[RFA 0/5] Prec: x86 segment register support

[Hui Zhu <teawater at gmail dot com>]

Hi,

In before, the prec cannot support the x86 segment register.  Because
GDB just have the interface to get the index the segment reg point to,
but cannot get the base in the segment.
Just the OS can get this value.

After I read some code of Linux kernel, I found that some code about
this issue in file common.c and segment.h, the x86-32 struct about it
is:
DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
#ifdef CONFIG_X86_64
	/*
	 * We need valid kernel segments for data and code in long mode too
	 * IRET will check the segment types  kkeil 2000/10/28
	 * Also sysret mandates a special GDT layout
	 *
	 * TLS descriptors are currently at a different place compared to i386.
	 * Hopefully nobody expects them at a fixed place (Wine?)
	 */
	[GDT_ENTRY_KERNEL32_CS]		= GDT_ENTRY_INIT(0xc09b, 0, 0xfffff),
	[GDT_ENTRY_KERNEL_CS]		= GDT_ENTRY_INIT(0xa09b, 0, 0xfffff),
	[GDT_ENTRY_KERNEL_DS]		= GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
	[GDT_ENTRY_DEFAULT_USER32_CS]	= GDT_ENTRY_INIT(0xc0fb, 0, 0xfffff),
	[GDT_ENTRY_DEFAULT_USER_DS]	= GDT_ENTRY_INIT(0xc0f3, 0, 0xfffff),
	[GDT_ENTRY_DEFAULT_USER_CS]	= GDT_ENTRY_INIT(0xa0fb, 0, 0xfffff),
#else
	[GDT_ENTRY_KERNEL_CS]		= GDT_ENTRY_INIT(0xc09a, 0, 0xfffff),
	[GDT_ENTRY_KERNEL_DS]		= GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
	[GDT_ENTRY_DEFAULT_USER_CS]	= GDT_ENTRY_INIT(0xc0fa, 0, 0xfffff),
	[GDT_ENTRY_DEFAULT_USER_DS]	= GDT_ENTRY_INIT(0xc0f2, 0, 0xfffff),
	/*
	 * Segments used for calling PnP BIOS have byte granularity.
	 * They code segments and data segments have fixed 64k limits,
	 * the transfer segment sizes are set at run time.
	 */
	/* 32-bit code */
	[GDT_ENTRY_PNPBIOS_CS32 18]	= GDT_ENTRY_INIT(0x409a, 0, 0xffff),
	/* 16-bit code */
	[GDT_ENTRY_PNPBIOS_CS16 19]	= GDT_ENTRY_INIT(0x009a, 0, 0xffff),
	/* 16-bit data */
	[GDT_ENTRY_PNPBIOS_DS]		= GDT_ENTRY_INIT(0x0092, 0, 0xffff),
	/* 16-bit data */
	[GDT_ENTRY_PNPBIOS_TS1]		= GDT_ENTRY_INIT(0x0092, 0, 0),
	/* 16-bit data */
	[GDT_ENTRY_PNPBIOS_TS2]		= GDT_ENTRY_INIT(0x0092, 0, 0),
	/*
	 * The APM segments have byte granularity and their bases
	 * are set at run time.  All have 64k limits.
	 */
	/* 32-bit code */
	[GDT_ENTRY_APMBIOS_BASE]	= GDT_ENTRY_INIT(0x409a, 0, 0xffff),
	/* 16-bit code */
	[GDT_ENTRY_APMBIOS_BASE+1]	= GDT_ENTRY_INIT(0x009a, 0, 0xffff),
	/* data */
	[GDT_ENTRY_APMBIOS_BASE+2]	= GDT_ENTRY_INIT(0x4092, 0, 0xffff),

	[GDT_ENTRY_ESPFIX_SS]		= GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
	[GDT_ENTRY_PERCPU]		= GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
	GDT_STACK_CANARY_INIT
#endif
} };

/*
 * The layout of the per-CPU GDT under Linux:
 *
 *   0 - null
 *   1 - reserved
 *   2 - reserved
 *   3 - reserved
 *
 *   4 - unused			<==== new cacheline
 *   5 - unused
 *
 *  ------- start of TLS (Thread-Local Storage) segments:
 *
 *   6 - TLS segment #1			[ glibc's TLS segment ]
 *   7 - TLS segment #2			[ Wine's %fs Win32 segment ]
 *   8 - TLS segment #3
 *   9 - reserved
 *  10 - reserved
 *  11 - reserved
 *
 *  ------- start of kernel segments:
 *
 *  12 - kernel code segment		<==== new cacheline
 *  13 - kernel data segment
 *  14 - default user CS
 *  15 - default user DS
 *  16 - TSS
 *  17 - LDT
 *  18 - PNPBIOS support (16->32 gate)
 *  19 - PNPBIOS support
 *  20 - PNPBIOS support
 *  21 - PNPBIOS support
 *  22 - PNPBIOS support
 *  23 - APM BIOS support
 *  24 - APM BIOS support
 *  25 - APM BIOS support
 *
 *  26 - ESPFIX small SS
 *  27 - per-cpu			[ offset to per-cpu data area ]
 *  28 - stack_canary-20		[ for stack protector ]
 *  29 - unused
 *  30 - unused
 *  31 - TSS for double fault handler
 */

So, most of base is 0.  Just 6 7 8 can be change.

And I found linux kernel x86-32 have system calls set_thread_area and
get_thread_area to set them's value.
I think let inferior call "get_thread_area" is support each target is
more better.  But I found ptrace have a interface
PTRACE_GET_THREAD_AREA to get the message of thread_area message and
ps_get_thread_area function of GDB use it.
So I choice call ps_get_thread_area to get the segment base.


For the amd64 linux, I did some small check about it, it didn't affect
by segment register issue most of time.  And it have PTRACE_ARCH_PRCTL
to support get base message.  When SSE patch is OK.  I will add patch
for them.

Thanks,
Hui