Can I use printf in dl-load.c?

Can I use printf in dl-load.c?

[Yihan Dang]

Hello,


I'm trying to use printf to get maplength in dl-load.c:_dl_map_object_from_fd, and this is how the code looks like:


    /* add stdio at the very beginning */
&nbsp; &nbsp; #include <stdio.h&gt;
&nbsp; &nbsp; ......
&nbsp; &nbsp; /* Length of the sections to be loaded.&nbsp; */
&nbsp; &nbsp; maplength = loadcmds[nloadcmds - 1].allocend - loadcmds[0].mapstart;


&nbsp; &nbsp; /* code added */
&nbsp; &nbsp; printf("The maplength of shared library %s is %ld\n", name, maplength);


&nbsp; &nbsp; /* Now process the load commands and map segments into memory.
&nbsp; &nbsp; &nbsp; &nbsp;This is responsible for filling in:
&nbsp; &nbsp; &nbsp; &nbsp;l_map_start, l_map_end, l_addr, l_contiguous, l_text_end, l_phdr
&nbsp; &nbsp; &nbsp;*/

&nbsp; &nbsp; errstring = _dl_map_segments (l, fd, header, type, loadcmds, nloadcmds,
				&nbsp; maplength, has_holes, loader);



And this is the error message I got:
gcc&nbsp; &nbsp;-nostdlib -nostartfiles -r -o /home/dyh/expr/glibc/build/elf/librtld.map.o -Wl,--defsym='__stack_chk_fail=0' -Wl,--defsym='__stack_chk_fail_local=0' \
	'-Wl,-(' /home/dyh/expr/glibc/build/elf/dl-allobjs.os /home/dyh/expr/glibc/build/libc_pic.a -lgcc '-Wl,-)' -Wl,-Map,/home/dyh/expr/glibc/build/elf/librtld.mapT
/home/dyh/expr/glibc/build/libc_pic.a(dl-error.os): In function `__GI__dl_signal_exception':
/home/dyh/expr/glibc/elf/dl-error-skeleton.c:91: multiple definition of `_dl_signal_exception'
/home/dyh/expr/glibc/build/elf/dl-allobjs.os:/home/dyh/expr/glibc/elf/dl-error-skeleton.c:91: first defined here
/home/dyh/expr/glibc/build/libc_pic.a(dl-error.os): In function `__GI__dl_signal_error':
/home/dyh/expr/glibc/elf/dl-error-skeleton.c:109: multiple definition of `_dl_signal_error'
/home/dyh/expr/glibc/build/elf/dl-allobjs.os:/home/dyh/expr/glibc/elf/dl-error-skeleton.c:109: first defined here
/home/dyh/expr/glibc/build/libc_pic.a(dl-error.os): In function `__GI__dl_catch_exception':
/home/dyh/expr/glibc/elf/dl-error-skeleton.c:175: multiple definition of `_dl_catch_exception'
/home/dyh/expr/glibc/build/elf/dl-allobjs.os:/home/dyh/expr/glibc/elf/dl-error-skeleton.c:175: first defined here
/home/dyh/expr/glibc/build/libc_pic.a(dl-error.os): In function `__GI__dl_catch_error':
/home/dyh/expr/glibc/elf/dl-error-skeleton.c:213: multiple definition of `_dl_catch_error'
/home/dyh/expr/glibc/build/elf/dl-allobjs.os:/home/dyh/expr/glibc/elf/dl-error-skeleton.c:213: first defined here
/home/dyh/expr/glibc/build/libc_pic.a(init-first.os):(.data+0x0): multiple definition of `__libc_multiple_libcs'
/home/dyh/expr/glibc/build/elf/dl-allobjs.os:(.bss+0xe0): first defined here
collect2: error: ld returned 1 exit status
Makefile:437: recipe for target '/home/dyh/expr/glibc/build/elf/librtld.map' failed
make[2]: *** [/home/dyh/expr/glibc/build/elf/librtld.map] Error 1
make[2]: Leaving directory '/home/dyh/expr/glibc/elf'
Makefile:215: recipe for target 'elf/subdir_lib' failed
make[1]: *** [elf/subdir_lib] Error 2
make[1]: Leaving directory '/home/dyh/expr/glibc'
Makefile:9: recipe for target 'all' failed
make: *** [all] Error 2



I had a hard time understanding the messages. Why strcmp in <string.h&gt; can be used in this file, but printf in <stdio.h&gt; can't?


I know that I can use readelf to calculate the virtual space a shared library takes up, but I'm really confused about this error.




Thanks in advance,
Yihan

Re: Can I use printf in dl-load.c?

[Florian Weimer]

* Yihan Dang via Libc-help:

> I'm trying to use printf to get maplength in
> dl-load.c:_dl_map_object_from_fd, and this is how the code looks like:

You need to use _dl_printf.  Please note that it does not support all
formatting directives.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill

Re: Can I use printf in dl-load.c?

[Yihan Dang]

Hi Florian, 
Thank you very much for your reply!


I've checked ldsodefs.h and found _dl_printf there. I guess using functions in stdio.h may cause some problems so there is a seperate printf here.


Also, I did not find any function related to scanf in ldsodefs.h, does that mean there is no way to achieve the similar functionality?


Thank you in advance,
Yihan

---Original---
From: "Florian Weimer"<fweimer@redhat.com&gt;
Date: Thu, Jan 14, 2021 16:19 PM
To: "Yihan Dang via Libc-help"<libc-help@sourceware.org&gt;;
Cc: "Yihan Dang"<qcloud1223@qq.com&gt;;
Subject: Re: Can I use printf in dl-load.c?


* Yihan Dang via Libc-help:

&gt; I'm trying to use printf to get maplength in
&gt; dl-load.c:_dl_map_object_from_fd, and this is how the code looks like:

You need to use _dl_printf.&nbsp; Please note that it does not support all
formatting directives.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill

Re: Can I use printf in dl-load.c?

[Florian Weimer]

* Yihan Dang:

> Also, I did not find any function related to scanf in ldsodefs.h, does
> that mean there is no way to achieve the similar functionality?

Yes, there is no scanf in ld.so.  Perhaps you can use _dl_strtoul
instead.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill