gdb/i386-linux-nat.c

   1 /* Native-dependent code for GNU/Linux i386.
   2
   3    Copyright (C) 1999-2023 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #include "defs.h"
  21 #include "inferior.h"
  22 #include "gdbcore.h"
  23 #include "regcache.h"
  24 #include "elf/common.h"
  25 #include "nat/gdb_ptrace.h"
  26 #include <sys/uio.h>
  27 #include "gregset.h"
  28 #include "gdb_proc_service.h"
  29
  30 #include "i386-linux-nat.h"
  31 #include "i387-tdep.h"
  32 #include "i386-tdep.h"
  33 #include "i386-linux-tdep.h"
  34 #include "gdbsupport/x86-xstate.h"
  35
  36 #include "x86-linux-nat.h"
  37 #include "nat/linux-ptrace.h"
  38 #include "inf-ptrace.h"
  39
  40 struct i386_linux_nat_target final : public x86_linux_nat_target
  41 {
  42   /* Add our register access methods.  */
  43   void fetch_registers (struct regcache *, int) override;
  44   void store_registers (struct regcache *, int) override;
  45
  46   /* Override the default ptrace resume method.  */
  47   void low_resume (ptid_t ptid, int step, enum gdb_signal sig) override;
  48 };
  49
  50 static i386_linux_nat_target the_i386_linux_nat_target;
  51
  52 /* The register sets used in GNU/Linux ELF core-dumps are identical to
  53    the register sets in `struct user' that is used for a.out
  54    core-dumps, and is also used by `ptrace'.  The corresponding types
  55    are `elf_gregset_t' for the general-purpose registers (with
  56    `elf_greg_t' the type of a single GP register) and `elf_fpregset_t'
  57    for the floating-point registers.
  58
  59    Those types used to be available under the names `gregset_t' and
  60    `fpregset_t' too, and this file used those names in the past.  But
  61    those names are now used for the register sets used in the
  62    `mcontext_t' type, and have a different size and layout.  */
  63
  64 /* Which ptrace request retrieves which registers?
  65    These apply to the corresponding SET requests as well.  */
  66
  67 #define GETREGS_SUPPLIES(regno) \
  68   ((0 <= (regno) && (regno) <= 15) || (regno) == I386_LINUX_ORIG_EAX_REGNUM)
  69
  70 #define GETFPXREGS_SUPPLIES(regno) \
  71   (I386_ST0_REGNUM <= (regno) && (regno) < I386_SSE_NUM_REGS)
  72
  73 #define GETXSTATEREGS_SUPPLIES(regno) \
  74   (I386_ST0_REGNUM <= (regno) && (regno) < I386_PKEYS_NUM_REGS)
  75
  76 /* Does the current host support the GETREGS request?  */
  77 int have_ptrace_getregs =
  78 #ifdef HAVE_PTRACE_GETREGS
  79   1
  80 #else
  81   0
  82 #endif
  83 ;
  84
  85 /* Does the current host support the GETFPXREGS request?  The header
  86    file may or may not define it, and even if it is defined, the
  87    kernel will return EIO if it's running on a pre-SSE processor.
