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binutils-gdb/gdb/i386-tdep.h
Andrew Burgess c87df013cb gdb/i386/linux: fix possible register number conflict 
I noticed something that seemed really strange with the i386 register
numbering.

In i386-linux-tdep.h we setup I386_LINUX_ORIG_EAX_REGNUM based on
I386_PKRU_REGNUM.

However, in i386-tdep.h, enum i386_regnum ends like this:

  enum i386_regnum
  {
    ...
    I386_ZMM7H_REGNUM = I386_ZMM0H_REGNUM + 7,
    I386_PKRU_REGNUM,
    I386_PL3_SSP_REGNUM,
    I386_FSBASE_REGNUM,
    I386_GSBASE_REGNUM
  };

So I386_LINUX_ORIG_EAX_REGNUM will have the same value as
I386_PL3_SSP_REGNUM.

The I386_PL3_SSP_REGNUM was added in commit:

  commit 63b862be76
  AuthorDate: Fri Mar 29 16:38:50 2019 +0100
  CommitDate: Fri Aug 29 17:02:09 2025 +0000

      gdb, gdbserver: Add support of Intel shadow stack pointer register.

And before that, I386_FSBASE_REGNUM and I386_GSBASE_REGNUM were added
in commit:

  commit 1163a4b7a3
  AuthorDate: Tue Mar 12 13:39:02 2019 -0700
  CommitDate: Tue Mar 12 13:39:02 2019 -0700

      Support the fs_base and gs_base registers on i386.

So the SSP overlap is new, but the fs/gs base overlap has existed for
years, so why did it not cause any problems?

I think the explanation is that on i386, the fs/gs base are only used
for FreeBSD, all the calls to i386_target_description that pass true
for the segments argument are from fbsd files.  As a result, its fine
if there's numbering overlap between these i386 registers and some
Linux specific i386 registers.

OK, but what about the new SSP (shadow stack pointer) register?

I think in this case we would see problems, if the shadow stack was
supported for i386.  Here's what the docs say:

     The ‘org.gnu.gdb.i386.pl3_ssp’ feature is optional.  It should
  describe the user mode register ‘pl3_ssp’ which has 64 bits on amd64, 32
  bits on amd64 with 32-bit pointer size (X32) and 32 bits on i386.
  Following the restriction of the Linux kernel, only GDB for amd64
  targets makes use of this feature for now.

And indeed, if we look for callers of x86_supply_ssp, which supplies
the shadow stack pointer register, this is only called from amd64
specific code, either the native register fetching, or the core file
loading.  There's no calls from i386 code.

And so, again, we have register number overlap, but we avoid any
issues by not making use of these registers for i386 linux.

Here's my question: Is this super clever design aimed at saving 12
bytes (3 * 4-byte registers) of space in the i386 regcache?  Or is
this an accident where we happen to have gotten lucky?

If it's the first, then I really think there should be some comments
explaining what's going on.

If it's the second, then maybe we should fix this before it trips us
up?

This commit takes the second approach by doing the following:

  1. In i386-tdep.h move all the *_NUM_REGS constants to be members of
     'enum i386_regnum'.  The I386_NUM_REGS value can be automatically
     calculated based off the (current) last enum entry, and the
     other *_NUM_REGS constants are calculated just as they previously
     were, but are moved to keep them all together.

  2. In i386-linux-tdep.h, I386_LINUX_ORIG_EAX_REGNUM and
     I386_LINUX_NUM_REGS are moved into a new enum i386_linux_regnum,
     the name of which is inspired by i386_regnum with the addition
     of the linux tag.  The first entry in this new enum starts from
     I386_NUM_REGS rather than I386_PKRU_REGNUM.  The
     I386_LINUX_NUM_REGS will be calculated automatically by the
     compiler.

  3. In amd64-linux-nat.c, I extend amd64_linux_gregset32_reg_offset
     so that it now has entries for the 3 registers that are no longer
     aliasing, this stops an assert from the end of the file
     triggering:

     gdb_assert (ARRAY_SIZE (amd64_linux_gregset32_reg_offset)
                 == amd64_native_gregset32_num_regs);

     As I386_LINUX_NUM_REGS has now increased by 3.

  4. Given (3) I wondered why there was no assert being triggered from
     the i386 code as i386_linux_gregset_reg_offset, in i386-linux-tdep.c
     is clearly also wrong now.

     So, In i386-linux-tdep.c I've added a new assertion at the end of
     the file.

     And then I've fixed i386_linux_gregset_reg_offset by adding the 3
     new registers.

