x86/asm: Deobfuscate segment.h

This file just defines a number of constants, and a few macros
and inline functions. It is particularly badly written.

For example, it is not trivial to see how descriptors are
numbered (you'd expect that should be easy, right?).

This change deobfuscates it via the following changes:

Group all GDT_ENTRY_foo together (move intervening stuff away).

Number them explicitly: use a number, not PREV_DEFINE+1, +2, +3:
I want to immediately see that GDT_ENTRY_PNPBIOS_CS32 is 18.
Seeing (GDT_ENTRY_KERNEL_BASE+6) instead is not useful.

The above change allows to remove GDT_ENTRY_KERNEL_BASE
and GDT_ENTRY_PNPBIOS_BASE, which weren't used anywhere else.

After a group of GDT_ENTRY_foo, define all selector values.

Remove or improve some comments. In particular:
Comment deleted as stating the obvious:
    /*
     * The GDT has 32 entries
     */
    #define GDT_ENTRIES 32

"The segment offset needs to contain a RPL. Grr. -AK"
    changed to
"Selectors need to also have a correct RPL (+3 thingy)"

"GDT layout to get 64bit syscall right (sysret hardcodes gdt
offsets)" expanded into a description *how exactly* sysret
hardcodes them.

Patch was tested to compile and not change vmlinux.o
on 32-bit and 64-bit builds (verified with objdump).

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/segment.h

@@ -15,12 +15,10 @@
 /* Simple and small GDT entries for booting only */
 
 #define GDT_ENTRY_BOOT_CS	2
+#define GDT_ENTRY_BOOT_DS	3
+#define GDT_ENTRY_BOOT_TSS	4
 #define __BOOT_CS		(GDT_ENTRY_BOOT_CS * 8)
-
-#define GDT_ENTRY_BOOT_DS	(GDT_ENTRY_BOOT_CS + 1)
 #define __BOOT_DS		(GDT_ENTRY_BOOT_DS * 8)
-
-#define GDT_ENTRY_BOOT_TSS	(GDT_ENTRY_BOOT_CS + 2)
 #define __BOOT_TSS		(GDT_ENTRY_BOOT_TSS * 8)
 
 #define SEGMENT_RPL_MASK	0x3 /*
@@ -80,97 +78,86 @@
 #define GDT_ENTRY_TLS_MIN	6
 #define GDT_ENTRY_TLS_MAX 	(GDT_ENTRY_TLS_MIN + GDT_ENTRY_TLS_ENTRIES - 1)
 
+#define GDT_ENTRY_KERNEL_CS		12
+#define GDT_ENTRY_KERNEL_DS		13
 #define GDT_ENTRY_DEFAULT_USER_CS	14
-
 #define GDT_ENTRY_DEFAULT_USER_DS	15
+#define GDT_ENTRY_TSS			16
+#define GDT_ENTRY_LDT			17
+#define GDT_ENTRY_PNPBIOS_CS32		18
+#define GDT_ENTRY_PNPBIOS_CS16		19
+#define GDT_ENTRY_PNPBIOS_DS		20
+#define GDT_ENTRY_PNPBIOS_TS1		21
+#define GDT_ENTRY_PNPBIOS_TS2		22
+#define GDT_ENTRY_APMBIOS_BASE		23
+
+#define GDT_ENTRY_ESPFIX_SS		26
+#define GDT_ENTRY_PERCPU		27
+#define GDT_ENTRY_STACK_CANARY		28
 