  88
  89    My instinct is to attach this to some architecture- or
  90    target-specific data structure, but really, a particular GDB
  91    process can only run on top of one kernel at a time.  So it's okay
  92    for this to be a simple variable.  */
  93 int have_ptrace_getfpxregs =
  94 #ifdef HAVE_PTRACE_GETFPXREGS
  95   -1
  96 #else
  97   0
  98 #endif
  99 ;
 100 \f
 101
 102 /* Accessing registers through the U area, one at a time.  */
 103
 104 /* Fetch one register.  */
 105
 106 static void
 107 fetch_register (struct regcache *regcache, int regno)
 108 {
 109   pid_t tid;
 110   int val;
 111
 112   gdb_assert (!have_ptrace_getregs);
 113   if (i386_linux_gregset_reg_offset[regno] == -1)
 114     {
 115       regcache->raw_supply (regno, NULL);
 116       return;
 117     }
 118
 119   tid = get_ptrace_pid (regcache->ptid ());
 120
 121   errno = 0;
 122   val = ptrace (PTRACE_PEEKUSER, tid,
 123                 i386_linux_gregset_reg_offset[regno], 0);
 124   if (errno != 0)
 125     error (_("Couldn't read register %s (#%d): %s."),
 126            gdbarch_register_name (regcache->arch (), regno),
 127            regno, safe_strerror (errno));
 128
 129   regcache->raw_supply (regno, &val);
 130 }
 131
 132 /* Store one register.  */
 133
 134 static void
 135 store_register (const struct regcache *regcache, int regno)
 136 {
 137   pid_t tid;
 138   int val;
 139
 140   gdb_assert (!have_ptrace_getregs);
 141   if (i386_linux_gregset_reg_offset[regno] == -1)
 142     return;
 143
 144   tid = get_ptrace_pid (regcache->ptid ());
 145
 146   errno = 0;
 147   regcache->raw_collect (regno, &val);
 148   ptrace (PTRACE_POKEUSER, tid,
 149           i386_linux_gregset_reg_offset[regno], val);
 150   if (errno != 0)
 151     error (_("Couldn't write register %s (#%d): %s."),
 152            gdbarch_register_name (regcache->arch (), regno),
 153            regno, safe_strerror (errno));
 154 }
 155 \f
 156
 157 /* Transfering the general-purpose registers between GDB, inferiors
 158    and core files.  */
 159
 160 /* Fill GDB's register array with the general-purpose register values
 161    in *GREGSETP.  */
 162
 163 void
 164 supply_gregset (struct regcache *regcache, const elf_gregset_t *gregsetp)
 165 {
 166   const gdb_byte *regp = (const gdb_byte *) gregsetp;
 167   int i;
 168
 169   for (i = 0; i < I386_NUM_GREGS; i++)
 170     regcache->raw_supply (i, regp + i386_linux_gregset_reg_offset[i]);
 171
 172   if (I386_LINUX_ORIG_EAX_REGNUM
 173         < gdbarch_num_regs (regcache->arch ()))
 174     regcache->raw_supply
 175       (I386_LINUX_ORIG_EAX_REGNUM,
 176        regp + i386_linux_gregset_reg_offset[I386_LINUX_ORIG_EAX_REGNUM]);
 177 }
 178
 179 /* Fill register REGNO (if it is a general-purpose register) in
 180    *GREGSETPS with the value in GDB's register array.  If REGNO is -1,
 181    do this for all registers.  */
 182
 183 void
 184 fill_gregset (const struct regcache *regcache,
 185               elf_gregset_t *gregsetp, int regno)
 186 {
 187   gdb_byte *regp = (gdb_byte *) gregsetp;
 188   int i;
 189
 190   for (i = 0; i < I386_NUM_GREGS; i++)
 191     if (regno == -1 || regno == i)
 192       regcache->raw_collect (i, regp + i386_linux_gregset_reg_offset[i]);
 193
 194   if ((regno == -1 || regno == I386_LINUX_ORIG_EAX_REGNUM)
 195       && I386_LINUX_ORIG_EAX_REGNUM
 196            < gdbarch_num_regs (regcache->arch ()))
 197     regcache->raw_collect
 198       (I386_LINUX_ORIG_EAX_REGNUM,
 199        regp + i386_linux_gregset_reg_offset[I386_LINUX_ORIG_EAX_REGNUM]);
 200 }
 201
 202 #ifdef HAVE_PTRACE_GETREGS
 203
 204 /* Fetch all general-purpose registers from process/thread TID and
 205    store their values in GDB's register array.  */
 206
 207 static void
 208 fetch_regs (struct regcache *regcache, int tid)
 209 {
 210   elf_gregset_t regs;
 211   elf_gregset_t *regs_p = &regs;
 212
 213   if (ptrace (PTRACE_GETREGS, tid, 0, (int) &regs) < 0)
 214     {
 215       if (errno == EIO)
 216         {