With these changes made I believe that the register number for the
$orig_eax register on i386 GNU/Linux targets should no longer be
aliasing with the SSP register.

For the reasons given above, I don't think this fixes any actual bugs,
it's more just a, lets not have unnecessary, and undocumented,
register number aliasing.

This change is visible using 'maint print registers', check out the
register number of $orig_eax before and after, it should now be +3
from where it was (changed from 72 to 75).

I did worry briefly about gdbservers that might not support XML target
descriptions and instead rely on a fixed GDB register numbering.
Though, if I'm honest, I have very little sympathy for such gdbservers
these days.  Still, they could, potentially be tripped up by this
change.  However, this is not the first time in recent years that the
value of I386_LINUX_ORIG_EAX_REGNUM has changed.  This commit also
adjusted the register number:

  commit 51547df62c
  Date:   Wed Feb 1 12:22:27 2017 +0100

      Add support for Intel PKRU register to GDB and GDBserver.

And I'm not aware of any bug reports that came from this, we certainly
didn't feel the need to adjust the register number back again.  So I'm
guessing that this renumbering will also go without issue.

Other than that, there should be no user visible changes after this
commit.

Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
2025-09-23 18:08:32 +01:00

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/* Target-dependent code for the i386.
Copyright (C) 2001-2025 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef GDB_I386_TDEP_H
#define GDB_I386_TDEP_H
#include "gdbarch.h"
#include "infrun.h"
#include "expression.h"
#include "gdbsupport/x86-xstate.h"
class frame_info_ptr;
struct gdbarch;
struct reggroup;
struct regset;
struct regcache;
/* GDB's i386 target supports both the 32-bit Intel Architecture
(IA-32) and the 64-bit AMD x86-64 architecture. Internally it uses
a similar register layout for both.
- General purpose registers
- FPU data registers
- FPU control registers
- SSE data registers
- SSE control register
The general purpose registers for the x86-64 architecture are quite
different from IA-32. Therefore, gdbarch_fp0_regnum
determines the register number at which the FPU data registers
start. The number of FPU data and control registers is the same
for both architectures. The number of SSE registers however,
differs and is determined by the num_xmm_regs member of `struct
gdbarch_tdep'. */
/* Convention for returning structures. */
enum struct_return
{
pcc_struct_return, /* Return "short" structures in memory. */
reg_struct_return /* Return "short" structures in registers. */
};
/* i386 architecture specific information. */
struct i386_gdbarch_tdep : gdbarch_tdep_base
{
/* General-purpose registers. */
int *gregset_reg_offset = 0;
int gregset_num_regs = 0;
size_t sizeof_gregset = 0;
/* Floating-point registers. */
size_t sizeof_fpregset = 0;
/* Register number for %st(0). The register numbers for the other
registers follow from this one. Set this to a value >= 0 if FPU is
present. */
int st0_regnum = -1;
/* Number of MMX registers. */
int num_mmx_regs = 0;
/* Register number for %mm0. Set this to a value >= 0 if MMX is
supported. */
int mm0_regnum = -1;
/* Number of pseudo YMM registers. */
int num_ymm_regs = 0;
/* Register number for %ymm0. Set this to a value >= 0 if pseudo YMM
registers are supported. */
int ymm0_regnum = -1;
/* Number of AVX512 OpMask registers (K-registers) */
int num_k_regs = 0;
/* Register number for %k0. Set this to a value >= 0 if AVX512 OpMask
is supported. */
int k0_regnum = -1;
/* Number of pseudo ZMM registers ($zmm0-$zmm31). */
int num_zmm_regs = 0;
/* Register number for %zmm0. Set this to a value >= 0 if pseudo ZMM
registers are supported. */
int zmm0_regnum = -1;
/* Number of byte registers. */
int num_byte_regs = 0;
/* Register pseudo number for %al. If supported, set this to a
