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.

1 files changed, 10 insertions, 10 deletions

diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index 5be4984fb57..05bf03973fc 100644
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -79,7 +79,7 @@ static void
 s390_write_pc (struct regcache *regcache, CORE_ADDR pc)
 {
   struct gdbarch *gdbarch = regcache->arch ();
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
 
   regcache_cooked_write_unsigned (regcache, tdep->pc_regnum, pc);
 
@@ -269,7 +269,7 @@ s390_iterate_over_regset_sections (struct gdbarch *gdbarch,
 				   void *cb_data,
 				   const struct regcache *regcache)
 {
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   const int gregset_size = (tdep->abi == ABI_LINUX_S390 ?
 			    s390_sizeof_gregset : s390x_sizeof_gregset);
 
@@ -390,7 +390,7 @@ s390_sigtramp_frame_unwind_cache (struct frame_info *this_frame,
 				  void **this_prologue_cache)
 {
   struct gdbarch *gdbarch = get_frame_arch (this_frame);
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   int word_size = gdbarch_ptr_bit (gdbarch) / 8;
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
   struct s390_sigtramp_unwind_cache *info;
@@ -561,7 +561,7 @@ s390_linux_get_syscall_number (struct gdbarch *gdbarch,
 			       thread_info *thread)
 {
   struct regcache *regs = get_thread_regcache (thread);
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
   ULONGEST pc;
   ULONGEST svc_number = -1;
@@ -594,7 +594,7 @@ static int
 s390_all_but_pc_registers_record (struct regcache *regcache)
 {
   struct gdbarch *gdbarch = regcache->arch ();
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   int i;
 
   for (i = 0; i < 16; i++)
@@ -804,7 +804,7 @@ static int
 s390_linux_syscall_record (struct regcache *regcache, LONGEST syscall_native)
 {
   struct gdbarch *gdbarch = regcache->arch ();
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   int ret;
   enum gdb_syscall syscall_gdb;
 
@@ -855,7 +855,7 @@ static int
 s390_linux_record_signal (struct gdbarch *gdbarch, struct regcache *regcache,
 			  enum gdb_signal signal)
 {
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
   /* There are two kinds of signal frames on s390. rt_sigframe is always
      the larger one, so don't even bother with sigframe.  */
   const int sizeof_rt_sigframe = (tdep->abi == ABI_LINUX_ZSERIES ?
@@ -1119,7 +1119,7 @@ s390_init_linux_record_tdep (struct linux_record_tdep *record_tdep,
 static void
 s390_linux_init_abi_any (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
 
   tdep->s390_syscall_record = s390_linux_syscall_record;
 
@@ -1154,7 +1154,7 @@ s390_linux_init_abi_any (struct gdbarch_info info, struct gdbarch *gdbarch)
 static void
 s390_linux_init_abi_31 (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
 
   tdep->abi = ABI_LINUX_S390;
 
@@ -1170,7 +1170,7 @@ s390_linux_init_abi_31 (struct gdbarch_info info, struct gdbarch *gdbarch)
 static void
 s390_linux_init_abi_64 (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
-  s390_gdbarch_tdep *tdep = (s390_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  s390_gdbarch_tdep *tdep = gdbarch_tdep<s390_gdbarch_tdep> (gdbarch);
 
   tdep->abi = ABI_LINUX_ZSERIES;