[PATCH][gdb/breakpoint, PIE] Handle setting breakpoint on label without address

[Tom de Vries <tdevries@suse.de>]

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[ was: Re: [PATCH][gdb/breakpoint] Handle setting breakpoint on label
without address ]

On 8/28/20 12:31 PM, Tom de Vries wrote:
> On 8/27/20 3:49 PM, Tom de Vries wrote:
>> On 8/27/20 2:41 PM, Pedro Alves wrote:
>>> On 8/27/20 12:52 PM, Tom de Vries wrote:
>>>> Hi,
>>>>
>>>> Consider test-case test.c:
>>>> ...
>>>> $ cat test.c
>>>> int main (void) {
>>>>   return 0;
>>>>  L1:
>>>>   (void)0;
>>>> }
>>>> ...
>>>>
>>>> Compiled with debug info:
>>>> ...
>>>> $ gcc test.c -g
>>>> ...
>>>>
>>>> When attempting to set a breakpoint at L1, which is a label without address:
>>>> ...
>>>>  <1><f4>: Abbrev Number: 2 (DW_TAG_subprogram)
>>>>     <f5>   DW_AT_name        : main
>>>>  <2><115>: Abbrev Number: 3 (DW_TAG_label)
>>>>     <116>   DW_AT_name        : L1
>>>>     <119>   DW_AT_decl_file   : 1
>>>>     <11a>   DW_AT_decl_line   : 5
>>>>  <2><11b>: Abbrev Number: 0
>>>
>>> Is this a debug info bug,
>>
>> Strictly speaking, this is a debug info bug.  The standard says that:
>> ...
>> The label entry has a DW_AT_low_pc attribute whose value is the address
>> of the first executable instruction for the location identified by the
>> label in the source program.
>> ...
>>
>> But I interpret the missing DW_AT_low_pc attribute as: there is a label
>> in the source, but the corresponding code has been optimized out.
>>
>>> or is the debug info telling us that the
>>> address of the label is the same as the line number's address?
>>>
>>> How about looking up the line number address instead of throwing
>>> an error?
>>>
>>
>> Well, in this particular case, that wouldn't help.
>>
>> With L1 at line 3:
>> ...
>> $ cat -n test.c
>>      1  int main (void) {
>>      2    return 0;
>>      3   L1:
>>      4    (void)0;
>>      5  }
>>      6
>> ...
>> there's no corresponding address:
>> ...
>> $ readelf -wL a.out
>> CU: test.c:
>> File name                            Line number    Starting address
>> View    Stmt
>> test.c                                         1            0x400497
>>            x
>> test.c                                         2            0x40049b
>>            x
>> test.c                                         5            0x4004a0
>>            x
>> test.c                                         -            0x4004a2
>> ...
>>
>> My suspicion is that this won't be useful in general.
>>
> 
> I've pushed this as attached below, with the test-case updated to work
> around PR26546 - "[pie] Setting breakpoint on missing label sets
> breakpoint at offset 0 in NULL section" (
> https://sourceware.org/bugzilla/show_bug.cgi?id=26546 ).

Which is fixed by the patch below.

Any comments?

Thanks,
- Tom



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[gdb/breakpoint, PIE] Handle setting breakpoint on label without address

When adding:
...
if ![runto_main] then {
    fail "can't run to main"
    return 0
}
...
to test-case gdb.base/label-without-address.exp and running it with target
board unix/-fPIE/-pie, we run into:
...
(gdb) break main:L1^M
Breakpoint 2 at 0x555555554000: file label-without-address.c, line 22.^M
...
That is, for a label with optimized-out address, we set a breakpoint at the
relocation base.

The root cause is that the dwarf reader, despite finding that attribute
DW_AT_low_pc is missing, still tags the L1 symbol as having LOC_LABEL, which
means it has a valid address, which defaults to 0.

Fix this by instead tagging the L1 symbol with LOC_OPTIMIZED_OUT.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26546
	* dwarf2/read.c (new_symbol): Tag label symbol without DW_AT_low_pc as
	LOC_OPTIMIZED_OUT instead of LOC_LABEL.

gdb/testsuite/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26546
	* gdb.base/label-without-address.exp: Runto main first.

---
 gdb/dwarf2/read.c                                | 4 +++-
 gdb/testsuite/gdb.base/label-without-address.exp | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 0ac8533263..b37f7e7a2f 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -21447,10 +21447,12 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	      addr = attr->value_as_address ();
 	      addr = gdbarch_adjust_dwarf2_addr (gdbarch, addr + baseaddr);
 	      SET_SYMBOL_VALUE_ADDRESS (sym, addr);
+	      SYMBOL_ACLASS_INDEX (sym) = LOC_LABEL;
 	    }
+	  else
+	    SYMBOL_ACLASS_INDEX (sym) = LOC_OPTIMIZED_OUT;
 	  SYMBOL_TYPE (sym) = objfile_type (objfile)->builtin_core_addr;
 	  SYMBOL_DOMAIN (sym) = LABEL_DOMAIN;
-	  SYMBOL_ACLASS_INDEX (sym) = LOC_LABEL;
 	  add_symbol_to_list (sym, cu->list_in_scope);
 	  break;
 	case DW_TAG_subprogram:
diff --git a/gdb/testsuite/gdb.base/label-without-address.exp b/gdb/testsuite/gdb.base/label-without-address.exp
index 0fcb1fd19a..c688149cf3 100644
--- a/gdb/testsuite/gdb.base/label-without-address.exp
+++ b/gdb/testsuite/gdb.base/label-without-address.exp
@@ -19,6 +19,11 @@ if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
     return -1
 }
 
+if ![runto_main] then {
+    fail "can't run to main"
+    return 0
+}
+
 set supported 0
 gdb_test_multiple "l main:L1" "" {
     -wrap -re "No label \"L1\" defined in function \"main\"\." {