value >= 0. */
int al_regnum = -1;
/* Number of pseudo word registers. */
int num_word_regs = 0;
/* Register number for %ax. If supported, set this to a value >= 0. */
int ax_regnum = -1;
/* Number of pseudo dword registers. */
int num_dword_regs = 0;
/* Register number for %eax. Set this to a value >= 0 if pseudo dword
registers are supported. */
int eax_regnum = -1;
/* Number of core registers. */
int num_core_regs = 0;
/* Number of SSE registers. */
int num_xmm_regs = 0;
/* Number of SSE registers added in AVX512. */
int num_xmm_avx512_regs = 0;
/* Register number of XMM16, the first XMM register added in AVX512.
Set this to a value >= 0 if XMM registers are supported. */
int xmm16_regnum = -1;
/* Number of YMM registers added in AVX512. */
int num_ymm_avx512_regs = 0;
/* Register number of YMM16, the first YMM register added in AVX512.
Set this to a value >= 0 if YMM registers are supported. */
int ymm16_regnum = -1;
/* Bits of the extended control register 0 (the XFEATURE_ENABLED_MASK
register), excluding the x87 bit, which are supported by this GDB. */
uint64_t xcr0 = 0;
/* Offset of XCR0 in XSAVE extended state. */
int xsave_xcr0_offset = 0;
/* Layout of the XSAVE area extended region. */
x86_xsave_layout xsave_layout;
/* Register names. */
const char * const *register_names = nullptr;
/* Register number for %ymm0h. Set this to a value >= 0 if they are
supported. */
int ymm0h_regnum = -1;
/* Upper YMM register names. Only used for tdesc_numbered_register. */
const char * const *ymmh_register_names = nullptr;
/* Register number for %ymm16h. Set this to a value >= 0 if they are
supported. */
int ymm16h_regnum = -1;
/* YMM16-31 register names. Only used for tdesc_numbered_register. */
const char * const *ymm16h_register_names = nullptr;
/* Register number for %zmm0h. Set this to a value >= 0 if ZMM_HI256
registers are supported. */
int zmm0h_regnum = -1;
/* OpMask register names. */
const char * const *k_register_names = nullptr;
/* ZMM register names. Only used for tdesc_numbered_register. */
const char * const *zmmh_register_names = nullptr;
/* XMM16-31 register names. Only used for tdesc_numbered_register. */
const char * const *xmm_avx512_register_names = nullptr;
/* YMM16-31 register names. Only used for tdesc_numbered_register. */
const char * const *ymm_avx512_register_names = nullptr;
/* Number of PKEYS registers. */
int num_pkeys_regs = 0;
/* Register number for PKRU register. If supported, set this to a value
>= 0. */
int pkru_regnum = -1;
/* PKEYS register names. */
const char * const *pkeys_register_names = nullptr;
/* Register number for the shadow stack pointer register. If supported,
set this to a value >= 0. */
int ssp_regnum = -1;
/* Register number for %fsbase. If supported, set this to a value
>= 0. */
int fsbase_regnum = -1;
/* Target description. */
const struct target_desc *tdesc = nullptr;
/* Register group function. */
gdbarch_register_reggroup_p_ftype *register_reggroup_p = nullptr;
/* Offset of saved PC in jmp_buf. */
int jb_pc_offset = 0;
/* Convention for returning structures. */
enum struct_return struct_return {};
/* Address range where sigtramp lives. */
CORE_ADDR sigtramp_start = 0;
CORE_ADDR sigtramp_end = 0;
/* Detect sigtramp. */
int (*sigtramp_p) (const frame_info_ptr &) = nullptr;
/* Get address of sigcontext for sigtramp. */
CORE_ADDR (*sigcontext_addr) (const frame_info_ptr &) = nullptr;
/* Offset of registers in `struct sigcontext'. */
int *sc_reg_offset = 0;
int sc_num_regs = 0;
/* Offset of saved PC and SP in `struct sigcontext'. Usage of these
is deprecated, please use `sc_reg_offset' instead. */
int sc_pc_offset = 0;
int sc_sp_offset = 0;
/* ISA-specific data types. */
struct type *i386_mmx_type = nullptr;
struct type *i386_ymm_type = nullptr;
struct type *i386_zmm_type = nullptr;
struct type *i387_ext_type = nullptr;
/* Process record/replay target. */
/* The map for registers because the AMD64's registers order
in GDB is not same as I386 instructions. */
