[PATCH] gdb: handle DW_AT_entry_pc pointing at an empty sub-range

[Andrew Burgess <aburgess@redhat.com>]

The test gdb.cp/step-and-next-inline.exp creates a test binary called
step-and-next-inline-no-header.  This test includes a function
`tree_check` which is inlined 3 times.

When testing with some older versions of gcc (I've tried 8.4.0, 9.3.1)
we see the following DWARF representing one of the inline instances of
tree_check:

 <2><8d9>: Abbrev Number: 38 (DW_TAG_inlined_subroutine)
    <8da>   DW_AT_abstract_origin: <0x9ee>
    <8de>   DW_AT_entry_pc    : 0x401165
    <8e6>   DW_AT_GNU_entry_view: 0
    <8e7>   DW_AT_ranges      : 0x30
    <8eb>   DW_AT_call_file   : 1
    <8ec>   DW_AT_call_line   : 52
    <8ed>   DW_AT_call_column : 10
    <8ee>   DW_AT_sibling     : <0x92d>

 ...

 <1><9ee>: Abbrev Number: 46 (DW_TAG_subprogram)
    <9ef>   DW_AT_external    : 1
    <9ef>   DW_AT_name        : (indirect string, offset: 0xe8): tree_check
    <9f3>   DW_AT_decl_file   : 1
    <9f4>   DW_AT_decl_line   : 38
    <9f5>   DW_AT_decl_column : 1
    <9f6>   DW_AT_linkage_name: (indirect string, offset: 0x2f2): _Z10tree_checkP4treei
    <9fa>   DW_AT_type        : <0x9e8>
    <9fe>   DW_AT_inline      : 3       (declared as inline and inlined)
    <9ff>   DW_AT_sibling     : <0xa22>

 ...

 Contents of the .debug_ranges section:

    Offset   Begin    End
    ...
    00000030 0000000000401165 0000000000401165 (start == end)
    00000030 0000000000401169 0000000000401173
    00000030 0000000000401040 0000000000401045
    00000030 <End of list>
    ...

Notice that one of the sub-ranges of tree-check is empty, this is the
line marked 'start == end'.  As the end address is the first address
after the range, this range cover absolutely no code.

But notice too that the DW_AT_entry_pc for the inline instance points
at this empty range.

Further, notice that despite the ordering of the sub-ranges, the empty
range is actually in the middle of the region defined by the lowest
address to the highest address.  The ordering is not a problem, the
DWARF spec doesn't require that ranges be in any particular order.

However, this empty range is causing issues with GDB newly acquire
DW_AT_entry_pc support.

GDB already rejects, and has done for a long time, empty sub-ranges,
after all, the DWARF spec is clear that such a range covers no code.

The recent DW_AT_entry_pc patch also had GDB reject an entry-pc which
was outside of the low/high bounds of a block.

But in this case, the entry-pc value is within the bounds of a block,
it's just not within any useful sub-range.  As a consequence, GDB is
storing the entry-pc value, and making use of it, but when GDB stops,
and tries to work out which block the inferior is in, it fails to spot
that the inferior is within tree_check, and instead reports the
function into which tree_check was inlined.

I've tested with newer versions of gcc (12.2.0 and 14.2.0) and with
these versions gcc is still generating the empty sub-range, but now
this empty sub-range is no longer the entry point.  Here's the
corresponding ranges table from gcc 14.2.0:

  Contents of the .debug_rnglists section:

   Table at Offset: 0:
    Length:          0x56
    DWARF version:   5
    Address size:    8
    Segment size:    0
    Offset entries:  0
      Offset   Begin    End
      ...
      00000021 0000000000401165 000000000040116f
      0000002b 0000000000401040 (base address)
      00000034 0000000000401040 0000000000401040  (start == end)
      00000037 0000000000401041 0000000000401046
      0000003a <End of list>
      ...

