The C and C++ Include Header Files

/usr/include/x86_64-linux-gnu/asm/ucontext.h


$ cat -n /usr/include/x86_64-linux-gnu/asm/ucontext.h

     1	/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
     2	#ifndef _ASM_X86_UCONTEXT_H
     3	#define _ASM_X86_UCONTEXT_H
     4	
     5	/*
     6	 * Indicates the presence of extended state information in the memory
     7	 * layout pointed by the fpstate pointer in the ucontext's sigcontext
     8	 * struct (uc_mcontext).
     9	 */
    10	#define UC_FP_XSTATE	0x1
    11	
    12	#ifdef __x86_64__
    13	/*
    14	 * UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on
    15	 * kernels that save SS in the sigcontext.  All kernels that set
    16	 * UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp
    17	 * regardless of SS (i.e. they implement espfix).
    18	 *
    19	 * Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS
    20	 * when delivering a signal that came from 64-bit code.
    21	 *
    22	 * Sigreturn restores SS as follows:
    23	 *
    24	 * if (saved SS is valid || UC_STRICT_RESTORE_SS is set ||
    25	 *     saved CS is not 64-bit)
    26	 *         new SS = saved SS  (will fail IRET and signal if invalid)
    27	 * else
    28	 *         new SS = a flat 32-bit data segment
    29	 *
    30	 * This behavior serves three purposes:
    31	 *
    32	 * - Legacy programs that construct a 64-bit sigcontext from scratch
    33	 *   with zero or garbage in the SS slot (e.g. old CRIU) and call
    34	 *   sigreturn will still work.
    35	 *
    36	 * - Old DOSEMU versions sometimes catch a signal from a segmented
    37	 *   context, delete the old SS segment (with modify_ldt), and change
    38	 *   the saved CS to a 64-bit segment.  These DOSEMU versions expect
    39	 *   sigreturn to send them back to 64-bit mode without killing them,
    40	 *   despite the fact that the SS selector when the signal was raised is
    41	 *   no longer valid.  UC_STRICT_RESTORE_SS will be clear, so the kernel
    42	 *   will fix up SS for these DOSEMU versions.
    43	 *
    44	 * - Old and new programs that catch a signal and return without
    45	 *   modifying the saved context will end up in exactly the state they
    46	 *   started in, even if they were running in a segmented context when
    47	 *   the signal was raised..  Old kernels would lose track of the
    48	 *   previous SS value.
    49	 */
    50	#define UC_SIGCONTEXT_SS	0x2
    51	#define UC_STRICT_RESTORE_SS	0x4
    52	#endif
    53	
    54	#include <asm-generic/ucontext.h>
    55	
    56	#endif /* _ASM_X86_UCONTEXT_H */