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The GNU C library
=================
This will be a port of the GNU C library to the Hurd system running on
L4.
Configuring and Building
------------------------
This directory is only built if you specify "--enable-libc" at
configure time.
Before you do this, you should first compile and install the software
with "--disable-libc" (which is the default), to make sure that the
Hurd and libl4 header files are installed and available to the
compiler.
Because the GNU C library is huge, it is not shipped with this source
package. You have to retrieve it manually. This can be done
semi-automatically by entering in the BUILD directory (after
configuring this source tree):
$ mkdir hurd-l4-build
$ cd hurd-l4-build
$ ../hurd-l4/configure --prefix=/l4 --with-libc \
--enable-maintainer-mode --build=i686-linux --host=i686-gnu
$ make -C libc libc-cvs
This will check out the right glibc version via CVS. It is
recommended that you save a copy of this version and copy it to the
libc directory instead downloading it the next time around, to save
bandwidth and reduced the stress on the CVS server.
$ cp -a libc ~/libc-for-hurd-l4
(next time around:)
$ cp -a ~/libc-for-hurd-l4 libc/libc
$ ../hurd-l4/configure --prefix=/l4 --with-libc \
--enable-maintainer-mode --build=i686-linux --host=i686-gnu
$ make
The Makefile target libc-cvs will always download a tested, working
version of the C library. But you can copy any version to the libc
directory and try your luck.
The downloaded or installed version of the GNU C library source _will_
be modified when you build the package! So, please, make a copy
before running "make". The Makefile rules are a bit simple and
reverting the modifications is not fully supported. So, if the
patches or add-ons are modified, you will probably need to rebuild
from scratch. The clean rules are not tested.
Hacking
-------
If you want to hack this, have fun! Without fun, it is a daunting task.
Startup
-------
When a new task is executed, the startup code for ELF
(wortel/startup.c) will set up the following initial configuration
before passing control to the entry point of the ELF executable:
1. The ELF sections marked as LOAD are mapped into the virtual memory
of the program.
FIXME: This is wrong. The physmem server may drop those mappings
at any time. Instead:
The pager should be set to the physical memory server, and
the physical memory server should act as a fallback pager that maps
in the ELF sections on demand. For this, the client must be
associated with a special memory object in physmem. The memory object
for this must then be set up in advance. This memory object will
have something like the following layout:
ELF memory object:
START END ALLOC MAX ACCESS
0x00100000 - 0x00108000 anon, init'd with startup code rw-
0x08000000 - 0x08015fff shared (from the exec'd file), r-x
0x08016000 - 0x08016fff c-o-w (from the exec'd file), rw-
0x08017000 - 0x08017fff anon (but see below), rw-
bytes 0x00017000 - 0x00017C30 would be initialized
with content from the exec'd file
This example corresponds with the following ELF header:
LOAD off 0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**12
filesz 0x00015474 memsz 0x00015474 flags r-x
LOAD off 0x00015480 vaddr 0x00016480 paddr 0x00016480 align 2**12
filesz 0x000017b0 memsz 0x000019b8 flags rw-
2. The entry point will be invoked as if it were a function which gets
a pointer to the Hurd startup data as its first argument.
You can find the Hurd startup data layout in wortel/startup.h.
FIXME: The stack layout described in Process Initialization, p. 3-26
in System V Application Binary Interface - Intel386 Architecture
Processor Supplement, Fourth Edition
http://www.caldera.com/developers/devspecs/abi386-4.pdf
is like this:
High: unspecified
info block (arg strings, env strings, aux info, etc)
unspecified
null aux vector
aux vectors (2-word entries)
0 word
env pointers
0 word
arg pointers
4(%esp) 0 word
0(%esp) arg count
Low: undefined
Also specified:
%ebp: unspecified, should be set to 0 by user.
%esp: see above
%edx: used for shared linking
%cs,%ds,%es,%ss: set up by kernel
Aux vector: We can use it, at least to specify a pointer to the hurd
startup data in the info block. (Although it may be misleading to
support an aux vector at all).
Initialization
--------------
(Also see http://ldp.paradoxical.co.uk/LDP/LGNET/issue84/hawk.html)
All the following only applies to statically linked programs.
Dynamically linked programs work differently.
We produce an empty executable like this:
$ cd libc-build
$ echo 'main(){}' > main.c
$ gcc -g -static -nodefaultlibs -nostartfiles -o main csu/crt0.o csu/crti.o `gcc -print-file-name=crtbeginT.o` main.c -Wl,--start-group -lgcc -lgcc_eh ./libc.a -Wl,--end-group `gcc -print-file-name=crtend.o` csu/crtn.o
./libc.a(exit.o)(.text+0x9b): In function `exit':
/space/home/marcus/gnu/hurd/work/hurd-l4-build/libc/libc/stdlib/exit.c:82: warning: warning: _exit is not implemented and will always fail
./libc.a(libc-start.o)(.text+0x101): In function `__libc_start_main':
../sysdeps/generic/libc-start.c:249: warning: warning: __exit_thread is not implemented and will always fail
Now we can analyze the resulting empty main program with objdump and gdb.
To make gdb find the source, use:
(gdb) directory ../libc/elf
(gdb) set print symbol-filename on
I.1. Entry point.
Generic Code:
The entry point of a program is usually defined by the location of the
symbol _start:
$ objdump --syms main | grep ' _start'
08048120 g F .text 00000000 _start
(gdb) print _start
$2 = {
<text variable, no debug info>} 0x8048120 <_start at ../sysdeps/i386/elf/start.S:47>
(gdb) l _start
42 %edx Contains a function pointer [...]
[...]
_start is defined in crt1.o for ELF programs.
In Hurd/L4:
_start is renamed to _start1, and the actual entry point _start is is
in libc/hurd-l4/sysdeps/l4/hurd/i386/static-start.S (for static
programs), which is linked into crt0.c (see
libc/hurd-l4/sysdeps/l4/hurd/Makeconfig and Makefile).
_start first calls _hurd_pre_start, which initializes the components
glibc itself depends upon (libl4, libhurd-mm, ...).
_start is defined in crt0.o for statically linked programs, and in
crt1.o for dynamically linked programs.
From here, observe the program flow by reading the source code,
looking at the disassembled code (objdump -x) and find symbols and
source files with gdb.
I.2. _start
sysdeps/i386/elf/start.S::_start()
(gdb) print _start
$4 = {
<text variable, no debug info>} 0x8048120 <_start at ../sysdeps/i386/elf/start.S:47>
FIXME: Expects argc, argv (and envp) on top of stack.
See: Process Initialization, p. 3-26 in System V Application Binary
Interface - Intel386 Architecture Processor Supplement, Fourth Edition
http://www.caldera.com/developers/devspecs/abi386-4.pdf
I.3. __libc_start_main
sysdeps/generic/libc-start.c::__libc_start_main()
(gdb) print __libc_start_main
$3 = {int (int (*)(int, char **, char **), int, char **, void (*)(void),
void (*)(void), void (*)(void),
void *)} 0x8048250 <__libc_start_main at ../sysdeps/generic/libc-start.c:100>
II TLS
TLS, TSD, DTV. See Drepper's paper on TLS support. Initialization is
done in sysdeps/generic/libc-tls.c, which requires that _dl_phdr and
_dl_phnum are initialized, which happens in init-first.c on the Hurd
(in _dl_aux_init from the aux vector on Linux). This is because the
PT_TLS program header must be found to determine the tdata and tbss
initializer location and sizes.
Support is in nptl/sysdeps/l4/hurd/i386/tls.h. We do not allow
%gs:OFFSET where OFFSET is not 0.
- Marcus
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