The DW_AT_entry_pc is 0x401165, but this is not the empty sub-range,
as a result, when GDB stops at the entry-pc, GDB will correctly spot
that the inferior is in the tree_check function.

The fix I propose here is, instead of rejecting entry-pc values that
are outside the block's low/high range, instead reject entry-pc values
that are not inside any of the block's sub-ranges.

Now, on the older versions of GDB, GDB will ignore the prescribed
entry-pc, and will instead select a suitable default entry-pc based on
either the block's low-pc value, or the first address of the first
range.

I have extended the gdb.cp/step-and-next-inline.exp test to check this
case, but this does depend on the compiler version being used (newer
compilers will always pass, even without the fix).

So I have also added a DWARF assembler test to cover this case.
---
 gdb/dwarf2/read.c                             |  24 ++-
 gdb/testsuite/gdb.cp/step-and-next-inline.exp |  26 +++
 .../gdb.dwarf2/dw2-entry-pc-in-empty-range.c  |  57 ++++++
 .../dw2-entry-pc-in-empty-range.exp           | 186 ++++++++++++++++++
 4 files changed, 292 insertions(+), 1 deletion(-)
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.c
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 5f0b0d4e5d6..c178b13d96d 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -11338,6 +11338,28 @@ dwarf2_die_base_address (struct die_info *die, struct block *block,
   return {};
 }
 
+/* Return true if ADDR is within any of the ranges covered by BLOCK.  If
+   there are no sub-ranges then just check against the block's start and
+   end addresses, otherwise, check each sub-range covered by the block.  */
+
+static bool
+dwarf2_addr_in_block_ranges (CORE_ADDR addr, struct block *block)
+{
+  if (block->ranges ().size () == 0)
+    return addr >= block->start () && addr < block->end ();
+
+  /* Check if ADDR is within any of the block's sub-ranges.  */
+  for (const blockrange &br : block->ranges ())
+    {
+      if (addr >= br.start () && addr < br.end ())
+	return true;
+    }
+
+  /* ADDR is not within any of the block's sub-ranges.  */
+  return false;
+}
+
+
 /* Set the entry PC for BLOCK which represents DIE from CU.  Relies on the
    range information (if present) already having been read from DIE and
    stored into BLOCK.  */
@@ -11398,7 +11420,7 @@ dwarf2_record_block_entry_pc (struct die_info *die, struct block *block,
 
 	 To avoid this, ignore entry-pc values that are outside the block's
 	 range, GDB will then select a suitable default entry-pc.  */
-      if (entry_pc >= block->start () && entry_pc < block->end ())
+      if (dwarf2_addr_in_block_ranges (entry_pc, block))
 	block->set_entry_pc (entry_pc);
       else
 	complaint (_("in %s, DIE %s, DW_AT_entry_pc (%s) outside "
diff --git a/gdb/testsuite/gdb.cp/step-and-next-inline.exp b/gdb/testsuite/gdb.cp/step-and-next-inline.exp
index af1719dc53a..e16c2cca82d 100644
--- a/gdb/testsuite/gdb.cp/step-and-next-inline.exp
+++ b/gdb/testsuite/gdb.cp/step-and-next-inline.exp
@@ -55,6 +55,32 @@ proc do_test { use_header } {
 
     set main_location [gdb_get_line_number "Beginning of main" $srcfile]
 
