gdb/i386-linux-tdep.c

   1 /* Target-dependent code for Linux running on i386's, for GDB.
   2    Copyright 2000, 2001 Free Software Foundation, Inc.
   3
   4    This file is part of GDB.
   5
   6    This program is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program; if not, write to the Free Software
  18    Foundation, Inc., 59 Temple Place - Suite 330,
  19    Boston, MA 02111-1307, USA.  */
  20
  21 #include "defs.h"
  22 #include "gdbcore.h"
  23 #include "frame.h"
  24 #include "value.h"
  25 #include "regcache.h"
  26
  27 /* For i386_linux_skip_solib_resolver.  */
  28 #include "symtab.h"
  29 #include "symfile.h"
  30 #include "objfiles.h"
  31
  32 #include "solib-svr4.h"         /* For struct link_map_offsets.  */
  33
  34 \f
  35 /* Recognizing signal handler frames.  */
  36
  37 /* Linux has two flavors of signals.  Normal signal handlers, and
  38    "realtime" (RT) signals.  The RT signals can provide additional
  39    information to the signal handler if the SA_SIGINFO flag is set
  40    when establishing a signal handler using `sigaction'.  It is not
  41    unlikely that future versions of Linux will support SA_SIGINFO for
  42    normal signals too.  */
  43
  44 /* When the i386 Linux kernel calls a signal handler and the
  45    SA_RESTORER flag isn't set, the return address points to a bit of
  46    code on the stack.  This function returns whether the PC appears to
  47    be within this bit of code.
  48
  49    The instruction sequence for normal signals is
  50        pop    %eax
  51        mov    $0x77,%eax
  52        int    $0x80
  53    or 0x58 0xb8 0x77 0x00 0x00 0x00 0xcd 0x80.
  54
  55    Checking for the code sequence should be somewhat reliable, because
  56    the effect is to call the system call sigreturn.  This is unlikely
  57    to occur anywhere other than a signal trampoline.
  58
  59    It kind of sucks that we have to read memory from the process in
  60    order to identify a signal trampoline, but there doesn't seem to be
  61    any other way.  The IN_SIGTRAMP macro in tm-linux.h arranges to
  62    only call us if no function name could be identified, which should
  63    be the case since the code is on the stack.
  64
  65    Detection of signal trampolines for handlers that set the
  66    SA_RESTORER flag is in general not possible.  Unfortunately this is
  67    what the GNU C Library has been doing for quite some time now.
  68    However, as of version 2.1.2, the GNU C Library uses signal
  69    trampolines (named __restore and __restore_rt) that are identical
  70    to the ones used by the kernel.  Therefore, these trampolines are
  71    supported too.  */
  72
  73 #define LINUX_SIGTRAMP_INSN0 (0x58)     /* pop %eax */
  74 #define LINUX_SIGTRAMP_OFFSET0 (0)
  75 #define LINUX_SIGTRAMP_INSN1 (0xb8)     /* mov $NNNN,%eax */
  76 #define LINUX_SIGTRAMP_OFFSET1 (1)
  77 #define LINUX_SIGTRAMP_INSN2 (0xcd)     /* int */
  78 #define LINUX_SIGTRAMP_OFFSET2 (6)
  79
  80 static const unsigned char linux_sigtramp_code[] =
  81 {
  82   LINUX_SIGTRAMP_INSN0,                                 /* pop %eax */
  83   LINUX_SIGTRAMP_INSN1, 0x77, 0x00, 0x00, 0x00,         /* mov $0x77,%eax */
  84   LINUX_SIGTRAMP_INSN2, 0x80                            /* int $0x80 */
  85 };
  86
  87 #define LINUX_SIGTRAMP_LEN (sizeof linux_sigtramp_code)
  88
  89 /* If PC is in a sigtramp routine, return the address of the start of
  90    the routine.  Otherwise, return 0.  */
  91
  92 static CORE_ADDR
  93 i386_linux_sigtramp_start (CORE_ADDR pc)
  94 {
  95   unsigned char buf[LINUX_SIGTRAMP_LEN];
  96
  97   /* We only recognize a signal trampoline if PC is at the start of
  98      one of the three instructions.  We optimize for finding the PC at
  99      the start, as will be the case when the trampoline is not the
 100      first frame on the stack.  We assume that in the case where the
 101      PC is not at the start of the instruction sequence, there will be
 102      a few trailing readable bytes on the stack.  */
 103
 104   if (read_memory_nobpt (pc, (char *) buf, LINUX_SIGTRAMP_LEN) != 0)
 105     return 0;
 106
 107   if (buf[0] != LINUX_SIGTRAMP_INSN0)
 108     {
 109       int adjust;
 110
 111       switch (buf[0])
 112         {
 113         case LINUX_SIGTRAMP_INSN1:
 114           adjust = LINUX_SIGTRAMP_OFFSET1;
 115           break;
 116         case LINUX_SIGTRAMP_INSN2:
 117           adjust = LINUX_SIGTRAMP_OFFSET2;
 118           break;
 119         default:
 120           return 0;
 121         }
 122
 123       pc -= adjust;
 124
 125       if (read_memory_nobpt (pc, (char *) buf, LINUX_SIGTRAMP_LEN) != 0)
 126         return 0;
 127     }
 128
 129   if (memcmp (buf, linux_sigtramp_code, LINUX_SIGTRAMP_LEN) != 0)
 130     return 0;
 131
 132   return pc;
 133 }
 134
 135 /* This function does the same for RT signals.  Here the instruction
 136    sequence is
 137        mov    $0xad,%eax
 138        int    $0x80
 139    or 0xb8 0xad 0x00 0x00 0x00 0xcd 0x80.
