Fix buffer overflow regression due to minsym malloc-ed instead of obstack-ed.

Valgrind detects the following error in a bunch of tests,
e.g. in gdb.base/foll-fork.exp.

==15155== VALGRIND_GDB_ERROR_BEGIN
==15155== Invalid read of size 8
==15155==    at 0x55BE04: minimal_symbol_upper_bound(bound_minimal_symbol) (minsyms.c:1504)
==15155==    by 0x3B2E9C: find_pc_partial_function(unsigned long, char const**, unsigned long*, unsigned long*, block const**) (blockframe.c:340)
==15155==    by 0x3B3135: find_function_entry_range_from_pc(unsigned long, char const**, unsigned long*, unsigned long*) (blockframe.c:385)
==15155==    by 0x4F5597: fill_in_stop_func(gdbarch*, execution_control_state*) [clone .part.16] (infrun.c:4124)
==15155==    by 0x4FBE01: fill_in_stop_func (infrun.c:7636)
==15155==    by 0x4FBE01: process_event_stop_test(execution_control_state*) (infrun.c:6279)
...
==15155==  Address 0x715bec8 is 0 bytes after a block of size 2,952 alloc'd
==15155==    at 0x4C2E2B3: realloc (vg_replace_malloc.c:836)
==15155==    by 0x405F2C: xrealloc (common-utils.c:62)
==15155==    by 0x55BA4E: xresizevec<minimal_symbol> (poison.h:170)
==15155==    by 0x55BA4E: minimal_symbol_reader::install() (minsyms.c:1399)
==15155==    by 0x4981C7: elf_read_minimal_symbols (elfread.c:1165)
...

This seems to be a regression created by:
    commit 042d75e42c5572f333e0e06dabd3c5c4afab486c
    Author:     Tom Tromey <tom@tromey.com>
    AuthorDate: Sat Mar 2 12:29:48 2019 -0700
    Commit:     Tom Tromey <tom@tromey.com>
    CommitDate: Fri Mar 15 16:02:10 2019 -0600

        Allocate minimal symbols with malloc

Before this commit, the array of 'struct minimal_symbol'
contained a last element that was a "null symbol".  The comment in
minimal_symbol_reader::install was:
      /* We also terminate the minimal symbol table with a "null symbol",
         which is *not* included in the size of the table.  This makes it
         easier to find the end of the table when we are handed a pointer
         to some symbol in the middle of it.  Zero out the fields in the
         "null symbol" allocated at the end of the array.  Note that the
         symbol count does *not* include this null symbol, which is why it
         is indexed by mcount and not mcount-1.  */

      memset (&msymbols[mcount], 0, sizeof (struct minimal_symbol));

However, minimal_symbol_upper_bound was still based on the assumption
that the array of minsym is terminated by a minsym with a null symbol:
it is looping with:
  for (i = 1; MSYMBOL_LINKAGE_NAME (msymbol + i) != NULL; i++)

Replace this NULL comparison by a logic that calculates how
many msymbol are following the msymbols from which we are starting from.

(Re-)tested on debian/amd64, natively and under valgrind.

gdb/ChangeLog
2019-03-24  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	    Tom Tromey  <tromey@adacore.com>

	* minsyms.c (minimal_symbol_upper_bound): Fix buffer overflow.

2 files changed, 15 insertions, 8 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 4ef8a5c9780..0c658e2cfe8 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2019-03-24  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+	    Tom Tromey  <tromey@adacore.com>
+
+	* minsyms.c (minimal_symbol_upper_bound): Fix buffer overflow.
+
 2019-03-26  Joel Brobecker  <brobecker@adacore.com>
 
 	* gdb-gdb.py.in (StructMainTypePrettyPrinter.bound_img): New method.
diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index 777f545d84a..51b65f51421 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -1445,11 +1445,10 @@ find_solib_trampoline_target (struct frame_info *frame, CORE_ADDR pc)
 CORE_ADDR
 minimal_symbol_upper_bound (struct bound_minimal_symbol minsym)
 {
-  int i;
   short section;
   struct obj_section *obj_section;
   CORE_ADDR result;
-  struct minimal_symbol *msymbol;
+  struct minimal_symbol *iter, *msymbol;
 
   gdb_assert (minsym.minsym != NULL);
 
@@ -1464,21 +1463,24 @@ minimal_symbol_upper_bound (struct bound_minimal_symbol minsym)
      other sections, to find the next symbol in this section with a
      different address.  */
 
+  struct minimal_symbol *past_the_end
+    = (minsym.objfile->per_bfd->msymbols.get ()
+       + minsym.objfile->per_bfd->minimal_symbol_count);
   msymbol = minsym.minsym;
   section = MSYMBOL_SECTION (msymbol);
-  for (i = 1; MSYMBOL_LINKAGE_NAME (msymbol + i) != NULL; i++)
+  for (iter = msymbol + 1; iter != past_the_end; ++iter)
     {
-      if ((MSYMBOL_VALUE_RAW_ADDRESS (msymbol + i)
+      if ((MSYMBOL_VALUE_RAW_ADDRESS (iter)
 	   != MSYMBOL_VALUE_RAW_ADDRESS (msymbol))
-	  && MSYMBOL_SECTION (msymbol + i) == section)
+	  && MSYMBOL_SECTION (iter) == section)
 	break;
     }
 
   obj_section = MSYMBOL_OBJ_SECTION (minsym.objfile, minsym.minsym);
-  if (MSYMBOL_LINKAGE_NAME (msymbol + i) != NULL
-      && (MSYMBOL_VALUE_ADDRESS (minsym.objfile, msymbol + i)
+  if (iter != past_the_end
+      && (MSYMBOL_VALUE_ADDRESS (minsym.objfile, iter)
 	  < obj_section_endaddr (obj_section)))
-    result = MSYMBOL_VALUE_ADDRESS (minsym.objfile, msymbol + i);
+    result = MSYMBOL_VALUE_ADDRESS (minsym.objfile, iter);
   else
     /* We got the start address from the last msymbol in the objfile.
        So the end address is the end of the section.  */