Add common abstrations for segmentation

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/asm/asm.s

@@ -173,6 +173,36 @@ _x86_64_asm_load_gs:
     mov %di, %gs
     retq
 
+.global _x86_64_asm_get_ss
+.p2align 4
+_x86_64_asm_get_ss:
+    mov %ss, %ax
+    retq
+
+.global _x86_64_asm_get_ds
+.p2align 4
+_x86_64_asm_get_ds:
+    mov %ds, %ax
+    retq
+
+.global _x86_64_asm_get_es
+.p2align 4
+_x86_64_asm_get_es:
+    mov %es, %ax
+    retq
+
+.global _x86_64_asm_get_fs
+.p2align 4
+_x86_64_asm_get_fs:
+    mov %fs, %ax
+    retq
+
+.global _x86_64_asm_get_gs
+.p2align 4
+_x86_64_asm_get_gs:
+    mov %gs, %ax
+    retq
+
 .global _x86_64_asm_swapgs
 .p2align 4
 _x86_64_asm_swapgs:

src/asm/mod.rs

@@ -114,6 +114,36 @@ extern "C" {
     )]
     pub(crate) fn x86_64_asm_load_gs(sel: u16);
 
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_get_ss"
+    )]
+    pub(crate) fn x86_64_asm_get_ss() -> u16;
+
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_get_ds"
+    )]
+    pub(crate) fn x86_64_asm_get_ds() -> u16;
+
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_get_es"
+    )]
+    pub(crate) fn x86_64_asm_get_es() -> u16;
+
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_get_fs"
+    )]
+    pub(crate) fn x86_64_asm_get_fs() -> u16;
+
+    #[cfg_attr(
+        any(target_env = "gnu", target_env = "musl"),
+        link_name = "_x86_64_asm_get_gs"
+    )]
+    pub(crate) fn x86_64_asm_get_gs() -> u16;
+
     #[cfg_attr(
         any(target_env = "gnu", target_env = "musl"),
         link_name = "_x86_64_asm_swapgs"