 140
 141    The effect is to call the system call rt_sigreturn.  */
 142
 143 #define LINUX_RT_SIGTRAMP_INSN0 (0xb8)  /* mov $NNNN,%eax */
 144 #define LINUX_RT_SIGTRAMP_OFFSET0 (0)
 145 #define LINUX_RT_SIGTRAMP_INSN1 (0xcd)  /* int */
 146 #define LINUX_RT_SIGTRAMP_OFFSET1 (5)
 147
 148 static const unsigned char linux_rt_sigtramp_code[] =
 149 {
 150   LINUX_RT_SIGTRAMP_INSN0, 0xad, 0x00, 0x00, 0x00,      /* mov $0xad,%eax */
 151   LINUX_RT_SIGTRAMP_INSN1, 0x80                         /* int $0x80 */
 152 };
 153
 154 #define LINUX_RT_SIGTRAMP_LEN (sizeof linux_rt_sigtramp_code)
 155
 156 /* If PC is in a RT sigtramp routine, return the address of the start
 157    of the routine.  Otherwise, return 0.  */
 158
 159 static CORE_ADDR
 160 i386_linux_rt_sigtramp_start (CORE_ADDR pc)
 161 {
 162   unsigned char buf[LINUX_RT_SIGTRAMP_LEN];
 163
 164   /* We only recognize a signal trampoline if PC is at the start of
 165      one of the two instructions.  We optimize for finding the PC at
 166      the start, as will be the case when the trampoline is not the
 167      first frame on the stack.  We assume that in the case where the
 168      PC is not at the start of the instruction sequence, there will be
 169      a few trailing readable bytes on the stack.  */
 170
 171   if (read_memory_nobpt (pc, (char *) buf, LINUX_RT_SIGTRAMP_LEN) != 0)
 172     return 0;
 173
 174   if (buf[0] != LINUX_RT_SIGTRAMP_INSN0)
 175     {
 176       if (buf[0] != LINUX_RT_SIGTRAMP_INSN1)
 177         return 0;
 178
 179       pc -= LINUX_RT_SIGTRAMP_OFFSET1;
 180
 181       if (read_memory_nobpt (pc, (char *) buf, LINUX_RT_SIGTRAMP_LEN) != 0)
 182         return 0;
 183     }
 184
 185   if (memcmp (buf, linux_rt_sigtramp_code, LINUX_RT_SIGTRAMP_LEN) != 0)
 186     return 0;
 187
 188   return pc;
 189 }
 190
 191 /* Return whether PC is in a Linux sigtramp routine.  */
 192
 193 int
 194 i386_linux_in_sigtramp (CORE_ADDR pc, char *name)
 195 {
 196   if (name)
 197     return STREQ ("__restore", name) || STREQ ("__restore_rt", name);
 198
 199   return (i386_linux_sigtramp_start (pc) != 0
 200           || i386_linux_rt_sigtramp_start (pc) != 0);
 201 }
 202
 203 /* Assuming FRAME is for a Linux sigtramp routine, return the address
 204    of the associated sigcontext structure.  */
 205
 206 CORE_ADDR
 207 i386_linux_sigcontext_addr (struct frame_info *frame)
 208 {
 209   CORE_ADDR pc;
 210
 211   pc = i386_linux_sigtramp_start (frame->pc);
 212   if (pc)
 213     {
 214       CORE_ADDR sp;
 215
 216       if (frame->next)
 217         /* If this isn't the top frame, the next frame must be for the
 218            signal handler itself.  The sigcontext structure lives on
