From c3efaf0afd9d37004c42cdfd3ce0c1bfa979c45e Mon Sep 17 00:00:00 2001 From: Andrew Burgess Date: Mon, 12 Dec 2022 14:05:22 +0000 Subject: [PATCH] gdb: fix crash when getting the value of a label symbol When the source program contains a goto label, it turns out it's actually pretty hard for a user to find out more about that label. For example: (gdb) p some_label No symbol "some_label" in current context. (gdb) disassemble some_label No symbol "some_label" in current context. (gdb) x/10i some_label No symbol "some_label" in current context. (gdb) break some_label Breakpoint 2 at 0x401135: file /tmp/py-label-symbol-value.c, line 35. In all cases, some_label is a goto label within the current frame. Only placing a breakpoint on the label worked. This all seems a little strange to me, it feels like asking about a goto label would not be an unreasonable thing for a user to do. This commit doesn't fix any of the above issues, I mention them just to provide a little context for why the following issue has probably not been seen before. It turns out there is one way a user can access the symbol for a goto label, through the Python API: python frame = gdb.selected_frame() python frame_pc = frame.pc() python block = gdb.current_progspace().block_for_pc(frame_pc) python symbol,_ = gdb.lookup_symbol('some_label', block, gdb.SYMBOL_LABEL_DOMAIN) python print(str(symbol.value())) ../../src/gdb/findvar.c:204: internal-error: store_typed_address: Assertion `type->is_pointer_or_reference ()' failed. The problem is that label symbols are created using the builtin_core_addr type, which is a pure integer type. When GDB tries to fetch the value of a label symbol then we end up in findvar.c, in the function language_defn::read_var_value, in the LOC_LABEL case. From here store_typed_address is called to store the address of the label into a value object with builtin_core_addr type. The problem is that store_typed_address requires that the destination type be a pointer or reference, which the builtin_core_addr type is not. Now it's not clear what type a goto label address should have, but GCC has an extension that allows users to take the address of a goto label (using &&), in that case the result is of type 'void *'. I propose that when we convert the CORE_ADDR value to a GDB value object, we use builtin_func_ptr type instead of builtin_core_addr, this means the result will be of type 'void (*) ()'. The benefit of this approach is that when gdbarch_address_to_pointer is called the target type will be correctly identified as a pointer to code, which should mean any architecture specific adjustments are done correctly. We can then cast the new value to 'void *' type with a call to value_cast_pointer, this should not change the values bit representation, but will just update the type. After this asking for the value of a label symbol works just fine: (gdb) python print(str(symbol.value())) 0x401135 And the type is maybe what we'd expect: (gdb) python print(str(symbol.value().type)) void * --- gdb/findvar.c | 40 ++++++++++++------- .../gdb.python/py-label-symbol-value.c | 38 ++++++++++++++++++ .../gdb.python/py-label-symbol-value.exp | 39 ++++++++++++++++++ 3 files changed, 103 insertions(+), 14 deletions(-) create mode 100644 gdb/testsuite/gdb.python/py-label-symbol-value.c create mode 100644 gdb/testsuite/gdb.python/py-label-symbol-value.exp diff --git a/gdb/findvar.c b/gdb/findvar.c index e609358df08..a5e27035c15 100644 --- a/gdb/findvar.c +++ b/gdb/findvar.c @@ -593,20 +593,32 @@ language_defn::read_var_value (struct symbol *var, return v; case LOC_LABEL: - /* Put the constant back in target format. */ - v = allocate_value (type); - if (overlay_debugging) - { - struct objfile *var_objfile = var->objfile (); - addr = symbol_overlayed_address (var->value_address (), - var->obj_section (var_objfile)); - store_typed_address (value_contents_raw (v).data (), type, addr); - } - else - store_typed_address (value_contents_raw (v).data (), type, - var->value_address ()); - VALUE_LVAL (v) = not_lval; - return v; + { + /* Put the constant back in target format. */ + if (overlay_debugging) + { + struct objfile *var_objfile = var->objfile (); + addr = symbol_overlayed_address (var->value_address (), + var->obj_section (var_objfile)); + } + else + addr = var->value_address (); + + /* First convert the CORE_ADDR to a function pointer type, this + ensures the gdbarch knows what type of pointer we are + manipulating when value_from_pointer is called. */ + type = builtin_type (var->arch ())->builtin_func_ptr; + v = value_from_pointer (type, addr); + + /* But we want to present the value as 'void *', so cast it to the + required type now, this will not change the values bit + representation. */ + struct type *void_ptr_type + = builtin_type (var->arch ())->builtin_data_ptr; + v = value_cast_pointers (void_ptr_type, v, 0); + VALUE_LVAL (v) = not_lval; + return v; + } case LOC_CONST_BYTES: if (is_dynamic_type (type)) diff --git a/gdb/testsuite/gdb.python/py-label-symbol-value.c b/gdb/testsuite/gdb.python/py-label-symbol-value.c new file mode 100644 index 00000000000..94bdae6fd30 --- /dev/null +++ b/gdb/testsuite/gdb.python/py-label-symbol-value.c @@ -0,0 +1,38 @@ +/* This testcase is part of GDB, the GNU debugger. + + Copyright 2022 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 . */ + +volatile int global_var = 1; + +int +get_value () +{ + return global_var; +} + +int +main (void) +{ + int value = get_value (); + if (value > 0) + goto some_label; + + return 1; + + some_label: + + return 0; +} diff --git a/gdb/testsuite/gdb.python/py-label-symbol-value.exp b/gdb/testsuite/gdb.python/py-label-symbol-value.exp new file mode 100644 index 00000000000..44321e5f71d --- /dev/null +++ b/gdb/testsuite/gdb.python/py-label-symbol-value.exp @@ -0,0 +1,39 @@ +# Copyright 2022 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 . + +# Check that GDB handles the user asking for the value of a label +# symbol (i.e. a symbol for a goto label). + +load_lib gdb-python.exp +standard_testfile + +if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } { + return -1 +} + +# Skip all tests if Python scripting is not enabled. +if { [skip_python_tests] } { continue } + +if ![runto_main] { + return -1 +} + +# Use Python to print the value of the 'some_label' symbol. +gdb_test "python frame = gdb.selected_frame()" +gdb_test "python frame_pc = frame.pc()" +gdb_test "python block = gdb.current_progspace().block_for_pc(frame_pc)" +gdb_test "python symbol,_ = gdb.lookup_symbol('some_label', block, gdb.SYMBOL_LABEL_DOMAIN)" +gdb_test "python print(str(symbol.value()))" "$hex " +gdb_test "python print(str(symbol.value().type))" "void \\*" -- 2.30.2