linux-kernel - RE: X86 fpu registers in a signal handler's ucontext

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Date:	Tue, 30 Apr 2013 16:42:00 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	"Warlich, Christof" <christof.warlich@...mens.com>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: X86 fpu registers in a signal handler's ucontext

Warlich, Christof writes:
 > Mikael Pettersson <mikpe@...uu.se> writes:
 > > Write to the fpstate ->mxcsr and ->swd fields in the sigaction handler's uc_mcontext.
 > 
 > To me, "sigaction handler's uc_mcontext" sounds like userspace, which really confuses me:
 > Even in most recent glibc-2.17, uc_mcontext is of type mcontext_t, which us defined as:
 > 
 > typedef struct {
 >   gregset_t gregs;
 >   fpregset_t fpregs;
 > } mcontext_t;
 > 
 > typedef struct fpregset {
 >     union {
 > 	struct fpchip_state {
 > 	    int state[27];
 > 	    int status;
 > 	} fpchip_state;
 > 	struct fp_emul_space {
 > 	    char fp_emul[246];
 > 	    char fp_epad[2];
 > 	} fp_emul_space;
 > 	int f_fpregs[62];
 >     } fp_reg_set;
 >     long int f_wregs[33];
 > } fpregset_t;
 > 
 > So there is no fpstate whatsoever.

You're looking at the wrong header: glibc-2.17/sysdeps/i386/sys/ucontext.h is legacy SVR4.
Instead look at glibc-2.17/sysdeps/unix/sysv/linux/x86/sys/ucontext.h.

(If the old one got installed on your system then something there is seriously wrong.)

The only quirk is that to access ->mxcsr on 32-bit you have to check if the high half of
->status contains FXSR_MAGIC and if so you need to cast that pointer to struct _fpstate*.
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