[RFA] fetch result of locdesc expressions as integer (not address)

Joel Brobecker
2011-10-03T21:10:48Z

This is a problem that showed up on AVR as well. The debugger
crashes while trying to print the contents of any struct value.
I was able to reduce the testcase as follow:

% cat foo.c
struct blob
{
int a;
int b;
};

struct blob global_blob = {1234, 5678};

int
main (void)
{
global_blob.a++; /* Stop here */
return 0;
}

To reproduce:

% gdb foo
(gdb) target sim
(gdb) load foo
(gdb) start
Starting program: /[...]/foo

Temporary breakpoint 1, main () at foo.c:13
13 global_blob.a++;
(gdb) p global_blob
[SEGV]

The problem is that the debugger is treating the result of
the DWARF location expressions as addresses, whereas this is
just an offset in this case. I think that this was an unintentional
side-effect of simplifying the code that fetches the result
from the DWARF expression computation stack. We had a bit of
code that used to fetch it, and turn it into a struct value.
And we replaced it by one call to a function that seemed to
be doing the same: dwarf_expr_fetch_address. The problem is
that dwarf_expr_fetch_address treats the result as an address,
and thus applies the integer_to_address gdbarch method. We do
not want that for struct field offsets...

gdb/ChangeLog:

* dwarf2read.c (decode_locdesc): Fetch the result of
the expression evaluation as an integer rather than
an address.

Tested on x86_64-linux, no regression.
Is that OK?

Thanks,
--
Joel

---
gdb/dwarf2read.c | 14 +++++++++++++-
1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index fc6a4d5..6f768a4 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -14175,7 +14175,19 @@ decode_locdesc (struct dwarf_block *blk, struct dwarf2_cu *cu)
case DWARF_VALUE_MEMORY:
case DWARF_VALUE_STACK:
{
- CORE_ADDR address = dwarf_expr_fetch_address (ctx, 0);
+ /* Fetch the result of the expression as an integer,
+ not as an address. We don't know whether it is an
+ address or not; for instance, it could be an expression
+ that returns the offset of a field inside a struct.
+ If we were to fetch the result as an address, we would
+ end up applying the integer_to_address gdbarch method.
+ That would be wrong in the case of an offset. */
+ struct value *val = dwarf_expr_fetch (ctx, 0);
+ enum bfd_endian byte_order = gdbarch_byte_order (ctx->gdbarch);
+ CORE_ADDR address =
+ extract_unsigned_integer (value_contents (val),
+ TYPE_LENGTH (value_type (val)),
+ byte_order);

do_cleanups (old_chain);
return address;
--
1.7.1