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binutils-gdb/gdb/i386-obsd-tdep.c
Andrew Burgess 08106042d9 gdb: move the type cast into gdbarch_tdep 
I built GDB for all targets on a x86-64/GNU-Linux system, and
then (accidentally) passed GDB a RISC-V binary, and asked GDB to "run"
the binary on the native target.  I got this error:

  (gdb) show architecture
  The target architecture is set to "auto" (currently "i386").
  (gdb) file /tmp/hello.rv32.exe
  Reading symbols from /tmp/hello.rv32.exe...
  (gdb) show architecture
  The target architecture is set to "auto" (currently "riscv:rv32").
  (gdb) run
  Starting program: /tmp/hello.rv32.exe
  ../../src/gdb/i387-tdep.c:596: internal-error: i387_supply_fxsave: Assertion `tdep->st0_regnum >= I386_ST0_REGNUM' failed.

What's going on here is this; initially the architecture is i386, this
is based on the default architecture, which is set based on the native
target.  After loading the RISC-V executable the architecture of the
current inferior is updated based on the architecture of the
executable.

When we "run", GDB does a fork & exec, with the inferior being
controlled through ptrace.  GDB sees an initial stop from the inferior
as soon as the inferior comes to life.  In response to this stop GDB
ends up calling save_stop_reason (linux-nat.c), which ends up trying
to read register from the inferior, to do this we end up calling
target_ops::fetch_registers, which, for the x86-64 native target,
calls amd64_linux_nat_target::fetch_registers.

After this I eventually end up in i387_supply_fxsave, different x86
based targets will end in different functions to fetch registers, but
it doesn't really matter which function we end up in, the problem is
this line, which is repeated in many places:

  i386_gdbarch_tdep *tdep = (i386_gdbarch_tdep *) gdbarch_tdep (arch);

The problem here is that the ARCH in this line comes from the current
inferior, which, as we discussed above, will be a RISC-V gdbarch, the
tdep field will actually be of type riscv_gdbarch_tdep, not
i386_gdbarch_tdep.  After this cast we are relying on undefined
behaviour, in my case I happen to trigger an assert, but this might
not always be the case.

The thing I tried that exposed this problem was of course, trying to
start an executable of the wrong architecture on a native target.  I
don't think that the correct solution for this problem is to detect,
at the point of cast, that the gdbarch_tdep object is of the wrong
type, but, I did wonder, is there a way that we could protect
ourselves from incorrectly casting the gdbarch_tdep object?

I think that there is something we can do here, and this commit is the
first step in that direction, though no actual check is added by this
commit.

This commit can be split into two parts:

 (1) In gdbarch.h and arch-utils.c.  In these files I have modified
 gdbarch_tdep (the function) so that it now takes a template argument,
 like this:

    template<typename TDepType>
    static inline TDepType *
    gdbarch_tdep (struct gdbarch *gdbarch)
    {
      struct gdbarch_tdep *tdep = gdbarch_tdep_1 (gdbarch);
      return static_cast<TDepType *> (tdep);
    }

  After this change we are no better protected, but the cast is now
  done within the gdbarch_tdep function rather than at the call sites,
  this leads to the second, much larger change in this commit,

  (2) Everywhere gdbarch_tdep is called, we make changes like this:

    -  i386_gdbarch_tdep *tdep = (i386_gdbarch_tdep *) gdbarch_tdep (arch);
    +  i386_gdbarch_tdep *tdep = gdbarch_tdep<i386_gdbarch_tdep> (arch);

There should be no functional change after this commit.

In the next commit I will build on this change to add an assertion in
gdbarch_tdep that checks we are casting to the correct type.
2022-07-21 15:19:42 +01:00

