This is me trying to figure out how to change the %fs register on Linux. :)

research/ldt.c

@@ -0,0 +1,104 @@
+// "man modify_ldt"
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <asm/ldt.h>
+
+int main(void)
+{
+    struct user_desc ldt[8 * 1024];
+    long rc;
+
+    //rc = syscall(123, (int) 0, (void *) ldt, (unsigned long) sizeof (ldt));
+    //printf("read syscall == %ld\n", rc);
+
+    char *str = malloc(64);
+    strcpy(str, "hello world!");
+    printf("&str == %p\n", str);
+
+    unsigned int fs = 0xFFFFFFFF;
+    __asm__ __volatile__ (
+        "xorl %%eax, %%eax \n\t"
+        "movw %%fs, %%ax \n\t"
+            : "=a" (fs)
+    );
+
+    printf("Current %%fs: 0x%X\n", fs);
+
+    struct user_desc *entry = ldt;
+    entry->entry_number = -1;
+    entry->base_addr = (unsigned long) str;
+    entry->limit = 64;
+    entry->seg_32bit = 1;
+    entry->contents = MODIFY_LDT_CONTENTS_DATA;
+    entry->read_exec_only = 0;
+    entry->limit_in_pages = 0;
+    entry->seg_not_present = 0;
+    entry->useable = 1;
+
+    printf("entry=%u, base=%p, limit=%u\n", entry->entry_number, (void *) entry->base_addr, entry->limit);
+
+// modify_ldt
+//    rc = syscall(SYS_modify_ldt, (int) 1, (void *) entry, (unsigned long) sizeof (*entry));
+//    printf("write syscall == %ld\n", rc);
+
+// set_thread_area
+    rc = syscall(SYS_set_thread_area, entry);
+    printf("write syscall == %ld\n", rc);
+
+    printf("entry==%d\n", (int) (entry->entry_number));
+
+// this doesn't give you a user_desc struct back, I think it gives you the real LDT entry...?
+//    rc = syscall(SYS_modify_ldt, (int) 0, (void *) entry, (unsigned long) sizeof (*entry) * 128);
+//    printf("read syscall == %ld\n", rc);
+
+//    long i;
+//    for (i = 0; i < 128; i++, entry++)
+//        printf("entry=%u, base=%p, limit=%u\n", entry->entry_number, (void *) entry->base_addr, entry->limit);
+
+    // I have no idea why this needs the "<< 3 | 3", but it's probably
+    //  specified in the Intel manuals. I got this from looking at how
+    //  Wine does it.
+    __asm__ __volatile__ (
+        //"pushw %%ax\n\t"
+        //"popw %%fs \n\t"
+        "movw %%ax,%%fs \n\t"
+            :
+            : "a" ((entry->entry_number << 3) | 3)
+    );
+
+    fs = 0xFFFFFFFF;
+    __asm__ __volatile__ (
+        "xorl %%eax, %%eax \n\t"
+        "movw %%fs, %%ax \n\t"
+            : "=a" (fs)
+    );
+
+    printf("New %%fs: 0x%X\n", fs);
+
+    char newstr[16];
+    memset(newstr, '\0', sizeof (newstr));
+    __asm__ __volatile__ (
+        "movl %%fs:0,%%eax\n\t"
+        "movl %%eax,(%%edi)\n\t"
+        "movl %%fs:4,%%eax\n\t"
+        "movl %%eax,4(%%edi)\n\t"
+        "movl %%fs:8,%%eax\n\t"
+        "movl %%eax,8(%%edi)\n\t"
+        :
+        : "D" (newstr)
+        : "eax"
+    );
+
+    printf("copied string == '%s'\n", newstr);
+
+    return 0;
+}
+
+// end of ldt.c ...
+