Correctly handle DW_LLE_start_end

When the code to handle DW_LLE_start_end was added (as part of some
DWARF 5 work), it was written to add the base address.  However, this
seems incorrect -- the DWARF standard describes this as an address,
not an offset from the base address.

This patch changes a couple of spots in dwarf2/loc.c to fix this
problem.  It then changes decode_debug_loc_addresses to return
DEBUG_LOC_OFFSET_PAIR instead, which preserves the previous semantics.

This only showed up on the RISC-V target internally, due to the
combination of DWARF 5 and a newer version of GCC.  I've updated a
couple of existing loclists test cases to demonstrate the bug.

4 files changed, 30 insertions, 7 deletions

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index eb128fa..b5936e1 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -139,7 +139,9 @@ decode_debug_loc_addresses (const gdb_byte *loc_ptr, const gdb_byte *buf_end,
   if (*low == 0 && *high == 0)
     return DEBUG_LOC_END_OF_LIST;
 
-  return DEBUG_LOC_START_END;
+  /* We want the caller to apply the base address, so we must return
+     DEBUG_LOC_OFFSET_PAIR here.  */
+  return DEBUG_LOC_OFFSET_PAIR;
 }
 
 /* Decode the addresses in .debug_loclists entry.
@@ -416,13 +418,15 @@ dwarf2_find_location_expression (struct dwarf2_loclist_baton *baton,
 	 .debug_addr which already has the DWARF "base address". We still add
 	 base_offset in case we're debugging a PIE executable. However, if the
 	 entry is DW_LLE_offset_pair from a DWO, add the base address as the
-	 operands are offsets relative to the applicable base address.  */
+	 operands are offsets relative to the applicable base address.
+	 If the entry is DW_LLE_start_end or DW_LLE_start_length, then
+	 it already is an address, and we don't need to add the base.  */
       if (baton->from_dwo && kind != DEBUG_LOC_OFFSET_PAIR)
 	{
 	  low += base_offset;
 	  high += base_offset;
 	}
-      else
+      else if (kind == DEBUG_LOC_OFFSET_PAIR)
 	{
 	  low += base_address;
 	  high += base_address;
@@ -3983,8 +3987,11 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
 	}
 
       /* Otherwise, a location expression entry.  */
-      low += base_address;
-      high += base_address;
+      if (kind == DEBUG_LOC_OFFSET_PAIR)
+	{
+	  low += base_address;
+	  high += base_address;
+	}
 
       low = gdbarch_adjust_dwarf2_addr (gdbarch, low);
       high = gdbarch_adjust_dwarf2_addr (gdbarch, high);
diff --git a/gdb/testsuite/gdb.dwarf2/loclists-multiple-cus.exp b/gdb/testsuite/gdb.dwarf2/loclists-multiple-cus.exp
index 7844628..a7af7fd 100644
--- a/gdb/testsuite/gdb.dwarf2/loclists-multiple-cus.exp
+++ b/gdb/testsuite/gdb.dwarf2/loclists-multiple-cus.exp
@@ -1,4 +1,4 @@
-# Copyright 2020 Free Software Foundation, Inc.
+# Copyright 2020, 2021 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
@@ -105,6 +105,9 @@ foreach_with_prefix is_64 {false true} {
 
 		# For variable foo.
 		list_ {
+		    # This should not affect the following addresses.
+		    base_address 0xffff
+
 		    # When in func1.
 		    start_length $func1_addr $func1_len {
 			DW_OP_constu 0x123456
diff --git a/gdb/testsuite/gdb.dwarf2/loclists-start-end.exp b/gdb/testsuite/gdb.dwarf2/loclists-start-end.exp
index e3c12e5..18ef2bf 100644
--- a/gdb/testsuite/gdb.dwarf2/loclists-start-end.exp
+++ b/gdb/testsuite/gdb.dwarf2/loclists-start-end.exp
@@ -96,6 +96,9 @@ foreach_with_prefix is_64 {false true} {
 
 		# For variable foo.
 		list_ {
+		    # This should not affect the following addresses.
+		    base_address 0xffff
+
 		    # When in func1.
 		    start_end $func1_addr "$func1_addr + $func1_len" {
 			DW_OP_constu 0x123456
diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
index b48cfad..774cac7 100644
--- a/gdb/testsuite/lib/dwarf.exp
+++ b/gdb/testsuite/lib/dwarf.exp
@@ -1894,9 +1894,10 @@ namespace eval Dwarf {
 	define_label $list_label
 
 	with_override Dwarf::start_length Dwarf::_loclists_start_length {
+	with_override Dwarf::base_address Dwarf::_loclists_base_address {
 	with_override Dwarf::start_end Dwarf::_loclists_start_end {
 	    uplevel $body
-	}}
+	}}}
 
 	# Emit end of list.
 	_op .byte 0x00 "DW_LLE_end_of_list"
@@ -1972,6 +1973,15 @@ namespace eval Dwarf {
 	incr _debug_loclists_locdesc_count
     }
 
+    # Emit a DW_LLE_base_address entry.
+    proc _loclists_base_address {addr} {
+	variable _debug_loclists_addr_size
+	variable _debug_loclists_locdesc_count
+	_op .byte 0x06 "DW_LLE_base_address"
+	_op .${_debug_loclists_addr_size}byte $addr "base_address"
+	incr _debug_loclists_locdesc_count
+    }
+
     # Emit a DWARF .debug_line unit.
     # OPTIONS is a list with an even number of elements containing
     # option-name and option-value pairs.