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/* Target-dependent code for OpenBSD/i386.
Copyright (C) 1988-2022 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/>. */
#include "defs.h"
#include "arch-utils.h"
#include "frame.h"
#include "frame-unwind.h"
#include "gdbcore.h"
#include "regcache.h"
#include "regset.h"
#include "symtab.h"
#include "objfiles.h"
#include "osabi.h"
#include "target.h"
#include "trad-frame.h"
#include "obsd-tdep.h"
#include "i386-tdep.h"
#include "i387-tdep.h"
#include "solib-svr4.h"
#include "bsd-uthread.h"
/* Support for signal handlers. */
/* Since OpenBSD 3.2, the sigtramp routine is mapped at a random page
in virtual memory. The randomness makes it somewhat tricky to
detect it, but fortunately we can rely on the fact that the start
of the sigtramp routine is page-aligned. We recognize the
trampoline by looking for the code that invokes the sigreturn
system call. The offset where we can find that code varies from
release to release.
By the way, the mapping mentioned above is read-only, so you cannot
place a breakpoint in the signal trampoline. */
/* Default page size. */
static const int i386obsd_page_size = 4096;
/* Offset for sigreturn(2). */
static const int i386obsd_sigreturn_offset[] = {
0x0a, /* OpenBSD 3.2 */
0x14, /* OpenBSD 3.6 */
0x3a, /* OpenBSD 3.8 */
-1
};
/* Return whether THIS_FRAME corresponds to an OpenBSD sigtramp
routine. */
static int
i386obsd_sigtramp_p (struct frame_info *this_frame)
{
CORE_ADDR pc = get_frame_pc (this_frame);
CORE_ADDR start_pc = (pc & ~(i386obsd_page_size - 1));
/* The call sequence invoking sigreturn(2). */
const gdb_byte sigreturn[] =
{
0xb8,
0x67, 0x00, 0x00, 0x00, /* movl $SYS_sigreturn, %eax */
0xcd, 0x80 /* int $0x80 */
};
size_t buflen = sizeof sigreturn;
const int *offset;
gdb_byte *buf;
const char *name;
/* If the function has a valid symbol name, it isn't a
trampoline. */
find_pc_partial_function (pc, &name, NULL, NULL);
if (name != NULL)
return 0;
/* If the function lives in a valid section (even without a starting
point) it isn't a trampoline. */
if (find_pc_section (pc) != NULL)
return 0;
/* Allocate buffer. */
buf = (gdb_byte *) alloca (buflen);
/* Loop over all offsets. */
for (offset = i386obsd_sigreturn_offset; *offset != -1; offset++)
{
/* If we can't read the instructions, return zero. */
if (!safe_frame_unwind_memory (this_frame, start_pc + *offset,
{buf, buflen}))
return 0;
/* Check for sigreturn(2). */
if (memcmp (buf, sigreturn, buflen) == 0)
return 1;
}
return 0;
}
/* Mapping between the general-purpose registers in `struct reg'
format and GDB's register cache layout. */
/* From <machine/reg.h>. */
static int i386obsd_r_reg_offset[] =
{
0 * 4, /* %eax */
1 * 4, /* %ecx */
2 * 4, /* %edx */
3 * 4, /* %ebx */
4 * 4, /* %esp */
5 * 4, /* %ebp */
6 * 4, /* %esi */
7 * 4, /* %edi */
8 * 4, /* %eip */
9 * 4, /* %eflags */
10 * 4, /* %cs */
11 * 4, /* %ss */
12 * 4, /* %ds */
13 * 4, /* %es */
14 * 4, /* %fs */
15 * 4 /* %gs */
};
/* Sigtramp routine location for OpenBSD 3.1 and earlier releases. */
CORE_ADDR i386obsd_sigtramp_start_addr = 0xbfbfdf20;
CORE_ADDR i386obsd_sigtramp_end_addr = 0xbfbfdff0;
/* From <machine/signal.h>. */
int i386obsd_sc_reg_offset[I386_NUM_GREGS] =
{
10 * 4, /* %eax */
9 * 4, /* %ecx */
8 * 4, /* %edx */
7 * 4, /* %ebx */
14 * 4, /* %esp */
6 * 4, /* %ebp */
5 * 4, /* %esi */
4 * 4, /* %edi */
11 * 4, /* %eip */
13 * 4, /* %eflags */
12 * 4, /* %cs */
15 * 4, /* %ss */
3 * 4, /* %ds */
2 * 4, /* %es */
1 * 4, /* %fs */
0 * 4 /* %gs */
};
/* From /usr/src/lib/libpthread/arch/i386/uthread_machdep.c. */
static int i386obsd_uthread_reg_offset[] =
{
11 * 4, /* %eax */
10 * 4, /* %ecx */
9 * 4, /* %edx */
8 * 4, /* %ebx */
-1, /* %esp */
6 * 4, /* %ebp */
5 * 4, /* %esi */
4 * 4, /* %edi */
12 * 4, /* %eip */