const int *record_regmap = nullptr;
/* Parse intx80 args. */
int (*i386_intx80_record) (struct regcache *regcache) = nullptr;
/* Parse sysenter args. */
int (*i386_sysenter_record) (struct regcache *regcache) = nullptr;
/* Parse syscall args. */
int (*i386_syscall_record) (struct regcache *regcache) = nullptr;
/* Regsets. */
const struct regset *fpregset = nullptr;
};
/* Floating-point registers. */
/* All FPU control registers (except for FIOFF and FOOFF) are 16-bit
(at most) in the FPU, but are zero-extended to 32 bits in GDB's
register cache. */
/* Return non-zero if REGNUM matches the FP register and the FP
register set is active. */
extern int i386_fp_regnum_p (struct gdbarch *, int);
extern int i386_fpc_regnum_p (struct gdbarch *, int);
/* Register numbers of various important registers. */
enum i386_regnum
{
I386_EAX_REGNUM, /* %eax */
I386_ECX_REGNUM, /* %ecx */
I386_EDX_REGNUM, /* %edx */
I386_EBX_REGNUM, /* %ebx */
I386_ESP_REGNUM, /* %esp */
I386_EBP_REGNUM, /* %ebp */
I386_ESI_REGNUM, /* %esi */
I386_EDI_REGNUM, /* %edi */
I386_EIP_REGNUM, /* %eip */
I386_EFLAGS_REGNUM, /* %eflags */
I386_CS_REGNUM, /* %cs */
I386_SS_REGNUM, /* %ss */
I386_DS_REGNUM, /* %ds */
I386_ES_REGNUM, /* %es */
I386_FS_REGNUM, /* %fs */
I386_GS_REGNUM, /* %gs */
I386_ST0_REGNUM, /* %st(0) */
I386_MXCSR_REGNUM = 40, /* %mxcsr */
I386_YMM0H_REGNUM, /* %ymm0h */
I386_YMM7H_REGNUM = I386_YMM0H_REGNUM + 7,
/* MPX is deprecated. Yet we keep this to not give the registers below
a new number. That could break older gdbservers. */
I386_BND0R_REGNUM,
I386_BND3R_REGNUM = I386_BND0R_REGNUM + 3,
I386_BNDCFGU_REGNUM,
I386_BNDSTATUS_REGNUM,
I386_K0_REGNUM, /* %k0 */
I386_K7_REGNUM = I386_K0_REGNUM + 7,
I386_ZMM0H_REGNUM, /* %zmm0h */
I386_ZMM7H_REGNUM = I386_ZMM0H_REGNUM + 7,
I386_PKRU_REGNUM,
I386_PL3_SSP_REGNUM,
I386_FSBASE_REGNUM,
I386_GSBASE_REGNUM,
I386_NUM_REGS, /* Calculated from last *_REGNUM entry. */
I386_SSE_NUM_REGS = I386_MXCSR_REGNUM + 1,
I386_AVX_NUM_REGS = I386_YMM7H_REGNUM + 1,
I386_AVX512_NUM_REGS = I386_ZMM7H_REGNUM + 1,
I386_PKEYS_NUM_REGS = I386_PKRU_REGNUM + 1
/* STOP! New *_REGNUM entries should be added before I386_NUM_REGS. */
};
/* Register numbers of RECORD_REGMAP. */
enum record_i386_regnum
{
X86_RECORD_REAX_REGNUM,
X86_RECORD_RECX_REGNUM,
X86_RECORD_REDX_REGNUM,
X86_RECORD_REBX_REGNUM,
X86_RECORD_RESP_REGNUM,
X86_RECORD_REBP_REGNUM,
X86_RECORD_RESI_REGNUM,
X86_RECORD_REDI_REGNUM,
X86_RECORD_R8_REGNUM,
X86_RECORD_R9_REGNUM,
X86_RECORD_R10_REGNUM,
X86_RECORD_R11_REGNUM,
X86_RECORD_R12_REGNUM,
X86_RECORD_R13_REGNUM,
X86_RECORD_R14_REGNUM,
X86_RECORD_R15_REGNUM,
X86_RECORD_REIP_REGNUM,
X86_RECORD_EFLAGS_REGNUM,
X86_RECORD_CS_REGNUM,
X86_RECORD_SS_REGNUM,
X86_RECORD_DS_REGNUM,
X86_RECORD_ES_REGNUM,
X86_RECORD_FS_REGNUM,
X86_RECORD_GS_REGNUM,
X86_RECORD_XMM0_REGNUM,
};
#define I386_NUM_GREGS 16
#define I386_NUM_XREGS 9
/* Size of the largest register. */
#define I386_MAX_REGISTER_SIZE 64
/* Types for i386-specific registers. */
extern struct type *i387_ext_type (struct gdbarch *gdbarch);
/* Checks of different registers. */
extern int i386_byte_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_word_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_dword_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_xmm_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_xmm_avx512_regnum_p (struct gdbarch * gdbarch, int regnum);
extern int i386_ymm_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_ymm_avx512_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_k_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_zmm_regnum_p (struct gdbarch *gdbarch, int regnum);
extern int i386_zmmh_regnum_p (struct gdbarch *gdbarch, int regnum);
extern bool i386_pkru_regnum_p (struct gdbarch *gdbarch, int regnum);
extern const char *i386_pseudo_register_name (struct gdbarch *gdbarch,
int regnum);
extern struct type *i386_pseudo_register_type (struct gdbarch *gdbarch,
int regnum);