 219            the stack, right after the signum argument.  */
 220         return frame->next->frame + 12;
 221
 222       /* This is the top frame.  We'll have to find the address of the
 223          sigcontext structure by looking at the stack pointer.  Keep
 224          in mind that the first instruction of the sigtramp code is
 225          "pop %eax".  If the PC is at this instruction, adjust the
 226          returned value accordingly.  */
 227       sp = read_register (SP_REGNUM);
 228       if (pc == frame->pc)
 229         return sp + 4;
 230       return sp;
 231     }
 232
 233   pc = i386_linux_rt_sigtramp_start (frame->pc);
 234   if (pc)
 235     {
 236       if (frame->next)
 237         /* If this isn't the top frame, the next frame must be for the
 238            signal handler itself.  The sigcontext structure is part of
 239            the user context.  A pointer to the user context is passed
 240            as the third argument to the signal handler.  */
 241         return read_memory_integer (frame->next->frame + 16, 4) + 20;
 242
 243       /* This is the top frame.  Again, use the stack pointer to find
 244          the address of the sigcontext structure.  */
 245       return read_memory_integer (read_register (SP_REGNUM) + 8, 4) + 20;
 246     }
 247
 248   error ("Couldn't recognize signal trampoline.");
 249   return 0;
 250 }
 251
 252 /* Offset to saved PC in sigcontext, from <asm/sigcontext.h>.  */
 253 #define LINUX_SIGCONTEXT_PC_OFFSET (56)
 254
 255 /* Assuming FRAME is for a Linux sigtramp routine, return the saved
 256    program counter.  */
 257
 258 static CORE_ADDR
 259 i386_linux_sigtramp_saved_pc (struct frame_info *frame)
 260 {
 261   CORE_ADDR addr;
 262   addr = i386_linux_sigcontext_addr (frame);
 263   return read_memory_integer (addr + LINUX_SIGCONTEXT_PC_OFFSET, 4);
 264 }
 265
 266 /* Offset to saved SP in sigcontext, from <asm/sigcontext.h>.  */
 267 #define LINUX_SIGCONTEXT_SP_OFFSET (28)
 268
 269 /* Assuming FRAME is for a Linux sigtramp routine, return the saved
 270    stack pointer.  */
 271
 272 static CORE_ADDR
 273 i386_linux_sigtramp_saved_sp (struct frame_info *frame)
 274 {
 275   CORE_ADDR addr;
 276   addr = i386_linux_sigcontext_addr (frame);
 277   return read_memory_integer (addr + LINUX_SIGCONTEXT_SP_OFFSET, 4);
 278 }
 279
 280 /* Return the saved program counter for FRAME.  */
 281
 282 CORE_ADDR
 283 i386_linux_frame_saved_pc (struct frame_info *frame)
 284 {
 285   if (frame->signal_handler_caller)
 286     return i386_linux_sigtramp_saved_pc (frame);
 287
 288   /* See comment in "i386/tm-linux.h" for an explanation what this
 289      "FRAMELESS_SIGNAL" stuff is supposed to do.
