From 45326f6fbe28ef5bac22dac447a4181c44cb945a Mon Sep 17 00:00:00 2001 From: Jan Kratochvil Date: Tue, 22 Jul 2014 22:15:27 +0200 Subject: [PATCH] Remove setting value address for reference entry value target data value. 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>: Abbrev Number: 13 (DW_TAG_GNU_call_site_parameter) DW_AT_location : 1 byte block: 55 (DW_OP_reg5 (rdi)) DW_AT_GNU_call_site_value: 2 byte block: 91 74 (DW_OP_fbreg: -12) 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 * 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> --- gdb/ChangeLog | 5 +++++ gdb/dwarf2loc.c | 9 --------- 2 files changed, 5 insertions(+), 9 deletions(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 73a13d3e576..687e2fe6494 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,8 @@ +2014-07-22 Jan Kratochvil + + * dwarf2loc.c (value_of_dwarf_reg_entry): Remove setting value address + for reference entry value target data value. + 2014-07-22 Jan Kratochvil * stack.c (read_frame_arg): Verify value_optimized_out before calling diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c index fcab9b91556..b1c7ee1a6dc 100644 --- a/gdb/dwarf2loc.c +++ b/gdb/dwarf2loc.c @@ -1312,7 +1312,6 @@ value_of_dwarf_reg_entry (struct type *type, struct frame_info *frame, struct value *outer_val, *target_val, *val; struct call_site_parameter *parameter; struct dwarf2_per_cu_data *caller_per_cu; - CORE_ADDR addr; parameter = dwarf_expr_reg_to_entry_parameter (frame, kind, kind_u, &caller_per_cu); @@ -1335,14 +1334,6 @@ value_of_dwarf_reg_entry (struct type *type, struct frame_info *frame, target_type, caller_frame, caller_per_cu); - /* value_as_address dereferences TYPE_CODE_REF. */ - addr = extract_typed_address (value_contents (outer_val), checked_type); - - /* The target entry value has artificial address of the entry value - reference. */ - VALUE_LVAL (target_val) = lval_memory; - set_value_address (target_val, addr); - release_value (target_val); val = allocate_computed_value (type, &entry_data_value_funcs, target_val /* closure */); -- 2.30.2