gdb: Handle malformed ELF, symbols in non-allocatable sections

I ended up debugging a malformed ELF where a section containing
executable code was not correctly marked as allocatable.  Before
realising the ELF was corrupted I tried to place a breakpoint on a
symbol in the non-allocatable, executable section, and GDB crashed.

Though trying to debug such an ELF clearly isn't going to go well I
would prefer, as far as possible, that any input, no matter how
corrupted, not crash GDB.

The crash occurs when trying to set a breakpoint on the name of a
function from the corrupted section.  GDB converts the symbol to a
symtab_and_line, and looks up a suitable section for this.

The problem is that the section is actually an obj_section, which is
stored in the table within the objfile, and we only initialise this
table for allocatable sections (see add_to_objfile_sections_full in
objfiles.c).  So, if the symbol is in a non-allocatable section then
we end up referencing an uninitialised obj_section.

Later we call get_sal_arch on the symtab_and_line, which calls
get_objfile_arch, which uses the objfile from the uninitialised
obj_section, which will be nullptr, at which point GDB crashes.

The fix I propose here is that when we setup the section references on
msymbols, we should check if the bfd_section being referenced is
allocatable or not.  If it is not then we should set the section
reference back to the default 0 section (see how MSYMBOL_OBJ_SECTION
and SYMBOL_OBJ_SECTION treat the 0 section index).

With this fix in place GDB no longer crashes.  Instead GDB creates the
breakpoint at the non-allocated address, and then fails, with an
error, when it tries to insert the breakpoint.

gdb/ChangeLog:

	* elfread.c (record_minimal_symbol): Set section index to 0 for
	non-allocatable sections.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-bad-elf-other.S: New file.
	* gdb.dwarf2/dw2-bad-elf.c: New file.
	* gdb.dwarf2/dw2-bad-elf.exp: New file.

Change-Id: Ie05436ab4c6a71440304d20ee639dfb021223f8b

6 files changed, 253 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 7a1c81da07f..fc4920ee643 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2020-01-13  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* elfread.c (record_minimal_symbol): Set section index to 0 for
+	non-allocatable sections.
+
 
 2020-01-13  Ali Tamur <tamur@google.com>
 
diff --git a/gdb/elfread.c b/gdb/elfread.c
index 9a6ce0ee371..453bca527e9 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -210,11 +210,16 @@ record_minimal_symbol (minimal_symbol_reader &reader,
       || ms_type == mst_text_gnu_ifunc)
     address = gdbarch_addr_bits_remove (gdbarch, address);
 
+  /* We only setup section information for allocatable sections.  Usually
+     we'd only expect to find msymbols for allocatable sections, but if the
+     ELF is malformed then this might not be the case.  In that case don't
+     create an msymbol that references an uninitialised section object.  */
+  int section_index = 0;
+  if ((bfd_section_flags (bfd_section) & SEC_ALLOC) == SEC_ALLOC)
+    section_index = gdb_bfd_section_index (objfile->obfd, bfd_section);
+
   struct minimal_symbol *result
-    = reader.record_full (name, copy_name, address,
-			  ms_type,
-			  gdb_bfd_section_index (objfile->obfd,
-						 bfd_section));
+    = reader.record_full (name, copy_name, address, ms_type, section_index);
   if ((objfile->flags & OBJF_MAINLINE) == 0
       && (ms_type == mst_data || ms_type == mst_bss))
     result->maybe_copied = 1;
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 4209c5757c3..6491d4d44b6 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2020-01-13  Andrew Burgess  <andrew.burgess@embecosm.com>
 
+	* gdb.dwarf2/dw2-bad-elf-other.S: New file.
+	* gdb.dwarf2/dw2-bad-elf.c: New file.
+	* gdb.dwarf2/dw2-bad-elf.exp: New file.
+
+2020-01-13  Andrew Burgess  <andrew.burgess@embecosm.com>
+
 	* lib/dwarf.exp (Dwarf::lines): Reset _line_saw_program and
 	_line_saw_file.
 