 290
 291      FIXME: kettenis/2001-03-26: That comment should eventually be
 292      moved to this file.  */
 293   if (FRAMELESS_SIGNAL (frame))
 294     {
 295       CORE_ADDR sp = i386_linux_sigtramp_saved_sp (frame->next);
 296       return read_memory_unsigned_integer (sp, 4);
 297     }
 298
 299   return read_memory_unsigned_integer (frame->frame + 4, 4);
 300 }
 301
 302 /* Immediately after a function call, return the saved pc.  */
 303
 304 CORE_ADDR
 305 i386_linux_saved_pc_after_call (struct frame_info *frame)
 306 {
 307   if (frame->signal_handler_caller)
 308     return i386_linux_sigtramp_saved_pc (frame);
 309
 310   return read_memory_integer (read_register (SP_REGNUM), 4);
 311 }
 312
 313 \f
 314
 315 /* Calling functions in shared libraries.  */
 316 /* Find the minimal symbol named NAME, and return both the minsym
 317    struct and its objfile.  This probably ought to be in minsym.c, but
 318    everything there is trying to deal with things like C++ and
 319    SOFUN_ADDRESS_MAYBE_TURQUOISE, ...  Since this is so simple, it may
 320    be considered too special-purpose for general consumption.  */
 321
 322 static struct minimal_symbol *
 323 find_minsym_and_objfile (char *name, struct objfile **objfile_p)
 324 {
 325   struct objfile *objfile;
 326
 327   ALL_OBJFILES (objfile)
 328     {
 329       struct minimal_symbol *msym;
 330
 331       ALL_OBJFILE_MSYMBOLS (objfile, msym)
 332         {
 333           if (SYMBOL_NAME (msym)
 334               && STREQ (SYMBOL_NAME (msym), name))
 335             {
 336               *objfile_p = objfile;
 337               return msym;
 338             }
 339         }
 340     }
 341
 342   return 0;
 343 }
 344
 345 static CORE_ADDR
 346 skip_hurd_resolver (CORE_ADDR pc)
 347 {
 348   /* The HURD dynamic linker is part of the GNU C library, so many
 349      GNU/Linux distributions use it.  (All ELF versions, as far as I
 350      know.)  An unresolved PLT entry points to "_dl_runtime_resolve",
 351      which calls "fixup" to patch the PLT, and then passes control to
 352      the function.
 353
 354      We look for the symbol `_dl_runtime_resolve', and find `fixup' in
 355      the same objfile.  If we are at the entry point of `fixup', then
 356      we set a breakpoint at the return address (at the top of the
 357      stack), and continue.
 358
 359      It's kind of gross to do all these checks every time we're
 360      called, since they don't change once the executable has gotten
 361      started.  But this is only a temporary hack --- upcoming versions
 362      of Linux will provide a portable, efficient interface for
 363      debugging programs that use shared libraries.  */
 364
 365   struct objfile *objfile;
 366   struct minimal_symbol *resolver
 367     = find_minsym_and_objfile ("_dl_runtime_resolve", &objfile);
 368
 369   if (resolver)
 370     {
 371       struct minimal_symbol *fixup
 372         = lookup_minimal_symbol ("fixup", 0, objfile);
 373
 374       if (fixup && SYMBOL_VALUE_ADDRESS (fixup) == pc)
 375         return (SAVED_PC_AFTER_CALL (get_current_frame ()));
 376     }
 377
 378   return 0;
 379 }
 380
 381 /* See the comments for SKIP_SOLIB_RESOLVER at the top of infrun.c.
 382    This function:
 383    1) decides whether a PLT has sent us into the linker to resolve
 384       a function reference, and
 385    2) if so, tells us where to set a temporary breakpoint that will
 386       trigger when the dynamic linker is done.  */
 387
 388 CORE_ADDR
 389 i386_linux_skip_solib_resolver (CORE_ADDR pc)
 390 {
 391   CORE_ADDR result;
 392
 393   /* Plug in functions for other kinds of resolvers here.  */
 394   result = skip_hurd_resolver (pc);
 395   if (result)
 396     return result;
 397
 398   return 0;
 399 }
 400
 401 /* Fetch (and possibly build) an appropriate link_map_offsets
 402    structure for native Linux/x86 targets using the struct offsets
 403    defined in link.h (but without actual reference to that file).
 404
 405    This makes it possible to access Linux/x86 shared libraries from a
 406    GDB that was not built on an Linux/x86 host (for cross debugging).  */
 407
 408 struct link_map_offsets *
 409 i386_linux_svr4_fetch_link_map_offsets (void)
 410 {
 411   static struct link_map_offsets lmo;
 412   static struct link_map_offsets *lmp = NULL;
 413
 414   if (lmp == NULL)
 415     {
 416       lmp = &lmo;
 417
 418       lmo.r_debug_size = 8;     /* The actual size is 20 bytes, but
 419                                    this is all we need.  */
 420       lmo.r_map_offset = 4;
 421       lmo.r_map_size   = 4;
 422
 423       lmo.link_map_size = 20;   /* The actual size is 552 bytes, but
 424                                    this is all we need.  */
 425       lmo.l_addr_offset = 0;
 426       lmo.l_addr_size   = 4;
 427
 428       lmo.l_name_offset = 4;
 429       lmo.l_name_size   = 4;
 430
 431       lmo.l_next_offset = 12;
 432       lmo.l_next_size   = 4;
 433
 434       lmo.l_prev_offset = 16;
 435       lmo.l_prev_size   = 4;
 436     }
 437
 438   return lmp;
 439 }