-#define GDT_ENTRY_KERNEL_BASE		(12)
-
-#define GDT_ENTRY_KERNEL_CS		(GDT_ENTRY_KERNEL_BASE+0)
-
-#define GDT_ENTRY_KERNEL_DS		(GDT_ENTRY_KERNEL_BASE+1)
-
-#define GDT_ENTRY_TSS			(GDT_ENTRY_KERNEL_BASE+4)
-#define GDT_ENTRY_LDT			(GDT_ENTRY_KERNEL_BASE+5)
-
-#define GDT_ENTRY_PNPBIOS_BASE		(GDT_ENTRY_KERNEL_BASE+6)
-#define GDT_ENTRY_APMBIOS_BASE		(GDT_ENTRY_KERNEL_BASE+11)
+#define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
-#define GDT_ENTRY_ESPFIX_SS		(GDT_ENTRY_KERNEL_BASE+14)
+#define __KERNEL_CS			(GDT_ENTRY_KERNEL_CS*8)
+#define __KERNEL_DS			(GDT_ENTRY_KERNEL_DS*8)
+#define __USER_DS			(GDT_ENTRY_DEFAULT_USER_DS*8+3)
+#define __USER_CS			(GDT_ENTRY_DEFAULT_USER_CS*8+3)
 #define __ESPFIX_SS			(GDT_ENTRY_ESPFIX_SS*8)
-
-#define GDT_ENTRY_PERCPU		(GDT_ENTRY_KERNEL_BASE+15)
+#define PNP_CS32   (GDT_ENTRY_PNPBIOS_CS32 * 8) /* segment for calling fn */
+#define PNP_CS16   (GDT_ENTRY_PNPBIOS_CS16 * 8) /* code segment for BIOS */
+/* "Is this PNP code selector (PNP_CS32 or PNP_CS16)?" */
+#define SEGMENT_IS_PNP_CODE(x)   (((x) & 0xf4) == PNP_CS32)
+#define PNP_DS     (GDT_ENTRY_PNPBIOS_DS * 8)	/* data segment for BIOS */
+#define PNP_TS1    (GDT_ENTRY_PNPBIOS_TS1 * 8)	/* transfer data segment */
+#define PNP_TS2    (GDT_ENTRY_PNPBIOS_TS2 * 8)	/* another data segment */
 #ifdef CONFIG_SMP
-#define __KERNEL_PERCPU (GDT_ENTRY_PERCPU * 8)
+#define __KERNEL_PERCPU			(GDT_ENTRY_PERCPU*8)
 #else
-#define __KERNEL_PERCPU 0
+#define __KERNEL_PERCPU			0
 #endif
-
-#define GDT_ENTRY_STACK_CANARY		(GDT_ENTRY_KERNEL_BASE+16)
 #ifdef CONFIG_CC_STACKPROTECTOR
 #define __KERNEL_STACK_CANARY		(GDT_ENTRY_STACK_CANARY*8)
 #else
 #define __KERNEL_STACK_CANARY		0
 #endif
 
-#define GDT_ENTRY_DOUBLEFAULT_TSS	31
-
-/*
- * The GDT has 32 entries
- */
 #define GDT_ENTRIES 32
 
-/* The PnP BIOS entries in the GDT */
-#define GDT_ENTRY_PNPBIOS_CS32		(GDT_ENTRY_PNPBIOS_BASE + 0)
-#define GDT_ENTRY_PNPBIOS_CS16		(GDT_ENTRY_PNPBIOS_BASE + 1)
-#define GDT_ENTRY_PNPBIOS_DS		(GDT_ENTRY_PNPBIOS_BASE + 2)
-#define GDT_ENTRY_PNPBIOS_TS1		(GDT_ENTRY_PNPBIOS_BASE + 3)
-#define GDT_ENTRY_PNPBIOS_TS2		(GDT_ENTRY_PNPBIOS_BASE + 4)
-
-/* The PnP BIOS selectors */
-#define PNP_CS32   (GDT_ENTRY_PNPBIOS_CS32 * 8)	/* segment for calling fn */
-#define PNP_CS16   (GDT_ENTRY_PNPBIOS_CS16 * 8)	/* code segment for BIOS */
-#define PNP_DS     (GDT_ENTRY_PNPBIOS_DS * 8)	/* data segment for BIOS */
-#define PNP_TS1    (GDT_ENTRY_PNPBIOS_TS1 * 8)	/* transfer data segment */
-#define PNP_TS2    (GDT_ENTRY_PNPBIOS_TS2 * 8)	/* another data segment */
+#else /* 64-bit: */
 