-1, /* %eflags */
13 * 4, /* %cs */
-1, /* %ss */
3 * 4, /* %ds */
2 * 4, /* %es */
1 * 4, /* %fs */
0 * 4 /* %gs */
};
/* Offset within the thread structure where we can find the saved
stack pointer (%esp). */
#define I386OBSD_UTHREAD_ESP_OFFSET 176
static void
i386obsd_supply_uthread (struct regcache *regcache,
int regnum, CORE_ADDR addr)
{
struct gdbarch *gdbarch = regcache->arch ();
enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
CORE_ADDR sp_addr = addr + I386OBSD_UTHREAD_ESP_OFFSET;
CORE_ADDR sp = 0;
gdb_byte buf[4];
int i;
gdb_assert (regnum >= -1);
if (regnum == -1 || regnum == I386_ESP_REGNUM)
{
int offset;
/* Fetch stack pointer from thread structure. */
sp = read_memory_unsigned_integer (sp_addr, 4, byte_order);
/* Adjust the stack pointer such that it looks as if we just
returned from _thread_machdep_switch. */
offset = i386obsd_uthread_reg_offset[I386_EIP_REGNUM] + 4;
store_unsigned_integer (buf, 4, byte_order, sp + offset);
regcache->raw_supply (I386_ESP_REGNUM, buf);
}
for (i = 0; i < ARRAY_SIZE (i386obsd_uthread_reg_offset); i++)
{
if (i386obsd_uthread_reg_offset[i] != -1
&& (regnum == -1 || regnum == i))
{
/* Fetch stack pointer from thread structure (if we didn't
do so already). */
if (sp == 0)
sp = read_memory_unsigned_integer (sp_addr, 4, byte_order);
/* Read the saved register from the stack frame. */
read_memory (sp + i386obsd_uthread_reg_offset[i], buf, 4);
regcache->raw_supply (i, buf);
}
}
}
static void
i386obsd_collect_uthread (const struct regcache *regcache,
int regnum, CORE_ADDR addr)
{
struct gdbarch *gdbarch = regcache->arch ();
enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
CORE_ADDR sp_addr = addr + I386OBSD_UTHREAD_ESP_OFFSET;
CORE_ADDR sp = 0;
gdb_byte buf[4];
int i;
gdb_assert (regnum >= -1);
if (regnum == -1 || regnum == I386_ESP_REGNUM)
{
int offset;
/* Calculate the stack pointer (frame pointer) that will be
stored into the thread structure. */
offset = i386obsd_uthread_reg_offset[I386_EIP_REGNUM] + 4;
regcache->raw_collect (I386_ESP_REGNUM, buf);
sp = extract_unsigned_integer (buf, 4, byte_order) - offset;
/* Store the stack pointer. */
write_memory_unsigned_integer (sp_addr, 4, byte_order, sp);
/* The stack pointer was (potentially) modified. Make sure we
build a proper stack frame. */
regnum = -1;
}
for (i = 0; i < ARRAY_SIZE (i386obsd_uthread_reg_offset); i++)
{
if (i386obsd_uthread_reg_offset[i] != -1
&& (regnum == -1 || regnum == i))
{
/* Fetch stack pointer from thread structure (if we didn't
calculate it already). */
if (sp == 0)
sp = read_memory_unsigned_integer (sp_addr, 4, byte_order);
/* Write the register into the stack frame. */
regcache->raw_collect (i, buf);
write_memory (sp + i386obsd_uthread_reg_offset[i], buf, 4);
}
}
}
/* Kernel debugging support. */
/* From <machine/frame.h>. Note that %esp and %ess are only saved in
a trap frame when entering the kernel from user space. */
static int i386obsd_tf_reg_offset[] =
{
10 * 4, /* %eax */
9 * 4, /* %ecx */
8 * 4, /* %edx */
7 * 4, /* %ebx */
-1, /* %esp */
6 * 4, /* %ebp */
5 * 4, /* %esi */
4 * 4, /* %edi */
13 * 4, /* %eip */
15 * 4, /* %eflags */
14 * 4, /* %cs */
-1, /* %ss */
3 * 4, /* %ds */
2 * 4, /* %es */
0 * 4, /* %fs */
1 * 4 /* %gs */
};
static struct trad_frame_cache *
i386obsd_trapframe_cache (struct frame_info *this_frame, void **this_cache)
{
struct gdbarch *gdbarch = get_frame_arch (this_frame);
enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
struct trad_frame_cache *cache;
CORE_ADDR func, sp, addr;
ULONGEST cs;
const char *name;
int i;
if (*this_cache)
return (struct trad_frame_cache *) *this_cache;
cache = trad_frame_cache_zalloc (this_frame);
*this_cache = cache;
func = get_frame_func (this_frame);
sp = get_frame_register_unsigned (this_frame, I386_ESP_REGNUM);
