mpv/loader/setup_FS.c

#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>

#ifdef __linux__
#include <asm/unistd.h>
#include <asm/ldt.h>
#else

#if defined(__NetBSD__) || defined(__FreeBSD__) || defined(__OpenBSD__)
#include <machine/sysarch.h>
#endif

#ifdef __svr4__
#include <sys/segment.h>
#include <sys/sysi86.h>
/* solaris x86: add missing prototype for sysi86() */
extern int sysi86(int, void*);
#define       TEB_SEL_IDX     NUMSYSLDTS
#endif

#define LDT_ENTRIES     8192
#define LDT_ENTRY_SIZE  8
#pragma pack(4)
struct modify_ldt_ldt_s {
        unsigned int  entry_number;
        unsigned long base_addr;
        unsigned int  limit;
        unsigned int  seg_32bit:1;
        unsigned int  contents:2;
        unsigned int  read_exec_only:1;
        unsigned int  limit_in_pages:1;
        unsigned int  seg_not_present:1;
        unsigned int  useable:1;
};

#define MODIFY_LDT_CONTENTS_DATA        0
#define MODIFY_LDT_CONTENTS_STACK       1
#define MODIFY_LDT_CONTENTS_CODE        2
#endif


/* user level (privilege level: 3) ldt (1<<2) segment selector */
#define       LDT_SEL(idx) ((idx) << 3 | 1 << 2 | 3)

#ifndef       TEB_SEL_IDX
#define       TEB_SEL_IDX     1
#endif

#define       TEB_SEL LDT_SEL(TEB_SEL_IDX)

void setup_FS_Segment()
{
#ifdef DETAILED_OUT
    printf("Setup FS...");fflush(stdout);
#endif
    __asm__ __volatile__(
	"movl %0,%%eax; movw %%ax, %%fs" : : "i" (TEB_SEL)
    );
#ifdef DETAILED_OUT
    printf("OK!\n");
#endif
}


/**
 *
 *  This should be performed before we create first thread. See remarks
 *  for write_ldt(), linux/kernel/ldt.c.
 *
 */

static void* fs_seg=NULL;

#ifdef __linux__
    /* XXX: why is this routine from libc redefined here? */
    /* NOTE: the redefined version ignores the count param, count is hardcoded as 16 */
    static int modify_ldt( int func, struct modify_ldt_ldt_s *ptr,
                                  unsigned long count )
    {
	int res;
#ifdef __PIC__
	__asm__ __volatile__( "pushl %%ebx\n\t"
                          "movl %2,%%ebx\n\t"
                          "int $0x80\n\t"
                          "popl %%ebx"
                          : "=a" (res)
                          : "0" (__NR_modify_ldt),
                            "r" (func),
                            "c" (ptr),
			    "d"(16)//sizeof(*ptr) from kernel point of view
			  :"esi"     );
#else
        __asm__ __volatile__("int $0x80"
                         : "=a" (res)
                         : "0" (__NR_modify_ldt),
                           "b" (func),
                           "c" (ptr),
			    "d"(16)
			   :"esi");
#endif  /* __PIC__ */
	if (res >= 0) return res;
	errno = -res;
        return -1;
    }
#endif

#if defined(__NetBSD__) || defined(__FreeBSD__) || defined(__OpenBSD__)
    static void LDT_EntryToBytes( unsigned long *buffer, const struct modify_ldt_ldt_s *content )
    {
	*buffer++ = ((content->base_addr & 0x0000ffff) << 16) |
             (content->limit & 0x0ffff);
        *buffer = (content->base_addr & 0xff000000) |
              ((content->base_addr & 0x00ff0000)>>16) |
              (content->limit & 0xf0000) |
              (content->contents << 10) |
              ((content->read_exec_only == 0) << 9) |
              ((content->seg_32bit != 0) << 22) |
              ((content->limit_in_pages != 0) << 23) |
              0xf000;
    }
#endif

void Setup_LDT_Keeper(){
    struct modify_ldt_ldt_s array;
    int fd;
    int ret;
    void* prev_struct;
    
    if(fs_seg) return; // already set!
    
    fd=open("/dev/zero", O_RDWR);
    fs_seg=mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE, MAP_PRIVATE,
	fd, 0);
    if(fs_seg==(void*)-1)
    {
	perror("ERROR: Couldn't allocate memory for fs segment");
	return;
    }	
    array.base_addr=(int)fs_seg;
    array.entry_number=TEB_SEL_IDX;
    array.limit=array.base_addr+getpagesize()-1;
    array.seg_32bit=1;
    array.read_exec_only=0;
    array.seg_not_present=0;
    array.contents=MODIFY_LDT_CONTENTS_DATA;
    array.limit_in_pages=0;
#ifdef __linux__
    ret=modify_ldt(0x1, &array, sizeof(struct modify_ldt_ldt_s));
    if(ret<0)
    {
	perror("install_fs");
	printf("Couldn't install fs segment, expect segfault\n");
    }	
#endif /*linux*/

#if defined(__NetBSD__) || defined(__FreeBSD__) || defined(__OpenBSD__)
    {
        unsigned long d[2];

        LDT_EntryToBytes( d, &array );
        ret = i386_set_ldt(array.entry_number, (union descriptor *)d, 1);
        if (ret < 0)
        {
            perror("install_fs");
	    printf("Couldn't install fs segment, expect segfault\n");
            printf("Did you reconfigure the kernel with \"options USER_LDT\"?\n");
        }
	printf("Set_LDT\n");
    }
#endif  /* __NetBSD__ || __FreeBSD__ || __OpenBSD__ */

#if defined(__svr4__)
 {
    struct ssd ssd;
    ssd.sel = TEB_SEL;
    ssd.bo = array.base_addr;
    ssd.ls = array.limit - array.base_addr;
    ssd.acc1 = ((array.read_exec_only == 0) << 1) |
               (array.contents << 2) |
               0xf0;   /* P(resent) | DPL3 | S */
    ssd.acc2 = 0x4;   /* byte limit, 32-bit segment */
    if (sysi86(SI86DSCR, &ssd) < 0) {
	perror("sysi86(SI86DSCR)");
	printf("Couldn't install fs segment, expect segfault\n");
    }
 }
#endif

    setup_FS_Segment();

    prev_struct=malloc(8);
    *(void**)array.base_addr=prev_struct;
    close(fd);
}

void Restore_LDT_Keeper()
{
    if(fs_seg==0) return;
    munmap((char*)fs_seg, getpagesize());
}