+    if {![runto_main]} {
+	return
+    }
+
+    gdb_breakpoint tree_check
+
+    # Check that GDB can correctly stop in `tree_check`.  On some
+    # targets. gcc will use DW_AT_ranges to represent the addresses of
+    # tree_check, and in some cases, will create an empty sub-range
+    # for some of the tree_check code.  To really confuse things, gcc
+    # will then set the DW_AT_entry_pc to point at the address of the
+    # empty sub-range.
+    #
+    # The result of this is that GDB would stop at the DW_AT_entry_pc,
+    # but then GDB would fail to realise that this address was inside
+    # tree_check.
+    for { set i 1 } { $i < 4 } { incr i } {
+	gdb_test "continue" \
+	    [multi_line \
+		 "Breakpoint $::decimal\\.$i, (?:$::hex in )?tree_check \\(\[^\r\n\]+\\) at \[^\r\n\]+/$hdrfile:$::decimal" \
+		 "$::decimal\\s+\[^\r\n\]+"] \
+	    "stop at tree_check, $i"
+    }
+
+    clean_restart $executable
+
     if ![runto $main_location qualified] {
 	return
     }
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.c b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.c
new file mode 100644
index 00000000000..cf43727f1fb
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.c
@@ -0,0 +1,57 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2024 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/>.  */
+
+volatile int global_var = 0;
+
+void
+foo (void)	/* foo decl line */
+{
+  /* This label is used to find the start of 'foo' when generating the
+     debug information.  Place nothing before it.  */
+  asm ("foo_label: .globl foo_label");
+  ++global_var;
+
+  asm ("foo_0: .globl foo_0");
+  ++global_var;		/* bar call line */
+
+  asm ("foo_1: .globl foo_1");
+  ++global_var;
+
+  asm ("foo_2: .globl foo_2");
+  ++global_var;
+
+  asm ("foo_3: .globl foo_3");
+  ++global_var;
+
+  asm ("foo_4: .globl foo_4");
+  ++global_var;
+
+  asm ("foo_5: .globl foo_5");
+  ++global_var;
+
+  asm ("foo_6: .globl foo_6");
+  ++global_var;
+
+  asm ("foo_7: .globl foo_7");
+}
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  foo ();
+}
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp
new file mode 100644
index 00000000000..79b1783b2ec
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-entry-pc-in-empty-range.exp
@@ -0,0 +1,186 @@
+# Copyright 2024 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/>.
+
+# Create an inline function which uses DW_AT_ranges and which has a
+# DW_AT_entry_pc.
+#
+# Within the function's ranges, create an empty sub-range, many
+# versions of gcc (8.x to at least 14.x) do this, and point the
+# DW_AT_entry_pc at this empty sub-range (at last 8.x to 9.x did
+# this).
+#
+# Now place a breakpoint on the inline function and run to the
+# breakpoint, check that GDB reports we are inside the inline
+# function.
+#
+# At one point GDB would use the entry-pc value as the breakpoint
+# location even though that address is not actually associated with
+# the inline function.  Now GDB will reject the entry-pc value and
+# select a suitable default entry-pc value instead, one which is
+# associated with the inline function.
+
+load_lib dwarf.exp
+
+require dwarf2_support
+
+standard_testfile
+
+# This compiles the source file and starts and stops GDB, so run it
+# before calling prepare_for_testing otherwise GDB will have exited.
+get_func_info foo
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list ${srcfile}]] } {
+    return
+}
+
+if ![runto_main] {
+    return
+}
+
+# Some label addresses, needed to match against the output later.
+foreach foo {foo_1 foo_2 foo_3 foo_4 foo_5 foo_6} {
+    set $foo [get_hexadecimal_valueof "&$foo" "UNKNOWN" \
