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, 9 insertions, 9 deletions

diff --git a/gdb/ppc-linux-tdep.c b/gdb/ppc-linux-tdep.c
index 2f28337..beef4bd 100644
--- a/gdb/ppc-linux-tdep.c
+++ b/gdb/ppc-linux-tdep.c
@@ -332,7 +332,7 @@ ppc_skip_trampoline_code (struct frame_info *frame, CORE_ADDR pc)
 {
   unsigned int insnbuf[POWERPC32_PLT_CHECK_LEN];
   struct gdbarch *gdbarch = get_frame_arch (frame);
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
   CORE_ADDR target = 0;
   int scan_limit, i;
@@ -898,7 +898,7 @@ ppc_linux_vsxregset (void)
 const struct regset *
 ppc_linux_cgprregset (struct gdbarch *gdbarch)
 {
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
 
   if (tdep->wordsize == 4)
     {
@@ -938,7 +938,7 @@ ppc_linux_collect_core_cpgrregset (const struct regset *regset,
 				   int regnum, void *buf, size_t len)
 {
   struct gdbarch *gdbarch = regcache->arch ();
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
 
   const struct regset *cgprregset = ppc_linux_cgprregset (gdbarch);
 
@@ -985,7 +985,7 @@ ppc_linux_iterate_over_regset_sections (struct gdbarch *gdbarch,
 					void *cb_data,
 					const struct regcache *regcache)
 {
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   int have_altivec = tdep->ppc_vr0_regnum != -1;
   int have_vsx = tdep->ppc_vsr0_upper_regnum != -1;
   int have_ppr = tdep->ppc_ppr_regnum != -1;
@@ -1170,7 +1170,7 @@ ppc_linux_sigtramp_cache (struct frame_info *this_frame,
   CORE_ADDR fpregs;
   int i;
   struct gdbarch *gdbarch = get_frame_arch (this_frame);
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
 
   base = get_frame_register_unsigned (this_frame,
@@ -1341,7 +1341,7 @@ ppc_linux_get_syscall_number (struct gdbarch *gdbarch,
 			      thread_info *thread)
 {
   struct regcache *regcache = get_thread_regcache (thread);
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
 
   /* Make sure we're in a 32- or 64-bit machine */
@@ -1419,7 +1419,7 @@ static int
 ppc_linux_syscall_record (struct regcache *regcache)
 {
   struct gdbarch *gdbarch = regcache->arch ();
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   ULONGEST scnum;
   enum gdb_syscall syscall_gdb;
   int ret;
@@ -1509,7 +1509,7 @@ ppc_linux_record_signal (struct gdbarch *gdbarch, struct regcache *regcache,
   const int SIGNAL_FRAMESIZE = 128;
   const int sizeof_rt_sigframe = 1440 * 2 + 8 * 2 + 4 * 6 + 8 + 8 + 128 + 512;
   ULONGEST sp;
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   int i;
 
   for (i = 3; i <= 12; i++)
@@ -2044,7 +2044,7 @@ static void
 ppc_linux_init_abi (struct gdbarch_info info,
 		    struct gdbarch *gdbarch)
 {
-  ppc_gdbarch_tdep *tdep = (ppc_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
   struct tdesc_arch_data *tdesc_data = info.tdesc_data;
   static const char *const stap_integer_prefixes[] = { "i", NULL };
   static const char *const stap_register_indirection_prefixes[] = { "(",