Re: [gentoo-musl] What is crtbeginP.o ?

[Anthony G. Basile]

On 6/10/16 4:45 AM, Lei Zhang wrote:
> Hi,
>
> I found this file on my musl based system built by GRS:
> $ ls /usr/lib/gcc/x86_64-gentoo-linux-musl/4.9.3 | grep crtbegin
> crtbegin.o
> crtbeginP.o
> crtbeginS.o
> crtbeginT.o
>
> while on my host:
> $ ls /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3 | grep crtbegin
> crtbegin.o
> crtbeginS.o
> crtbeginT.o
>
> What is crtbeginP.o on the former system? I found very little info of
> it on the internet.
>
> Also I'm trying to replace GCC's crtbegin/end with those from NetBSD.
> But NetBSD doesn't offer the *S or *T variants, which are used by
> "-static" and "-shared" options. What's special about them? Are they
> just compiled out of the same source with different flags?
>
> Any inputs are appreciated :)
>
> Lei
>

those are the end files and they are used during linking.  you can see
them in use if you create a little test program and build it using

gcc -v -o test test.c

the last line of the verbose output looks something like the following
on a glibc system:

 /usr/libexec/gcc/x86_64-pc-linux-gnu/4.9.3/collect2 -plugin
/usr/libexec/gcc/x86_64-pc-linux-gnu/4.9.3/liblto_plugin.so
-plugin-opt=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.9.3/lto-wrapper
-plugin-opt=-fresolution=/tmp/cchAQ9r3.res
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s --eh-frame-hdr -m elf_x86_64
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -o test
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../lib64/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3
-L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../lib64
-L/lib/../lib64 -L/usr/lib/../lib64
-L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../x86_64-pc-linux-gnu/lib
-L/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../.. /tmp/ccD0QtZd.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../lib64/crtn.o


sifting through the output you see Scrt1.o crti.o crtbeginS.o ...
crtendS.o crtn.o.  the end files deployed will depend on whether you are
doing pie and/or static and possibly other factors.  i don't know that
there is a standard meaning to the P, S or T.  i remember when zorry and
I work on this for the hardened compiler, we chose T for hardened.  you
can see the rules for deploying these files by looking at the gcc specs
using `gcc -dumpspecs`

so i recommend you play with different gcc compiler profiles using
gcc-config on the musl based system you built with GRS, and look at the
collect2 line to see which end files are deployed with each setting.
then i recommend trying to read the gcc spec file for each of the
compiler profiles.  then if you know some assembly, look at the code for
those end files which you can find in the musl source tree.  that'll
give you some idea what's going on there.

its difficult stuff and since you couldn't find much on the internet,
now's the time to get it there!  document what you find in a systematic way.

finally, i don't know clang.  i suspect you'll have two issues going
forward with that GRS system: 1) its hardened so how to do hardening
with clang.  does clang use gcc style specs?  2) how to get clang to
play nice with musl

btw i doubt strongly the netbsd end files would work.


-- 
Anthony G. Basile, Ph.D.
Gentoo Linux Developer [Hardened]
E-Mail    : blueness@g.o
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