Run Time linker and relocation of multiple local labels
Chris Johns
chrisj at rtems.org
Thu Apr 25 22:02:58 UTC 2019
On 26/4/19 2:04 am, dufault at hda.com wrote:
> In a given object file for "NOTYPE LOCAL” entries in a data segment with zero
> size that don’t start with ‘$’ could you append text based on the Num field for
> the rtems_rtl_obj symbol lookup in cpukit/libdl/rtl-sym.c? Right now it’s a
> linear lookup on e.g. “.LC2” and stops at the first hit. I haven’t checked that
> consumers of the lookup can generate the correct name, that is, always have the
> “Num” field value.
I would like to understand at the ELF level what is happening before considering
solutions. As I indicated in my other email if the symbols are local and in
different sections it could be something in the handling of the relocation
records for a section that needs to consider the symbol section used for local
symbols. If the symbols are in the same section and match then I would question
why `ld` is doing this.
>
> You’d wind up with mangled lookups such as:
>
> 22: 0000002c 0 NOTYPE LOCAL DEFAULT 3 .LC2.Num22
> 44: 0000013c 0 NOTYPE LOCAL DEFAULT 3 .LC2.Num44
>
> This is from one of my relocatable files grepping for NOTYPE LOCAL. I’ve
> verified large files also work.
This means the local symbol table now has different entries. How do you know
which one a relocation record is pointing too?
Chris
> Num: Value Size Type Bind Vis Ndx Name
> 0: 00000000 0 NOTYPE LOCAL DEFAULT UND
> 14: 00000000 0 NOTYPE LOCAL DEFAULT 3 $d
> 15: 00000000 0 NOTYPE LOCAL DEFAULT 4 $d
> 17: 00000000 0 NOTYPE LOCAL DEFAULT 6 $d
> 18: 00000000 2048 NOTYPE LOCAL DEFAULT 6 env_entries
> 20: 00000800 4 NOTYPE LOCAL DEFAULT 6 env_initialized
> 21: 00000008 0 NOTYPE LOCAL DEFAULT 3 .LC1
> 22: 0000002c 0 NOTYPE LOCAL DEFAULT 3 .LC2
> 23: 00000030 0 NOTYPE LOCAL DEFAULT 3 .LC3
> 25: 00000000 0 NOTYPE LOCAL DEFAULT 1 $t
> 28: 00000050 0 NOTYPE LOCAL DEFAULT 3 .LC4
> 29: 00000058 0 NOTYPE LOCAL DEFAULT 3 .LC5
> 30: 00000080 0 NOTYPE LOCAL DEFAULT 3 .LC6
> 32: 00000094 0 NOTYPE LOCAL DEFAULT 3 .LC7
> 33: 000000c4 0 NOTYPE LOCAL DEFAULT 3 .LC8
> 34: 000000cc 0 NOTYPE LOCAL DEFAULT 3 .LC9
> 35: 000000f4 0 NOTYPE LOCAL DEFAULT 3 .LC10
> 37: 00000130 0 NOTYPE LOCAL DEFAULT 3 $d
> 38: 0000000c 0 NOTYPE LOCAL DEFAULT 4 $d
> 40: 00000804 0 NOTYPE LOCAL DEFAULT 6 $d
> 41: 00000d14 520 NOTYPE LOCAL DEFAULT 6 putenv_trace_ptrs
> 42: 00000690 0 NOTYPE LOCAL DEFAULT 1 $t
> 43: 00000138 0 NOTYPE LOCAL DEFAULT 3 .LC1
> 44: 0000013c 0 NOTYPE LOCAL DEFAULT 3 .LC2
> 45: 0000014c 0 NOTYPE LOCAL DEFAULT 3 .LC3
> 47: 00000164 0 NOTYPE LOCAL DEFAULT 3 .LC4
>
>
>> On Apr 25, 2019, at 07:56 , Peter Dufault <dufault at hda.com
>> <mailto:dufault at hda.com>> wrote:
>>
>> I’m porting a large vxWorks application and I’m trying to download “ld -r”
>> files as you can on vxWorks using the run-time loader. At least on ARM I
>> can’t get it to work, it uses the first local address it sees for all occurrences.
>>
>> Example: Label .LC2 is used in two places, once for the “r” in an fopen() and
>> later for a format string. In both cases the address used is 0x419f14, the
>> address of “r”.
>>
>> First is the readelf output, then the disassembly, then debugging output from
>> the RTL.
>>
>> FIRST OCCURRENCE OF .LC2 used as “r” in fopen()
>> readelf:
>> 00000020 0000152f R_ARM_THM_MOVW_ABS_NC 00000008 .LC1
>> 00000024 00001530 R_ARM_THM_MOVT_ABS 00000008 .LC1
>> 0000002a 0000162f R_ARM_THM_MOVW_ABS_NC 0000002c .LC2
>> 0000002e 00001630 R_ARM_THM_MOVT_ABS 0000002c .LC2
>> 00000034 0000450a R_ARM_THM_CALL 00000000 fopen
>>
>> Disassemble:
>> if ((fd = fopen(string, "r")) == (0)) {
>> 2a: f240 0100 movw r1, #0
>> 2e: f2c0 0100 movt r1, #0
>> 32: 6878 ldr r0, [r7, #4]
>> 34: f7ff fffe bl 0 <fopen>
>>
>> RTL:
>> rtl: THM_MOVT_ABS/THM_MOVW_ABS_NC 0x341f2c0 @ 0x4196dc in bad
>> rtl: rel: sym:.LC2(22)=00419f14 type:47 off:0000002a
>> rtl: THM_MOVT_ABS/THM_MOVW_ABS_NC 0x7114f649 @ 0x4196e2 in bad
>> rtl: rel: sym:.LC2(22)=00419f14 type:48 off:0000002e
>> rtl: THM_MOVT_ABS/THM_MOVW_ABS_NC 0x141f2c0 @ 0x4196e6 in bad
>> rtl: rel: sym:fopen(69)=000d33c1 type:10 off:00000034
>>
>> SECOND OCCURRENCE of .LC2 used for the printf format string:
>>
>> readelf:
>> 000007a8 00002c2f R_ARM_THM_MOVW_ABS_NC 0000013c .LC2
>> 000007ac 00002c30 R_ARM_THM_MOVT_ABS 0000013c .LC2
>> 000007b0 0000330a R_ARM_THM_CALL 00000000 printf
>>
>> Disassemble:
>> 7a6: 68f9 ldr r1, [r7, #12]
>> 7a8: f240 0000 movw r0, #0
>> 7ac: f2c0 0000 movt r0, #0
>> 7b0: f7ff fffe bl 0 <printf>
>>
>> RTL:
>> rtl: rel: sym:.LC2(44)=00419f14 type:47 off:000007a8
>> rtl: THM_MOVT_ABS/THM_MOVW_ABS_NC 0x7014f649 @ 0x419e60 in
>> /opt/flatland/arch/arm-stubs-vxstubs/bin/sys-bad
>> rtl: rel: sym:.LC2(44)=00419f14 type:48 off:000007ac
>> rtl: THM_MOVT_ABS/THM_MOVW_ABS_NC 0x41f2c0 @ 0x419e64 in
>> /opt/flatland/arch/arm-stubs-vxstubs/bin/sys-bad
>> rtl: rel: sym:printf(51)=000daac9 type:10 off:000007b0
>>
>> Peter
>> -----------------
>> Peter Dufault
>> HD Associates, Inc. Software and System Engineering
>>
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>
> Peter
> -----------------
> Peter Dufault
> HD Associates, Inc. Software and System Engineering
>
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>
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