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

[Lei Zhang]

2016-06-10 17:15 GMT+08:00 Anthony G. Basile <blueness@g.o>:
> 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`

First, thanks for your input :)

I'm aware of crtbegin{S,T}.o, but it's the first time I see
crtbeginP.o. Even more strangely, on my host:
$ ls /usr/lib/gcc/x86_64-pc-linux-musl/4.9.3 | grep crtbegin
crtbegin.o
crtbeginS.o
crtbeginT.o

You can see crtbeginP.o is not used by the musl cross-compiler, but
only on the musl based system.

>
> 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

1) Unfortunately clang doesn't support specs file. I know little about
hardened Linux. Does it require special support from the compiler? And
FYI, I'm not going to compile Linux kernel with clang, which requires
some heavy patching.

2) That's exactly what I'm working on lately, and it turns out to be
easy enough :)

The hard part, however, is dealing with clang's dependence on GCC,
like those crtbegin/end files.

>
> btw i doubt strongly the netbsd end files would work.

They're recommended in LLVM's mailing list, and unfortunately I found
no better alternatives at the moment...  At least last time I gave
them a shot, they appeared to work pretty well.


Lei