[RFC][PATCH?] fixed some segfaults and bugs in mdebug support

[Zeck S <zeck654321@gmail.com>]

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First off, I apologize if I'm doing this process wrong. I have sent an
email to assign@gnu.org trying to get the paperwork required for copyright
assignment. I think that's the correct thing to do?

While I wait on that, I'm not sure exactly what is required for these
changes.

Here's what I fixed in mdebug support.

info sym funcName would segfault
The first problem was that no compunit_symtab was set for the global_block
on blockvectors in  new_symtab. This caused a crash in block.c.
initialize_block_iterator called get_block_compunit_symtab and the
assertion gdb_assert (gb->compunit_symtab != NULL); would fail.

info types would segfault
The second problem was memory corruption. struct global_block is a larger
and different type from plain block and blockvector is expected to have
index 0 be a global_block struct. This can be seen done correctly in jit.c
near /* Now add the special blocks */ under if (i == GLOBAL_BLOCK). Failing
to allocate this correctly leads to crashes for me (usually) in
set_compunit_symtab where the assertion  gdb_assert (gb->compunit_symtab ==
NULL); would randomly fail. This fix is also in new_symtab.

info line file:line did not work
The third problem was finding lines never worked because add_line never set
.is_stmt to true, so in symtab.c find_line_common never saw item->is_stmt
as true, do it always went down the /* Ignore non-statements. */ path in
its main loop.

I looked in the gdb/testsuite directory, and I don't see a directory for
mips or mdebug? Unsure how to set up a test for this. To make files with
mdebug symbols, I used the old IRIX IDO compiler running under a kind of
qemu setup used by N64 game reverse engineering projects. (N64 dev is why
I'm interested in this symbol format. I can connect vscode to gdb and gdb
to an n64 emulator with a gdb stub to debug with symbols)

diff --git a/gdb/mdebugread.c b/gdb/mdebugread.c
index 4b0a1eb255f..9cb30ce0acd 100644
--- a/gdb/mdebugread.c
+++ b/gdb/mdebugread.c
@@ -239,9 +239,6 @@ enum block_type { FUNCTION_BLOCK, NON_FUNCTION_BLOCK };
 static struct block *new_block (struct objfile *objfile,
  enum block_type, enum language);

-static struct block *new_global_block (struct objfile *objfile,
- enum block_type, enum language);
-
 static struct compunit_symtab *new_symtab (const char *, int, struct
objfile *);

 static struct linetable *new_linetable (int);
@@ -4545,7 +4542,6 @@ add_line (struct linetable *lt, int lineno, CORE_ADDR
adr, int last)
     return lineno;

   lt->item[lt->nitems].line = lineno;
-  lt->item[lt->nitems].is_stmt = 1;
   lt->item[lt->nitems++].set_unrelocated_pc (unrelocated_addr (adr << 2));
   return lineno;
 }
@@ -4638,10 +4634,9 @@ new_symtab (const char *name, int maxlines, struct
objfile *objfile)

   /* All symtabs must have at least two blocks.  */
   bv = new_bvect (2);
-  bv->set_block (GLOBAL_BLOCK, new_global_block (objfile,
NON_FUNCTION_BLOCK, lang));
+  bv->set_block (GLOBAL_BLOCK, new_block (objfile, NON_FUNCTION_BLOCK,
lang));
   bv->set_block (STATIC_BLOCK, new_block (objfile, NON_FUNCTION_BLOCK,
lang));
   bv->static_block ()->set_superblock (bv->global_block ());
-  bv->global_block ()->set_compunit_symtab(cust);
   cust->set_blockvector (bv);

   cust->set_debugformat ("ECOFF");
@@ -4740,21 +4735,6 @@ new_block (struct objfile *objfile, enum block_type
type,
   return retval;
 }

-static struct block *
-new_global_block (struct objfile *objfile, enum block_type type,
-   enum language language)
-{
-  struct block *retval = new (&objfile->objfile_obstack) global_block;
-
-  if (type == FUNCTION_BLOCK)
-    retval->set_multidict (mdict_create_linear_expandable (language));
-  else
-    retval->set_multidict (mdict_create_hashed_expandable (language));
-
-  return retval;
-}
-
-
 /* Create a new symbol with printname NAME.  */

 static struct symbol *

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