 217           /* The kernel we're running on doesn't support the GETREGS
 218              request.  Reset `have_ptrace_getregs'.  */
 219           have_ptrace_getregs = 0;
 220           return;
 221         }
 222
 223       perror_with_name (_("Couldn't get registers"));
 224     }
 225
 226   supply_gregset (regcache, (const elf_gregset_t *) regs_p);
 227 }
 228
 229 /* Store all valid general-purpose registers in GDB's register array
 230    into the process/thread specified by TID.  */
 231
 232 static void
 233 store_regs (const struct regcache *regcache, int tid, int regno)
 234 {
 235   elf_gregset_t regs;
 236
 237   if (ptrace (PTRACE_GETREGS, tid, 0, (int) &regs) < 0)
 238     perror_with_name (_("Couldn't get registers"));
 239
 240   fill_gregset (regcache, &regs, regno);
 241
 242   if (ptrace (PTRACE_SETREGS, tid, 0, (int) &regs) < 0)
 243     perror_with_name (_("Couldn't write registers"));
 244 }
 245
 246 #else
 247
 248 static void fetch_regs (struct regcache *regcache, int tid) {}
 249 static void store_regs (const struct regcache *regcache, int tid, int regno) {}
 250
 251 #endif
 252 \f
 253
 254 /* Transfering floating-point registers between GDB, inferiors and cores.  */
 255
 256 /* Fill GDB's register array with the floating-point register values in
 257    *FPREGSETP.  */
 258
 259 void
 260 supply_fpregset (struct regcache *regcache, const elf_fpregset_t *fpregsetp)
 261 {
 262   i387_supply_fsave (regcache, -1, fpregsetp);
 263 }
 264
 265 /* Fill register REGNO (if it is a floating-point register) in
 266    *FPREGSETP with the value in GDB's register array.  If REGNO is -1,
 267    do this for all registers.  */
 268
 269 void
 270 fill_fpregset (const struct regcache *regcache,
 271                elf_fpregset_t *fpregsetp, int regno)
 272 {
 273   i387_collect_fsave (regcache, regno, fpregsetp);
 274 }
 275
 276 #ifdef HAVE_PTRACE_GETREGS
 277
 278 /* Fetch all floating-point registers from process/thread TID and store
 279    thier values in GDB's register array.  */
 280
 281 static void
 282 fetch_fpregs (struct regcache *regcache, int tid)
 283 {
 284   elf_fpregset_t fpregs;
 285
 286   if (ptrace (PTRACE_GETFPREGS, tid, 0, (int) &fpregs) < 0)
 287     perror_with_name (_("Couldn't get floating point status"));
 288
 289   supply_fpregset (regcache, (const elf_fpregset_t *) &fpregs);
 290 }
 291
 292 /* Store all valid floating-point registers in GDB's register array
 293    into the process/thread specified by TID.  */
 294
 295 static void
 296 store_fpregs (const struct regcache *regcache, int tid, int regno)
 297 {
 298   elf_fpregset_t fpregs;
 299
 300   if (ptrace (PTRACE_GETFPREGS, tid, 0, (int) &fpregs) < 0)
 301     perror_with_name (_("Couldn't get floating point status"));
 302
 303   fill_fpregset (regcache, &fpregs, regno);
 304
 305   if (ptrace (PTRACE_SETFPREGS, tid, 0, (int) &fpregs) < 0)
 306     perror_with_name (_("Couldn't write floating point status"));
 307 }
 308
 309 #else
 310
 311 static void
 312 fetch_fpregs (struct regcache *regcache, int tid)
 313 {
 314 }
 315
 316 static void
 317 store_fpregs (const struct regcache *regcache, int tid, int regno)
 318 {
 319 }
 320
 321 #endif
 322 \f
 323
 324 /* Transfering floating-point and SSE registers to and from GDB.  */
 325
 326 /* Fetch all registers covered by the PTRACE_GETREGSET request from
 327    process/thread TID and store their values in GDB's register array.
 328    Return non-zero if successful, zero otherwise.  */
 329
 330 static int
 331 fetch_xstateregs (struct regcache *regcache, int tid)
 332 {
 333   char xstateregs[X86_XSTATE_MAX_SIZE];
 334   struct iovec iov;
 335
 336   if (have_ptrace_getregset != TRIBOOL_TRUE)
 337     return 0;
 338
 339   iov.iov_base = xstateregs;
 340   iov.iov_len = sizeof(xstateregs);
 341   if (ptrace (PTRACE_GETREGSET, tid, (unsigned int) NT_X86_XSTATE,
 342               &iov) < 0)
 343     perror_with_name (_("Couldn't read extended state status"));
 344
 345   i387_supply_xsave (regcache, -1, xstateregs);
 346   return 1;
