin <> | Date | Sat, 7 Jan 2023 10:10:18 +0800 |
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On 2023/1/6 7:14, Borislav Petkov wrote: > On Tue, Jan 03, 2023 at 11:05:01AM +0800, Miaohe Lin wrote: >> Yes, it still reproduces in my working server. It might be the problem of gcc >> version. > > What is that compiler you're using? Where did you get the package from? Does it > have some out-of-tree patches in it?
My compiler is gcc 7.3.0:
linux-rtfsc:/home/linmiaohe/linux # gcc --version gcc (GCC) 7.3.0 Copyright (C) 2017 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
linux-rtfsc:/home/linmiaohe/linux #
And I cloned the code from https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git. It's a clean git repo without applying any out-of-tree patch to it.
> >> I think it's because convert_to_fxsr() is only defined when CONFIG_X86_32 or >> CONFIG_IA32_EMULATION is enabled. > > No, convert_to_fxsr() is part of arch/x86/kernel/fpu/regset.c which is built-in > unconditionally. > > What happens is this here: > > bool fpu__restore_sig(void __user *buf, int ia32_frame) > > ... > > } else { > success = __fpu_restore_sig(buf, buf_fx, ia32_fxstate); > } > > That ia32_fxstate is false because > > ia32_frame &= (IS_ENABLED(CONFIG_X86_32) || > IS_ENABLED(CONFIG_IA32_EMULATION)); > > > neither of those config items are set... > > /* > * Only FXSR enabled systems need the FX state quirk. > * FRSTOR does not need it and can use the fast path. > */ > if (ia32_frame && use_fxsr()) { > buf_fx = buf + sizeof(struct fregs_state); > size += sizeof(struct fregs_state); > ia32_fxstate = true; > ^^^^^^^^^^^^^^^^^^^ > > ... so this doesn't happen. > > } > > Then, in __fpu_restore_sig() you have: > > if (likely(!ia32_fxstate)) { > /* Restore the FPU registers directly from user memory. */ > return restore_fpregs_from_user(buf_fx, user_xfeatures, fx_only, > state_size); > } > > and since ia32_fxstate is false, we return here, the compiler sees that > everything behind that code is dead code and eliminates it.
Many thanks for your explanation! It's really helpful. I checked generated asm code and I found calling to convert_to_fxsr is not eliminated as expected:
objdump -S arch/x86/kernel/fpu/signal.o
3fc: e9 00 00 00 00 jmpq 401 <__fpu_restore_sig+0x401> fpregs->xsave.header.xfeatures |= XFEATURE_MASK_FPSSE; 401: 48 83 8d 40 02 00 00 orq $0x3,0x240(%rbp) 408: 03 convert_to_fxsr(&fpregs->fxsave, &env); ^^^^^^^^^^^^^^^ 409: 48 8d 74 24 10 lea 0x10(%rsp),%rsi
linux-rtfsc:/home/linmiaohe/linux # objdump -S arch/x86/kernel/fpu/signal.o | grep convert_to_fxsr convert_to_fxsr(&fpregs->fxsave, &env); linux-rtfsc:/home/linmiaohe/linux #
And if I compile the code with below patch, convert_to_fxsr is eliminated:
linux-rtfsc:/home/linmiaohe/linux # objdump -S arch/x86/kernel/fpu/signal.o | grep convert_to_fxsr linux-rtfsc:/home/linmiaohe/linux #
diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h index 611fa41711af..77ea052a8967 100644 --- a/arch/x86/include/asm/fpu/signal.h +++ b/arch/x86/include/asm/fpu/signal.h @@ -20,8 +20,14 @@
extern void convert_from_fxsr(struct user_i387_ia32_struct *env, struct task_struct *tsk); +#if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION extern void convert_to_fxsr(struct fxregs_state *fxsave, const struct user_i387_ia32_struct *env); +#else +static inline void convert_to_fxsr(struct fxregs_state *fxsave, + const struct user_i387_ia32_struct *env) +{} +#endif
unsigned long fpu__alloc_mathframe(unsigned long sp, int ia32_frame, -- 2.27.0 > > Your compiler doesn't, apparently. > > It does remove it from regset.c, though, as it sees it is an unused function, > which leads to this undefined reference. > > So it looks like a funky compiler to me...
It seems another issue with old compiler. Sigh... :(
Thanks, Miaohe Lin
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