45326f6fbe
I cannot reproduce any wrong case having the code removed.
I just do not find it correct to have it disabled. But at the same time I do
like much / I do not find correct the code myself. It is a bit problematic to
have struct value describing a memory content which is no longer present
there.
What happens there:
------------------------------------------------------------------------------
volatile int vv;
static __attribute__((noinline)) int
bar (int &ref) {
ref = 20;
vv++; /* break-here */
return ref;
}
int main (void) {
int var = 10;
return bar (var);
}
------------------------------------------------------------------------------
<4><c7>: Abbrev Number: 13 (DW_TAG_GNU_call_site_parameter)
<c8> DW_AT_location : 1 byte block: 55 (DW_OP_reg5 (rdi))
<ca> DW_AT_GNU_call_site_value: 2 byte block: 91 74 (DW_OP_fbreg: -12)
<cd> DW_AT_GNU_call_site_data_value: 1 byte block: 3a (DW_OP_lit10)
------------------------------------------------------------------------------
gdb -ex 'b value_addr' -ex r --args ../gdb ./1 -ex 'watch vv' -ex r -ex 'p &ref@entry'
->
6 return ref;
bar (ref=@0x7fffffffd944: 20, ref@entry=@0x7fffffffd944: 10) at 1.C:25
------------------------------------------------------------------------------
At /* break-here */ struct value variable 'ref' is TYPE_CODE_REF.
With FSF GDB HEAD:
(gdb) x/gx arg1.contents
0x6004000a4ad0: 0x00007fffffffd944
(gdb) p ((struct value *)arg1.location.computed.closure).lval
$1 = lval_memory
(gdb) p/x ((struct value *)arg1.location.computed.closure).location.address
$3 = 0x7fffffffd944
With your #if0-ed code:
(gdb) x/gx arg1.contents
0x6004000a4ad0: 0x00007fffffffd944
(gdb) p ((struct value *)arg1.location.computed.closure).lval
$8 = not_lval
(gdb) p/x ((struct value *)arg1.location.computed.closure).location.address
$9 = 0x0
I do not see how to access
((struct value *)arg1.location.computed.closure).location.address
from GDB CLI. Trying
(gdb) p &ref@entry
will invoke value_addr()'s:
if (TYPE_CODE (type) == TYPE_CODE_REF)
/* Copy the value, but change the type from (T&) to (T*). We
keep the same location information, which is efficient, and
allows &(&X) to get the location containing the reference. */
and therefore the address gets fetched already from
arg1.contents
and not from
((struct value *)arg1.location.computed.closure).location.address
.
And for any other type than TYPE_CODE_REF this code you removed does not get
executed at all. This DW_AT_GNU_call_site_data_value DWARF was meant
primarily for Fortran but with -O0 entry values do not get produced
and with -Og and higher Fortran always optimizes out the passing by reference.
If you do not like the removed code there I am OK with removing it as I do not
know how to make it's use reproducible for user anyway. In the worst case
- if there really is some way how to exploit it - one should just get
Attempt to take address of value not located in memory.
instead of some wrong value and it may be easy to fix then.
gdb/
2014-07-22 Jan Kratochvil <jan.kratochvil@redhat.com>
* dwarf2loc.c (value_of_dwarf_reg_entry): Remove setting value address
for reference entry value target data value.
Message-ID: <20140720150727.GA18488@host2.jankratochvil.net>
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