 347 }
 348
 349 /* Store all valid registers in GDB's register array covered by the
 350    PTRACE_SETREGSET request into the process/thread specified by TID.
 351    Return non-zero if successful, zero otherwise.  */
 352
 353 static int
 354 store_xstateregs (const struct regcache *regcache, int tid, int regno)
 355 {
 356   char xstateregs[X86_XSTATE_MAX_SIZE];
 357   struct iovec iov;
 358
 359   if (have_ptrace_getregset != TRIBOOL_TRUE)
 360     return 0;
 361
 362   iov.iov_base = xstateregs;
 363   iov.iov_len = sizeof(xstateregs);
 364   if (ptrace (PTRACE_GETREGSET, tid, (unsigned int) NT_X86_XSTATE,
 365               &iov) < 0)
 366     perror_with_name (_("Couldn't read extended state status"));
 367
 368   i387_collect_xsave (regcache, regno, xstateregs, 0);
 369
 370   if (ptrace (PTRACE_SETREGSET, tid, (unsigned int) NT_X86_XSTATE,
 371               (int) &iov) < 0)
 372     perror_with_name (_("Couldn't write extended state status"));
 373
 374   return 1;
 375 }
 376
 377 #ifdef HAVE_PTRACE_GETFPXREGS
 378
 379 /* Fetch all registers covered by the PTRACE_GETFPXREGS request from
 380    process/thread TID and store their values in GDB's register array.
 381    Return non-zero if successful, zero otherwise.  */
 382
 383 static int
 384 fetch_fpxregs (struct regcache *regcache, int tid)
 385 {
 386   elf_fpxregset_t fpxregs;
 387
 388   if (! have_ptrace_getfpxregs)
 389     return 0;
 390
 391   if (ptrace (PTRACE_GETFPXREGS, tid, 0, (int) &fpxregs) < 0)
 392     {
 393       if (errno == EIO)
 394         {
 395           have_ptrace_getfpxregs = 0;
 396           return 0;
 397         }
 398
 399       perror_with_name (_("Couldn't read floating-point and SSE registers"));
 400     }
 401
 402   i387_supply_fxsave (regcache, -1, (const elf_fpxregset_t *) &fpxregs);
 403   return 1;
 404 }
 405
 406 /* Store all valid registers in GDB's register array covered by the
 407    PTRACE_SETFPXREGS request into the process/thread specified by TID.
 408    Return non-zero if successful, zero otherwise.  */
 409
 410 static int
 411 store_fpxregs (const struct regcache *regcache, int tid, int regno)
 412 {
 413   elf_fpxregset_t fpxregs;
 414
 415   if (! have_ptrace_getfpxregs)
 416     return 0;
 417
 418   if (ptrace (PTRACE_GETFPXREGS, tid, 0, &fpxregs) == -1)
 419     {
 420       if (errno == EIO)
 421         {
 422           have_ptrace_getfpxregs = 0;
 423           return 0;
 424         }
 425
 426       perror_with_name (_("Couldn't read floating-point and SSE registers"));
 427     }
 428
 429   i387_collect_fxsave (regcache, regno, &fpxregs);
 430
 431   if (ptrace (PTRACE_SETFPXREGS, tid, 0, &fpxregs) == -1)
 432     perror_with_name (_("Couldn't write floating-point and SSE registers"));
 433
 434   return 1;
 435 }
 436
 437 #else
 438
 439 static int
 440 fetch_fpxregs (struct regcache *regcache, int tid)
 441 {
 442   return 0;
 443 }
 444
 445 static int
 446 store_fpxregs (const struct regcache *regcache, int tid, int regno)
 447 {
 448   return 0;
 449 }
 450
 451 #endif /* HAVE_PTRACE_GETFPXREGS */
 452 \f
 453
 454 /* Transferring arbitrary registers between GDB and inferior.  */
 455
 456 /* Fetch register REGNO from the child process.  If REGNO is -1, do
 457    this for all registers (including the floating point and SSE
 458    registers).  */
 459
 460 void
 461 i386_linux_nat_target::fetch_registers (struct regcache *regcache, int regno)
 462 {
 463   pid_t tid;
 464
 465   /* Use the old method of peeking around in `struct user' if the
 466      GETREGS request isn't available.  */
 467   if (!have_ptrace_getregs)
 468     {
 469       int i;
 470
 471       for (i = 0; i < gdbarch_num_regs (regcache->arch ()); i++)
 472         if (regno == -1 || regno == i)
 473           fetch_register (regcache, i);
 474
 475       return;
 476     }
 477
 478   tid = get_ptrace_pid (regcache->ptid ());
 479
 480   /* Use the PTRACE_GETFPXREGS request whenever possible, since it
 481      transfers more registers in one system call, and we'll cache the
 482      results.  But remember that fetch_fpxregs can fail, and return
 483      zero.  */
 484   if (regno == -1)
 485     {
 486       fetch_regs (regcache, tid);
 487
 488       /* The call above might reset `have_ptrace_getregs'.  */
 489       if (!have_ptrace_getregs)
 490         {
 491           fetch_registers (regcache, regno);
 492           return;
 493         }
 494
