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/nds32-tdep.c b/gdb/nds32-tdep.c
index e95ad7c..11bfd2d 100644
--- a/gdb/nds32-tdep.c
+++ b/gdb/nds32-tdep.c
@@ -289,7 +289,7 @@ typedef BP_MANIPULATION (nds32_break_insn) nds32_breakpoint;
 static int
 nds32_dwarf2_reg_to_regnum (struct gdbarch *gdbarch, int num)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   const int FSR = 38;
   const int FDR = FSR + 32;
 
@@ -432,7 +432,7 @@ nds32_pseudo_register_read (struct gdbarch *gdbarch,
 			    readable_regcache *regcache, int regnum,
 			    gdb_byte *buf)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   gdb_byte reg_buf[8];
   int offset, fdr_regnum;
   enum register_status status;
@@ -471,7 +471,7 @@ nds32_pseudo_register_write (struct gdbarch *gdbarch,
 			     struct regcache *regcache, int regnum,
 			     const gdb_byte *buf)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   gdb_byte reg_buf[8];
   int offset, fdr_regnum;
 
@@ -608,7 +608,7 @@ static CORE_ADDR
 nds32_analyze_prologue (struct gdbarch *gdbarch, CORE_ADDR pc,
 			CORE_ADDR limit_pc, struct nds32_frame_cache *cache)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   /* Current scanning status.  */
   int in_prologue_bb = 0;
@@ -1169,7 +1169,7 @@ static int
 nds32_analyze_epilogue (struct gdbarch *gdbarch, CORE_ADDR pc,
 			struct nds32_frame_cache *cache)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   CORE_ADDR limit_pc;
   uint32_t insn, insn_len;
@@ -1220,7 +1220,7 @@ nds32_analyze_epilogue (struct gdbarch *gdbarch, CORE_ADDR pc,
 static int
 nds32_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR addr)
 {
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   int insn_type = INSN_NORMAL;
   int ret_found = 0;
@@ -1424,7 +1424,7 @@ nds32_push_dummy_call (struct gdbarch *gdbarch, struct value *function,
   int i;
   ULONGEST regval;
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   struct type *func_type = value_type (function);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   int abi_split = nds32_abi_split (tdep->elf_abi);
@@ -1652,7 +1652,7 @@ nds32_extract_return_value (struct gdbarch *gdbarch, struct type *type,
 			    struct regcache *regcache, gdb_byte *valbuf)
 {
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   int calling_use_fpr;
   int len;
@@ -1742,7 +1742,7 @@ nds32_store_return_value (struct gdbarch *gdbarch, struct type *type,
 			  struct regcache *regcache, const gdb_byte *valbuf)
 {
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
-  nds32_gdbarch_tdep *tdep = (nds32_gdbarch_tdep *) gdbarch_tdep (gdbarch);
+  nds32_gdbarch_tdep *tdep = gdbarch_tdep<nds32_gdbarch_tdep> (gdbarch);
   int abi_use_fpr = nds32_abi_use_fpr (tdep->elf_abi);
   int calling_use_fpr;
   int len;
@@ -1965,7 +1965,7 @@ nds32_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
        best_arch = gdbarch_list_lookup_by_info (best_arch->next, &info))
     {
       nds32_gdbarch_tdep *idep
-	= (nds32_gdbarch_tdep *) gdbarch_tdep (best_arch->gdbarch);
+	= gdbarch_tdep<nds32_gdbarch_tdep> (best_arch->gdbarch);
 
       if (idep->elf_abi != elf_abi)
 	continue;