+		  "get address for $foo label"]
+}
+
+# Some line numbers needed in the generated DWARF.
+set foo_decl_line [gdb_get_line_number "foo decl line"]
+set bar_call_line [gdb_get_line_number "bar call line"]
+
+if [is_ilp32_target] {
+    set ptr_type "data4"
+} else {
+    set ptr_type "data8"
+}
+
+# Setup the fake DWARF (see comment at top of this file for more
+# details).  Use DWARF_VERSION (either 4 or 5) to select which type of
+# ranges are created.  Compile the source and generated DWARF and run
+# the test.
+#
+# The ENTRY_LABEL is the label to use as the entry-pc value.  The
+# useful choices are 'foo_3', this label is for an empty sub-range, or
+# 'foo_4', this label is within the blocks low/high addresses, but is
+# not in any sub-range for the block at all.
+#
+# The 'foo_4' case is not something that has been seen generated by
+# any compiler, but it doesn't hurt to test.
+
+proc run_test { entry_label dwarf_version } {
+    set dw_testname "${::testfile}-${dwarf_version}"
+
+    set asm_file [standard_output_file "${dw_testname}.S"]
+    Dwarf::assemble $asm_file {
+	upvar dwarf_version dwarf_version
+	upvar entry_label entry_label
+
+	declare_labels lines_table inline_func ranges_label
+
+	cu { version $dwarf_version } {
+	    compile_unit {
+		{producer "gcc"}
+		{language @DW_LANG_C}
+		{name $::srcfile}
+		{comp_dir /tmp}
+		{stmt_list $lines_table DW_FORM_sec_offset}
+		{low_pc 0 addr}
+	    } {
+		inline_func: subprogram {
+		    {name bar}
+		    {inline @DW_INL_declared_inlined}
+		}
+		subprogram {
+		    {name foo}
+		    {decl_file 1 data1}
+		    {decl_line $::foo_decl_line data1}
+		    {decl_column 1 data1}
+		    {low_pc $::foo_start addr}
+		    {high_pc $::foo_len $::ptr_type}
+		    {external 1 flag}
+		} {
+		    inlined_subroutine {
+			{abstract_origin %$inline_func}
+			{call_file 1 data1}
+			{call_line $::bar_call_line data1}
+			{entry_pc $entry_label addr}
+			{ranges ${ranges_label} DW_FORM_sec_offset}
+		    }
+		}
+	    }
+	}
+
+	lines {version 2} lines_table {
+	    include_dir "$::srcdir/$::subdir"
+	    file_name "$::srcfile" 1
+	}
+
+	if { $dwarf_version == 5 } {
+	    rnglists {} {
+		table {} {
+		    ranges_label: list_ {
+			start_end foo_3 foo_3
+			start_end foo_1 foo_2
+			start_end foo_5 foo_6
+		    }
+		}
+	    }
+	} else {
+	    ranges { } {
+		ranges_label: sequence {
+		    range foo_3 foo_3
+		    range foo_1 foo_2
+		    range foo_5 foo_6
+		}
+	    }
+	}
+    }
+
+    if {[prepare_for_testing "failed to prepare" "${dw_testname}" \
+	     [list $::srcfile $asm_file] {nodebug}]} {
+	return false
+    }
+
+    if ![runto_main] {
+	return false
+    }
+
+    # Place a breakpoint on `bar` and run to the breakpoint.  Use
+    # gdb_test as we want full pattern matching against the stop
+    # location.
+    gdb_breakpoint bar
+    gdb_test "continue" \
+	"Breakpoint $::decimal, $::hex in bar \\(\\)"
+
+    # Inspect the block structure of `bar` at this location.  We are
+    # expecting that the empty range (that contained the entry-pc) has
+    # been removed from the block, and that the entry-pc has its
+    # default value.
+    gdb_test "maint info blocks" \
+	[multi_line \
+	     "\\\[\\(block \\*\\) $::hex\\\] $::foo_1\\.\\.$::foo_6" \
+	     "  entry pc: $::foo_1" \
+	     "  inline function: bar" \
+	     "  symbol count: $::decimal" \
+	     "  address ranges:" \
+	     "    $::foo_1\\.\\.$::foo_2" \
+	     "    $::foo_5\\.\\.$::foo_6"]
+}
+
+foreach_with_prefix entry_label { foo_3 foo_4 } {
+    foreach_with_prefix dwarf_version { 4 5 } {
+	run_test $entry_label $dwarf_version
+    }
+}

base-commit: 82eff6743b77908a502b4cf9030acc93caf69e74
-- 
2.25.4