 495       if (fetch_xstateregs (regcache, tid))
 496         return;
 497       if (fetch_fpxregs (regcache, tid))
 498         return;
 499       fetch_fpregs (regcache, tid);
 500       return;
 501     }
 502
 503   if (GETREGS_SUPPLIES (regno))
 504     {
 505       fetch_regs (regcache, tid);
 506       return;
 507     }
 508
 509   if (GETXSTATEREGS_SUPPLIES (regno))
 510     {
 511       if (fetch_xstateregs (regcache, tid))
 512         return;
 513     }
 514
 515   if (GETFPXREGS_SUPPLIES (regno))
 516     {
 517       if (fetch_fpxregs (regcache, tid))
 518         return;
 519
 520       /* Either our processor or our kernel doesn't support the SSE
 521          registers, so read the FP registers in the traditional way,
 522          and fill the SSE registers with dummy values.  It would be
 523          more graceful to handle differences in the register set using
 524          gdbarch.  Until then, this will at least make things work
 525          plausibly.  */
 526       fetch_fpregs (regcache, tid);
 527       return;
 528     }
 529
 530   internal_error (_("Got request for bad register number %d."), regno);
 531 }
 532
 533 /* Store register REGNO back into the child process.  If REGNO is -1,
 534    do this for all registers (including the floating point and SSE
 535    registers).  */
 536 void
 537 i386_linux_nat_target::store_registers (struct regcache *regcache, int regno)
 538 {
 539   pid_t tid;
 540
 541   /* Use the old method of poking around in `struct user' if the
 542      SETREGS request isn't available.  */
 543   if (!have_ptrace_getregs)
 544     {
 545       int i;
 546
 547       for (i = 0; i < gdbarch_num_regs (regcache->arch ()); i++)
 548         if (regno == -1 || regno == i)
 549           store_register (regcache, i);
 550
 551       return;
 552     }
 553
 554   tid = get_ptrace_pid (regcache->ptid ());
 555
 556   /* Use the PTRACE_SETFPXREGS requests whenever possible, since it
 557      transfers more registers in one system call.  But remember that
 558      store_fpxregs can fail, and return zero.  */
 559   if (regno == -1)
 560     {
 561       store_regs (regcache, tid, regno);
 562       if (store_xstateregs (regcache, tid, regno))
 563         return;
 564       if (store_fpxregs (regcache, tid, regno))
 565         return;
 566       store_fpregs (regcache, tid, regno);
 567       return;
 568     }
 569
 570   if (GETREGS_SUPPLIES (regno))
 571     {
 572       store_regs (regcache, tid, regno);
 573       return;
 574     }
 575
 576   if (GETXSTATEREGS_SUPPLIES (regno))
 577     {
 578       if (store_xstateregs (regcache, tid, regno))
 579         return;
 580     }
 581
 582   if (GETFPXREGS_SUPPLIES (regno))
 583     {
 584       if (store_fpxregs (regcache, tid, regno))
 585         return;
 586
 587       /* Either our processor or our kernel doesn't support the SSE
 588          registers, so just write the FP registers in the traditional
 589          way.  */
 590       store_fpregs (regcache, tid, regno);
 591       return;
 592     }
 593
 594   internal_error (_("Got request to store bad register number %d."), regno);
 595 }
 596 \f
 597
 598 /* Called by libthread_db.  Returns a pointer to the thread local
 599    storage (or its descriptor).  */
 600
 601 ps_err_e
 602 ps_get_thread_area (struct ps_prochandle *ph,
 603                     lwpid_t lwpid, int idx, void **base)
 604 {
 605   unsigned int base_addr;
 606   ps_err_e result;
 607
 608   result = x86_linux_get_thread_area (lwpid, (void *) idx, &base_addr);
 609
 610   if (result == PS_OK)
 611     *(int *) base = base_addr;
 612
 613   return result;
 614 }
 615 \f
 616
 617 /* The instruction for a GNU/Linux system call is:
 618        int $0x80
 619    or 0xcd 0x80.  */
 620
 621 static const unsigned char linux_syscall[] = { 0xcd, 0x80 };
 622
 623 #define LINUX_SYSCALL_LEN (sizeof linux_syscall)
 624
 625 /* The system call number is stored in the %eax register.  */
 626 #define LINUX_SYSCALL_REGNUM I386_EAX_REGNUM
 627
 628 /* We are specifically interested in the sigreturn and rt_sigreturn
 629    system calls.  */
 630
 631 #ifndef SYS_sigreturn
 632 #define SYS_sigreturn           0x77
 633 #endif
 634 #ifndef SYS_rt_sigreturn
 635 #define SYS_rt_sigreturn        0xad
 636 #endif
 637
 638 /* Offset to saved processor flags, from <asm/sigcontext.h>.  */
 639 #define LINUX_SIGCONTEXT_EFLAGS_OFFSET (64)
 640
 641 /* Resume execution of the inferior process.
