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c3efaf0afd
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 <main+35>
And the type is maybe what we'd expect:
(gdb) python print(str(symbol.value().type))
void *
39 lines
949 B
C
39 lines
949 B
C
/* This testcase is part of GDB, the GNU debugger.
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Copyright 2022 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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volatile int global_var = 1;
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int
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get_value ()
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{
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return global_var;
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}
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int
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main (void)
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{
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int value = get_value ();
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if (value > 0)
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goto some_label;
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return 1;
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some_label:
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return 0;
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}
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