[RFC] elf: fine-grained output LD_DEBUG log when symbol lookup error

[Lv Ying]

Hi, Florian.
Sorry to reply you too late, and thanks for your reply. I took some time to reproduce this problem.
And I resend this email again to cc libc-alpha@sourceware.org.

> Do you have a reproducer that shows what you want to fix?
Yes, here is the reproduce processing:
I want to get shrared object library search paths in openEuler 22.03(https://ftp.belnet.be/mirror/openeuler/openEuler-22.03-LTS/):


# export LD_DEBUG=libs
# whoami
     40503:    find library=libc.so.6 [0]; searching
     40503:     search cache=/etc/ld.so.cache
     40503:      trying file=/usr/lib64/libc.so.6
     40503:   
     40503:   
     40503:    calling init: /lib64/ld-linux-x86-64.so.2
     40503:   
     40503:   
     40503:    calling init: /usr/lib64/libc.so.6
     40503:   
     40503:   
     40503:    initialize program: whoami
     40503:   
     40503:   
     40503:    transferring control: whoami
     40503:   
     40503:    find library=libnss_sss.so.2 [0]; searching
     40503:     search cache=/etc/ld.so.cache
     40503:      trying file=/usr/lib64/libnss_sss.so.2
     40503:   
     40503:   
     40503:    calling init: /usr/lib64/libnss_sss.so.2
     40503:   
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endaliasent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endetherent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endprotoent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endrpcent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endsgent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_endspent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getaliasbyname_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getaliasent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getcanonname_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getetherent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_gethostbyaddr2_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_gethostbyname3_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_gethostbyname4_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_gethostton_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getntohost_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getprotobyname_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getprotobynumber_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getprotoent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getpublickey (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getrpcbyname_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getrpcbynumber_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getrpcent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getsecretkey (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getsgent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getsgnam_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getspent_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_getspnam_r (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_netname2user (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setaliasent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setetherent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setprotoent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setrpcent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setsgent (fatal)
     40503:    /usr/lib64/libnss_sss.so.2: error: symbol lookup error: undefined symbol: _nss_sss_setspent (fatal)
root


As ld.so man page(https://man7.org/linux/man-pages/man8/ld-linux.so.8.html) said, LD_DEBUG=libs, only display library search paths.
"symbol lookup error" debug info message should not mess the debuginfo.
Because there are LD_DEBUG=reloc and LD_DEBUG=all to display relocation processing.

 

> The current code looks like this:
>
>       if ((*ref == NULL || ELFW(ST_BIND) ((*ref)->st_info) != STB_WEAK)
> 	  && !(GLRO(dl_debug_mask) & DL_DEBUG_UNUSED))
> 	{
> 	  /* We could find no value for a strong reference.  */
> 	  const char *reference_name = undef_map ? undef_map->l_name : "";
> 	  const char *versionstr = version ? ", version " : "";
> 	  const char *versionname = (version && version->name
> 				     ? version->name : "");
> 	  struct dl_exception exception;
> 	  /* XXX We cannot translate the message.  */
> 	  _dl_exception_create_format
> 	    (&exception, DSO_FILENAME (reference_name),
> 	     "undefined symbol: %s%s%s",
> 	     undef_name, versionstr, versionname);
> 	  _dl_signal_cexception (0, &exception, N_("symbol lookup error"));
> 	  _dl_exception_free (&exception);
> 	}
>
> I think in general, we want to signal an exception here, and we can't
> mask that except in debug mode because it does not result in the right
> behavior.

As the above said, we do not mask that, we only display the exception in the corresponding
reloc LD_DEBUG mode.

Thanks
Lv Ying