-
-/*
- * Matching rules for certain types of segments.
- */
-
-/* Matches PNP_CS32 and PNP_CS16 (they must be consecutive) */
-#define SEGMENT_IS_PNP_CODE(x)   (((x) & 0xf4) == GDT_ENTRY_PNPBIOS_BASE * 8)
-
-
-#else
 #include <asm/cache.h>
 
-#define GDT_ENTRY_KERNEL32_CS 1
-#define GDT_ENTRY_KERNEL_CS 2
-#define GDT_ENTRY_KERNEL_DS 3
-
-#define __KERNEL32_CS   (GDT_ENTRY_KERNEL32_CS * 8)
-
+#define GDT_ENTRY_KERNEL32_CS	1
+#define GDT_ENTRY_KERNEL_CS	2
+#define GDT_ENTRY_KERNEL_DS	3
 /*
  * we cannot use the same code segment descriptor for user and kernel
  * -- not even in the long flat mode, because of different DPL /kkeil
- * The segment offset needs to contain a RPL. Grr. -AK
- * GDT layout to get 64bit syscall right (sysret hardcodes gdt offsets)
+ * GDT layout to get 64bit syscall/sysret right. sysret hardcodes selectors:
+ * if returning to 32-bit userspace: cs = STAR.SYSRET_CS,
+ * if returning to 64-bit userspace: cs = STAR.SYSRET_CS+16,
+ * ss = STAR.SYSRET_CS+8 (in either case)
+ * thus USER_DS should be between 32-bit and 64-bit code selectors:
  */
 #define GDT_ENTRY_DEFAULT_USER32_CS 4
 #define GDT_ENTRY_DEFAULT_USER_DS 5
 #define GDT_ENTRY_DEFAULT_USER_CS 6
-#define __USER32_CS   (GDT_ENTRY_DEFAULT_USER32_CS*8+3)
-#define __USER32_DS	__USER_DS
 
-#define GDT_ENTRY_TSS 8	/* needs two entries */
-#define GDT_ENTRY_LDT 10 /* needs two entries */
-#define GDT_ENTRY_TLS_MIN 12
-#define GDT_ENTRY_TLS_MAX 14
+#define GDT_ENTRY_TSS		8  /* needs two entries */
+#define GDT_ENTRY_LDT		10 /* needs two entries */
+#define GDT_ENTRY_TLS_MIN	12
+#define GDT_ENTRY_TLS_MAX	14
 
-#define GDT_ENTRY_PER_CPU 15	/* Abused to load per CPU data from limit */
-#define __PER_CPU_SEG	(GDT_ENTRY_PER_CPU * 8 + 3)
+#define GDT_ENTRY_PER_CPU	15 /* abused to load per CPU data from limit */
+
+/* Selectors need to also have a correct RPL (+3 thingy) */
+#define __KERNEL_CS	(GDT_ENTRY_KERNEL_CS*8)
+#define __KERNEL_DS	(GDT_ENTRY_KERNEL_DS*8)
+#define __USER_DS	(GDT_ENTRY_DEFAULT_USER_DS*8+3)
+#define __USER_CS	(GDT_ENTRY_DEFAULT_USER_CS*8+3)
+#define __KERNEL32_CS	(GDT_ENTRY_KERNEL32_CS*8)
+#define __USER32_CS	(GDT_ENTRY_DEFAULT_USER32_CS*8+3)
+#define __USER32_DS	__USER_DS
+#define __PER_CPU_SEG	(GDT_ENTRY_PER_CPU*8+3)
 
 /* TLS indexes for 64bit - hardcoded in arch_prctl */
 #define FS_TLS 0
@@ -183,10 +170,6 @@
 
 #endif
 
-#define __KERNEL_CS	(GDT_ENTRY_KERNEL_CS*8)
-#define __KERNEL_DS	(GDT_ENTRY_KERNEL_DS*8)
-#define __USER_DS	(GDT_ENTRY_DEFAULT_USER_DS*8+3)
-#define __USER_CS	(GDT_ENTRY_DEFAULT_USER_CS*8+3)
 #ifndef CONFIG_PARAVIRT
 #define get_kernel_rpl()  0
 #endif