linux/arch/x86/include/uapi/asm/ucontext.h

#ifndef _ASM_X86_UCONTEXT_H
#define _ASM_X86_UCONTEXT_H

/*
 * Indicates the presence of extended state information in the memory
 * layout pointed by the fpstate pointer in the ucontext's sigcontext
 * struct (uc_mcontext).
 */
#define UC_FP_XSTATE	0x1

#ifdef __x86_64__
/*
 * UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on
 * kernels that save SS in the sigcontext.  All kernels that set
 * UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp
 * regardless of SS (i.e. they implement espfix).
 *
 * Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS
 * when delivering a signal that came from 64-bit code.
 *
 * Sigreturn restores SS as follows:
 *
 * if (saved SS is valid || UC_STRICT_RESTORE_SS is set ||
 *     saved CS is not 64-bit)
 *         new SS = saved SS  (will fail IRET and signal if invalid)
 * else
 *         new SS = a flat 32-bit data segment
 *
 * This behavior serves three purposes:
 *
 * - Legacy programs that construct a 64-bit sigcontext from scratch
 *   with zero or garbage in the SS slot (e.g. old CRIU) and call
 *   sigreturn will still work.
 *
 * - Old DOSEMU versions sometimes catch a signal from a segmented
 *   context, delete the old SS segment (with modify_ldt), and change
 *   the saved CS to a 64-bit segment.  These DOSEMU versions expect
 *   sigreturn to send them back to 64-bit mode without killing them,
 *   despite the fact that the SS selector when the signal was raised is
 *   no longer valid.  UC_STRICT_RESTORE_SS will be clear, so the kernel
 *   will fix up SS for these DOSEMU versions.
 *
 * - Old and new programs that catch a signal and return without
 *   modifying the saved context will end up in exactly the state they
 *   started in, even if they were running in a segmented context when
 *   the signal was raised..  Old kernels would lose track of the
 *   previous SS value.
 */
#define UC_SIGCONTEXT_SS	0x2
#define UC_STRICT_RESTORE_SS	0x4
#endif

#include <asm-generic/ucontext.h>

#endif /* _ASM_X86_UCONTEXT_H */