Correct bounds check when working around GAS DWARF 5 directory table bug

Recent Go toolchains are causing GDB to crash on a relatively recent
workaround for a GAS bug:

commit a833790a62
Date:   Wed Nov 1 00:33:12 2023 +0100

    [gdb/symtab] Work around gas PR28629

In the original GAS bug, the first directory table entry did not contain
the current directory of the compilation. So the above commit added a
workaround fix to prepend the second directory table entry.

However recent Go toolchain compilations (specifically on aarch64)
only output a single directory table entry. Looking at the workaround:

       if (lh->version == 5 && lh->is_valid_file_index (1))
         {
           std::string dir = lh->include_dir_at (1);
           fnd.set_comp_dir (std::move (dir));
         }

`lh->is_valid_file_index (1)' is true, but since the directory table only
has one entry, `include_dir_at (1)' returns nullptr. Consequently the
std::string ctor will segfault. Since there are no guarantees that the file
and directory tables are the same size, a better bounds check is to simply
rely on `include_dir_at' to ensure a valid directory table entry.

I have updated the workaround commit's test, gdb.dwarf2/dw2-gas-workaround.exp
and tested on x86_64 and aarch64 RHEL 9 and Fedora 41.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/dwarf2/read.c

@@ -6124,7 +6124,7 @@ read_file_scope (struct die_info *die, struct dwarf2_cu *cu)
 	  sect_offset line_offset = (sect_offset) attr->as_unsigned ();
 	  line_header_up lh = dwarf_decode_line_header (line_offset, cu,
 							fnd.get_comp_dir ());
-	  if (lh->version == 5 && lh->is_valid_file_index (1))
+	  if (lh->version == 5 && lh->include_dir_at (1) != nullptr)
 	    {
 	      std::string dir = lh->include_dir_at (1);
 	      fnd.set_comp_dir (std::move (dir));

gdb/testsuite/gdb.dwarf2/dw2-gas-workaround.exp

@@ -20,7 +20,7 @@ load_lib dwarf.exp
 # This test can only be run on targets which support DWARF-2 and use gas.
 require dwarf2_support
 
-standard_testfile dw2-lines.c -dw2.S
+standard_testfile dw2-lines.c -dw2.S -dw2-one-diridx.S
 
 with_shared_gdb {
     set func_info_vars [get_func_info bar]
@@ -33,49 +33,61 @@ proc line_for { l } {
     return [expr $line + 1]
 }
 
-set asm_file [standard_output_file $srcfile2]
-Dwarf::assemble $asm_file {
-    declare_labels Llines
-    global srcdir subdir srcfile objdir
-    global func_info_vars
-    foreach var $func_info_vars {
-	global $var
-    }
+# A helper proc to create the DWARF assembly for the test.
+# If ONE_DIRIDX is true, then the directory table will be limited
+# to one entry.
+proc create_dwarf_assembly {source_file one_diridx} {
+    set asm_file [standard_output_file $source_file]
+    Dwarf::assemble $asm_file {
+	declare_labels Llines
+	global srcdir subdir srcfile objdir
+	global func_info_vars
+	upvar one_diridx one_diridx
+	foreach var $func_info_vars {
+	    global $var
+	}
 
-    cu { version 5 } {
-	compile_unit {
-	    DW_AT_language @DW_LANG_Mips_Assembler
-	    DW_AT_name $srcfile
-	    DW_AT_comp_dir $objdir
-	    DW_AT_stmt_list $Llines DW_FORM_sec_offset
-	    DW_AT_producer "GNU AS 2.35.2"
-	} {
-	    subprogram {
-		DW_AT_external 1 flag
-		DW_AT_name bar
-		DW_AT_low_pc $bar_start addr
-		DW_AT_high_pc "$bar_start + $bar_len" addr
+	cu { version 5 } {
+	    compile_unit {
+		DW_AT_language @DW_LANG_Mips_Assembler
+		DW_AT_name $srcfile
+		DW_AT_comp_dir $objdir
+		DW_AT_stmt_list $Llines DW_FORM_sec_offset
+		DW_AT_producer "GNU AS 2.35.2"
+	    } {
+		subprogram {
+		    DW_AT_external 1 flag
+		    DW_AT_name bar
+		    DW_AT_low_pc $bar_start addr
+		    DW_AT_high_pc "$bar_start + $bar_len" addr
+		}
+	    }
+	}
+
+	lines [list version 5] Llines {
+	    set diridx1 [include_dir "${srcdir}/${subdir}"]
+	    file_name "$srcfile" $diridx1
+	    if {!$one_diridx} {
+		set diridx2 [include_dir "${srcdir}/${subdir}"]
+		file_name "$srcfile" $diridx2
+	    } else {
+		file_name "$srcfile" $diridx1
+	    }
+	    program {
+		DW_LNE_set_address bar_label
+		line [line_for bar_label]
+		DW_LNS_copy
+
+		DW_LNE_set_address $bar_end
+		DW_LNE_end_sequence
 	    }
 	}
     }
 
-    lines [list version 5] Llines {
-	set diridx1 [include_dir "${srcdir}/${subdir}"]
-	set diridx2 [include_dir "${srcdir}/${subdir}"]
-	file_name "$srcfile" $diridx1
-	file_name "$srcfile" $diridx2
-
-	program {
-	    DW_LNE_set_address bar_label
-	    line [line_for bar_label]
-	    DW_LNS_copy
-
-	    DW_LNE_set_address $bar_end
-	    DW_LNE_end_sequence
-	}
-    }
+    return $asm_file
 }
 
+set asm_file [create_dwarf_assembly $srcfile2 false]
 if { [prepare_for_testing "failed to prepare" ${testfile} \
 	  [list $srcfile $asm_file] {nodebug}] } {
     return -1
@@ -90,3 +102,13 @@ gdb_test_multiple "ptype bar" "" {
 	pass $gdb_test_name
     }
 }
+
+# Test whether gdb crashes in the case where the number of
+# directory indexes is only one.
+set asm_file [create_dwarf_assembly $srcfile3 true]
+if {[prepare_for_testing "failed to prepare" ${testfile}-one-diridx \
+	 [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+gdb_test "ptype bar" ".*" "do not crash with only one directory table entry"