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bad-elf-other.S b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf-other.S
new file mode 100644
index 00000000000..197c13deb5d
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf-other.S
@@ -0,0 +1,29 @@
+/* Copyright 2019-2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+	.section ".other", "x"
+	.global some_func, some_func_end
+	.type   some_func, @function
+	nop
+	nop
+	nop
+	nop
+some_func:
+	.rept 64
+	.byte 0
+	.endr
+	.size some_func,.-some_func
+some_func_end:
+	nop
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.c b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.c
new file mode 100644
index 00000000000..8e6193ee597
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.c
@@ -0,0 +1,21 @@
+/* Copyright 2019-2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main ()
+{
+  asm ("main_label: .globl main_label");
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.exp b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.exp
new file mode 100644
index 00000000000..f6fe54a634f
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bad-elf.exp
@@ -0,0 +1,183 @@
+# Copyright 2019-2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Checks for a bug where a baddly formed ELF would cause GDB to crash.
+# A section containing executable code, for which there was DWARF is
+# accidentally marked as non-alloctable, GDB becomes unhappy.
+#
+# This test creates some fake DWARF pointing at some symbols in a
+# non-allocatable section that is still marked as executable.  We then
+# start GDB and try to place a breakpoint on the symbol in the
+# non-allocatable section.
+#
+# It is not expected that the final debug experience really makes
+# sense, the symbol is in a non-allocatable section after all, but GDB
+# absolutely shouldn't crash.  All we try to do after placing the
+# breakpoint is check that GDB is still alive.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile dw2-bad-elf.c dw2-bad-elf-other.S dw2-bad-elf-dwarf.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile3]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    declare_labels ranges_label_1 ranges_label_2 L1 L2
+
+    set main_result [function_range main ${srcdir}/${subdir}/${srcfile}]
+    set main_start [lindex $main_result 0]
+    set main_length [lindex $main_result 1]
+
+    set int_size [get_sizeof "int" 4]
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     dw2-bad-elf.c}
+	    {DW_AT_comp_dir ${srcdir}/${subdir}}
+	    {stmt_list $L1 DW_FORM_sec_offset}
+	    {ranges ${ranges_label_1} DW_FORM_sec_offset}
+	    {DW_AT_low_pc   0 addr}
+	} {
+	    declare_labels integer_label
+
+	    DW_TAG_subprogram {
+		{name main}
+		{low_pc $main_start addr}
+		{high_pc $main_length data8}
+		{DW_AT_type :$integer_label}
+		{DW_AT_decl_file 1 data1}
+		{DW_AT_decl_line 10 data1}
+	    }
+
+	    integer_label: DW_TAG_base_type {
+		{DW_AT_byte_size $int_size DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      integer}
+	    }
+	}
+    }
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     dw2-bad-elf-other.c}
+	    {DW_AT_comp_dir ${srcdir}/${subdir}}
+	    {stmt_list $L2 DW_FORM_sec_offset}
+	    {ranges ${ranges_label_2} DW_FORM_sec_offset}
+	    {DW_AT_low_pc   0 addr}
+	} {
+	    declare_labels integer_label
+
+	    DW_TAG_subprogram {
+		{name some_func}
+		{low_pc some_func addr}
+		{high_pc some_func_end addr}
+		{DW_AT_type :$integer_label}
+		{DW_AT_decl_file 2 data1}
+		{DW_AT_decl_line 5 data1}
+	    }
+
+	    integer_label: DW_TAG_base_type {
+		{DW_AT_byte_size $int_size DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      integer}
+	    }
+	}
+    }
+
+    ranges {is_64 [is_64_target]} {
+	ranges_label_1: sequence {
+	    {base [lindex $main_result 0]}
+	    {range 0 [lindex $main_result 1]}
+	}
+	ranges_label_2: sequence {
+	    {base some_func}
+	    {range 0 64}
+	}
+    }
+
+    lines {version 2} L1 {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	# Line data doens't need to be correct, just present.
+	program {
+	    {DW_LNE_set_address [lindex $main_result 0]}
+	    {DW_LNS_advance_line 10}
+	    {DW_LNS_copy}
+	    {DW_LNS_advance_pc [lindex $main_result 1]}
+	    {DW_LNS_advance_line 19}
+	    {DW_LNS_copy}
+	    {DW_LNE_end_sequence}
+	}
+    }
+
+    lines {version 2} L2 {
+	include_dir "${srcdir}/${subdir}"
+	file_name "dw2-bad-elf-other.c" 1
+
+	# Line data doens't need to be correct, just present.
+	program {
+	    {DW_LNE_set_address some_func}
+	    {DW_LNS_advance_line 5}
+	    {DW_LNS_copy}
+	    {DW_LNS_advance_pc 64}
+	    {DW_LNS_advance_line 8}
+	    {DW_LNS_copy}
+	    {DW_LNE_end_sequence}
+	}
+    }
+}
+
+if { [build_executable ${testfile}.exp ${testfile} \
+	  [list $srcfile $srcfile2 $asm_file] {nodebug}] } {
+    return -1
+}
+
+# Attempt to place a breakpoint on 'some_func', then check GDB is
+# still alive.  This test can optionally set a breakpoint on 'main'
+# first (based on GOTO_MAIN), the original bug behaved differently
+# when there was already a breakpoint set.
+proc run_test { goto_main } {
+    global binfile decimal hex
+
+    clean_restart ${binfile}
+
+    if { $goto_main } {
+	if ![runto_main] {
+	    return -1
+	}
+    }
+
+    # Place a breakpoint.
+    gdb_test "break some_func" \
+	"Breakpoint $decimal at $hex: file .*dw2-bad-elf-other\\.c, line 6\\."
+
+    # Check GDB is still alive.
+    gdb_test "echo hello\\n" "hello"
+}
+
+# Run the tests.
+foreach_with_prefix goto_main { 0 1 } {
+    run_test $goto_main
+}