ELF decoder for symbols

[Zhengyu Gu]

Hi Harish,

Yes, apparently we do not handle prelinked case. Could you please file a 
bug, so we can work on it.

Thanks,

-Zhengyu

On 8/4/2014 8:13 AM, Harish Babu wrote:
> Hi,
>
>     I have a question regarding the code for ELF files parsing(Linux).
>
>     Looking at the code below it appears the relative offset address is sent
> like the below code:
>
> os::dll_address_to_function_name() {
> ....
> Decoder::decode((address)(addr - (address)dlinfo.dli_fbase),
>                            buf, buflen, offset, dlinfo.dli_fname))
> ....
> }
>
> This works well for most of the libraries which are not prelinked at
> particular address where the symbol tables are relative offsets.
>
> But when the libraries are prelinked at an address this does not work well.
> Like libc,
> readelf -l /lib64/libc.so.6
> Program Headers:
>    Type           Offset             VirtAddr           PhysAddr
>                   FileSiz            MemSiz              Flags  Align
>    PHDR           0x0000000000000040 0x00000038e7400040 0x00000038e7400040
>                   0x0000000000000230 0x0000000000000230  R E    8
>    INTERP         0x000000000013ff60 0x00000038e753ff60 0x00000038e753ff60
>                   0x000000000000001c 0x000000000000001c  R      10
>        [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
>    LOAD           0x0000000000000000 0x00000038e7400000 0x00000038e7400000
>                   0x000000000016f150 0x000000000016f150  R E    200000
>    LOAD           0x000000000016f720 0x00000038e776f720 0x00000038e776f720
>                   0x0000000000004678 0x0000000000009188  RW     200000
>
>
> Where libc may(or may not) be loaded at a base address 0x38e7400000, and
> the addresses are the absolute address rather than relative offsets.
>
>     131: 00000038e74274b0   192 FUNC    LOCAL  DEFAULT   12 open_translit
>     132: 00000038e7773f78     4 OBJECT  LOCAL  DEFAULT   34 lock
>     133: 00000038e7427c82    31 FUNC    LOCAL  DEFAULT   12 _L_lock_107
>     134: 00000038e7427810    11 FUNC    LOCAL  DEFAULT   12 trans_compare
>     135: 00000038e7773f70     8 OBJECT  LOCAL  DEFAULT   34 search_tree
>     136: 00000038e7427ca1    31 FUNC    LOCAL  DEFAULT   12 _L_unlock_135
>
>
>
>
> For pthread(which is not prelinked to an address) which the current code
> deals with correctly there is only relative addresses in the ELF file:
>
>    121: 000000000000da48    19 FUNC    LOCAL  DEFAULT   14 sem_wait_cleanup
>     122: 000000000000dc15    19 FUNC    LOCAL  DEFAULT   14
> sem_timedwait_cleanup
>     123: 000000000000dc28    31 FUNC    LOCAL  DEFAULT   14
> sem_timedwait_cleanup2
>     124: 000000000000df50    33 FUNC    LOCAL  DEFAULT   14 unwind_cleanup
>     125: 000000000000df80   287 FUNC    LOCAL  DEFAULT   14 unwind_stop
>     126: 000000000000e800   237 FUNC    LOCAL  DEFAULT   14 do_fcntl
>
>
> So like I mentioned earlier, the code os::dll_address_to_function_name
> subtracts the base address where the library was loaded from the current
> pc. This results in relative offset which may not work well for the
> libraries which are prelinked to an address.
>
> Please let me know if I got it completely wrong.
>
> Thanks,
> Harish