find_pc_partial_function (func, &name, NULL, NULL);
if (name && startswith (name, "Xintr"))
addr = sp + 8; /* It's an interrupt frame. */
else
addr = sp;
for (i = 0; i < ARRAY_SIZE (i386obsd_tf_reg_offset); i++)
if (i386obsd_tf_reg_offset[i] != -1)
trad_frame_set_reg_addr (cache, i, addr + i386obsd_tf_reg_offset[i]);
/* Read %cs from trap frame. */
addr += i386obsd_tf_reg_offset[I386_CS_REGNUM];
cs = read_memory_unsigned_integer (addr, 4, byte_order);
if ((cs & I386_SEL_RPL) == I386_SEL_UPL)
{
/* Trap from user space; terminate backtrace. */
trad_frame_set_id (cache, outer_frame_id);
}
else
{
/* Construct the frame ID using the function start. */
trad_frame_set_id (cache, frame_id_build (sp + 8, func));
}
return cache;
}
static void
i386obsd_trapframe_this_id (struct frame_info *this_frame,
void **this_cache, struct frame_id *this_id)
{
struct trad_frame_cache *cache =
i386obsd_trapframe_cache (this_frame, this_cache);
trad_frame_get_id (cache, this_id);
}
static struct value *
i386obsd_trapframe_prev_register (struct frame_info *this_frame,
void **this_cache, int regnum)
{
struct trad_frame_cache *cache =
i386obsd_trapframe_cache (this_frame, this_cache);
return trad_frame_get_register (cache, this_frame, regnum);
}
static int
i386obsd_trapframe_sniffer (const struct frame_unwind *self,
struct frame_info *this_frame,
void **this_prologue_cache)
{
ULONGEST cs;
const char *name;
/* Check Current Privilege Level and bail out if we're not executing
in kernel space. */
cs = get_frame_register_unsigned (this_frame, I386_CS_REGNUM);
if ((cs & I386_SEL_RPL) == I386_SEL_UPL)
return 0;
find_pc_partial_function (get_frame_pc (this_frame), &name, NULL, NULL);
return (name && (strcmp (name, "calltrap") == 0
|| strcmp (name, "syscall1") == 0
|| startswith (name, "Xintr")
|| startswith (name, "Xsoft")));
}
static const struct frame_unwind i386obsd_trapframe_unwind = {
"i386 openbsd trap",
/* FIXME: kettenis/20051219: This really is more like an interrupt
frame, but SIGTRAMP_FRAME would print <signal handler called>,
which really is not what we want here. */
NORMAL_FRAME,
default_frame_unwind_stop_reason,
i386obsd_trapframe_this_id,
i386obsd_trapframe_prev_register,
NULL,
i386obsd_trapframe_sniffer
};
static void
i386obsd_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
{
i386_gdbarch_tdep *tdep = gdbarch_tdep<i386_gdbarch_tdep> (gdbarch);
/* Obviously OpenBSD is BSD-based. */
i386bsd_init_abi (info, gdbarch);
obsd_init_abi (info, gdbarch);
i386_elf_init_abi (info, gdbarch);
/* OpenBSD has a different `struct reg'. */
tdep->gregset_reg_offset = i386obsd_r_reg_offset;
tdep->gregset_num_regs = ARRAY_SIZE (i386obsd_r_reg_offset);
tdep->sizeof_gregset = 16 * 4;
/* OpenBSD uses -freg-struct-return by default. */
tdep->struct_return = reg_struct_return;
/* OpenBSD uses a different memory layout. */
tdep->sigtramp_start = i386obsd_sigtramp_start_addr;
tdep->sigtramp_end = i386obsd_sigtramp_end_addr;
tdep->sigtramp_p = i386obsd_sigtramp_p;
/* OpenBSD has a `struct sigcontext' that's different from the
original 4.3 BSD. */
tdep->sc_reg_offset = i386obsd_sc_reg_offset;
tdep->sc_num_regs = ARRAY_SIZE (i386obsd_sc_reg_offset);
/* OpenBSD provides a user-level threads implementation. */
bsd_uthread_set_supply_uthread (gdbarch, i386obsd_supply_uthread);
bsd_uthread_set_collect_uthread (gdbarch, i386obsd_collect_uthread);
/* Unwind kernel trap frames correctly. */
frame_unwind_prepend_unwinder (gdbarch, &i386obsd_trapframe_unwind);
/* OpenBSD ELF uses SVR4-style shared libraries. */
set_solib_svr4_fetch_link_map_offsets
(gdbarch, svr4_ilp32_fetch_link_map_offsets);
}
void _initialize_i386obsd_tdep ();
void
_initialize_i386obsd_tdep ()
{
gdbarch_register_osabi (bfd_arch_i386, 0, GDB_OSABI_OPENBSD,
i386obsd_init_abi);
}
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