extern value *i386_pseudo_register_read_value (gdbarch *gdbarch,
const frame_info_ptr &next_frame,
int regnum);
extern void i386_pseudo_register_write (gdbarch *gdbarch,
const frame_info_ptr &next_frame, int regnum,
gdb::array_view<const gdb_byte> buf);
extern int i386_ax_pseudo_register_collect (struct gdbarch *gdbarch,
struct agent_expr *ax,
int regnum);
/* Segment selectors. */
#define I386_SEL_RPL 0x0003 /* Requester's Privilege Level mask. */
#define I386_SEL_UPL 0x0003 /* User Privilege Level. */
#define I386_SEL_KPL 0x0000 /* Kernel Privilege Level. */
/* The length of the longest i386 instruction (according to
include/asm-i386/kprobes.h in Linux 2.6. */
#define I386_MAX_INSN_LEN (16)
/* Functions exported from i386-tdep.c. */
extern CORE_ADDR i386_pe_skip_trampoline_code (const frame_info_ptr &frame,
CORE_ADDR pc, char *name);
extern CORE_ADDR i386_skip_main_prologue (struct gdbarch *gdbarch,
CORE_ADDR pc);
/* The "push_dummy_call" gdbarch method, optionally with the thiscall
calling convention. */
extern CORE_ADDR i386_thiscall_push_dummy_call (struct gdbarch *gdbarch,
struct value *function,
struct regcache *regcache,
CORE_ADDR bp_addr,
int nargs, struct value **args,
CORE_ADDR sp,
function_call_return_method
return_method,
CORE_ADDR struct_addr,
bool thiscall);
/* Return whether the THIS_FRAME corresponds to a sigtramp routine. */
extern int i386_sigtramp_p (const frame_info_ptr &this_frame);
/* Return non-zero if REGNUM is a member of the specified group. */
extern int i386_register_reggroup_p (struct gdbarch *gdbarch, int regnum,
const struct reggroup *group);
/* Supply register REGNUM from the general-purpose register set REGSET
to register cache REGCACHE. If REGNUM is -1, do this for all
registers in REGSET. */
extern void i386_supply_gregset (const struct regset *regset,
struct regcache *regcache, int regnum,
const void *gregs, size_t len);
/* General-purpose register set. */
extern const struct regset i386_gregset;
/* Floating-point register set. */
extern const struct regset i386_fpregset;
/* Default iterator over core file register note sections. */
extern void
i386_iterate_over_regset_sections (struct gdbarch *gdbarch,
iterate_over_regset_sections_cb *cb,
void *cb_data,
const struct regcache *regcache);
typedef buf_displaced_step_copy_insn_closure
i386_displaced_step_copy_insn_closure;
extern displaced_step_copy_insn_closure_up i386_displaced_step_copy_insn
(struct gdbarch *gdbarch, CORE_ADDR from, CORE_ADDR to,
struct regcache *regs);
extern void i386_displaced_step_fixup
(struct gdbarch *gdbarch, displaced_step_copy_insn_closure *closure,
CORE_ADDR from, CORE_ADDR to, regcache *regs, bool completed_p);
/* Initialize a basic ELF architecture variant. */
extern void i386_elf_init_abi (struct gdbarch_info, struct gdbarch *);
/* Initialize a SVR4 architecture variant. */
extern void i386_svr4_init_abi (struct gdbarch_info, struct gdbarch *);
/* Convert SVR4 register number REG to the appropriate register number
used by GDB. */
extern int i386_svr4_reg_to_regnum (struct gdbarch *gdbarch, int reg);
extern int i386_process_record (struct gdbarch *gdbarch,
struct regcache *regcache, CORE_ADDR addr);
/* Return the target description for the specified xsave features as
defined in XSTATE_BV and SEGMENTS. */
extern const struct target_desc *i386_target_description
(uint64_t xstate_bv, bool segments);
/* Functions and variables exported from i386-bsd-tdep.c. */
extern void i386bsd_init_abi (struct gdbarch_info, struct gdbarch *);
extern CORE_ADDR i386obsd_sigtramp_start_addr;
extern CORE_ADDR i386obsd_sigtramp_end_addr;
extern int i386obsd_sc_reg_offset[];
extern int i386bsd_sc_reg_offset[];
/* SystemTap related functions. */
extern int i386_stap_is_single_operand (struct gdbarch *gdbarch,
const char *s);
extern expr::operation_up i386_stap_parse_special_token
(struct gdbarch *gdbarch, struct stap_parse_info *p);
#endif /* GDB_I386_TDEP_H */
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