 642    If STEP is nonzero, single-step it.
 643    If SIGNAL is nonzero, give it that signal.  */
 644
 645 void
 646 i386_linux_nat_target::low_resume (ptid_t ptid, int step, enum gdb_signal signal)
 647 {
 648   int pid = ptid.lwp ();
 649   int request;
 650
 651   if (catch_syscall_enabled () > 0)
 652    request = PTRACE_SYSCALL;
 653   else
 654     request = PTRACE_CONT;
 655
 656   if (step)
 657     {
 658       struct regcache *regcache = get_thread_regcache (this, ptid);
 659       struct gdbarch *gdbarch = regcache->arch ();
 660       enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
 661       ULONGEST pc;
 662       gdb_byte buf[LINUX_SYSCALL_LEN];
 663
 664       request = PTRACE_SINGLESTEP;
 665
 666       regcache_cooked_read_unsigned (regcache,
 667                                      gdbarch_pc_regnum (gdbarch), &pc);
 668
 669       /* Returning from a signal trampoline is done by calling a
 670          special system call (sigreturn or rt_sigreturn, see
 671          i386-linux-tdep.c for more information).  This system call
 672          restores the registers that were saved when the signal was
 673          raised, including %eflags.  That means that single-stepping
 674          won't work.  Instead, we'll have to modify the signal context
 675          that's about to be restored, and set the trace flag there.  */
 676
 677       /* First check if PC is at a system call.  */
 678       if (target_read_memory (pc, buf, LINUX_SYSCALL_LEN) == 0
 679           && memcmp (buf, linux_syscall, LINUX_SYSCALL_LEN) == 0)
 680         {
 681           ULONGEST syscall;
 682           regcache_cooked_read_unsigned (regcache,
 683                                          LINUX_SYSCALL_REGNUM, &syscall);
 684
 685           /* Then check the system call number.  */
 686           if (syscall == SYS_sigreturn || syscall == SYS_rt_sigreturn)
 687             {
 688               ULONGEST sp, addr;
 689               unsigned long int eflags;
 690
 691               regcache_cooked_read_unsigned (regcache, I386_ESP_REGNUM, &sp);
 692               if (syscall == SYS_rt_sigreturn)
 693                 addr = read_memory_unsigned_integer (sp + 8, 4, byte_order)
 694                   + 20;
 695               else
 696                 addr = sp;
 697
 698               /* Set the trace flag in the context that's about to be
 699                  restored.  */
 700               addr += LINUX_SIGCONTEXT_EFLAGS_OFFSET;
 701               read_memory (addr, (gdb_byte *) &eflags, 4);
 702               eflags |= 0x0100;
 703               write_memory (addr, (gdb_byte *) &eflags, 4);
 704             }
 705         }
 706     }
 707
 708   if (ptrace (request, pid, 0, gdb_signal_to_host (signal)) == -1)
 709     perror_with_name (("ptrace"));
 710 }
 711
 712 void _initialize_i386_linux_nat ();
 713 void
 714 _initialize_i386_linux_nat ()
 715 {
 716   linux_target = &the_i386_linux_nat_target;
 717
 718   /* Add the target.  */
 719   add_inf_child_target (linux_target);
 720 }