How to build libc.a with -fPIC for all archive members?

How to build libc.a with -fPIC for all archive members?

[Oliver Schneider]

Hi,

how can I build the static library from the musl source, but compiling
with -fPIC?

I tried the obvious:

CFLAGS=-fPIC ./configure --enable-gcc-wrapper --disable-shared

and then built. But I am getting the following linker error still
(albeit for a different member function and object file than before):

/usr/local/musl/lib/libc.a(memmove.o): relocation R_X86_64_PC32 against
symbol `memcpy' can not be used when making a shared object; recompile
with -fPIC
final link failed: Bad value

The reason I want to do this, is to have a statically linked .so. I.e.
an .so that has no external dependency other than the system calls. Or
is this known to conflict with the ever-present glibc in some way?

Thanks for any insight into how to resolve this,

// Oliver

Re: How to build libc.a with -fPIC for all archive members?

[Oliver Schneider]

So I piped the output of the compilation into a text file and then used
"grep -v fPIC" to see only lines that don't contain -fPIC (well fPIC),
but that yields a list where the "ar" command and a few auxiliary
commands are the only ones left.

So it appears all calls to the compiler (I use GCC) pass the -fPIC from
the ./configure invocation.

Still looking, but if someone has any input, that would be appreciated.

// Oliver

Re: How to build libc.a with -fPIC for all archive members?

[Oliver Schneider]

Okay, so the probable reason is that the assembly code isn't relocatable
or not marked as such. Looking into this further.

// Oliver

Re: How to build libc.a with -fPIC for all archive members?

[Szabolcs Nagy]

* Oliver Schneider <musl-mailinglist@f-prot.com> [2014-02-10 12:35:14 +0000]:
> how can I build the static library from the musl source, but compiling
> with -fPIC?
> 
> I tried the obvious:
> 
> CFLAGS=-fPIC ./configure --enable-gcc-wrapper --disable-shared

this should work

> and then built. But I am getting the following linker error still
> (albeit for a different member function and object file than before):
> 
> /usr/local/musl/lib/libc.a(memmove.o): relocation R_X86_64_PC32 against
> symbol `memcpy' can not be used when making a shared object; recompile
> with -fPIC
> final link failed: Bad value

i think you miss -Bsymbolic-functions linker flags when you link the .so

this avoids relocations of all .so internal function calls

(which should be always on when building any library unless you explicitly
document that a sepecific function may be hijacked (interposed) through
ldpreloading otherwise the mismatch between the library internal and
library external abi can break semantics)

> The reason I want to do this, is to have a statically linked .so. I.e.
> an .so that has no external dependency other than the system calls. Or
> is this known to conflict with the ever-present glibc in some way?

how would you use that? somehow it will need to be loaded but
the loader must not interfere with the static linked libc

or would you use the .so as the loader?

(klibc does something like that: they put all libc code in a .so, they don't
use -fPIC because they load the .so at fixed address and all binaries that
are dynlinked to it are compiled against a fixed .so that has its sha1 checksum
in its name, that way you can have something that looks like dynamic linking
but actually it's more like a set of split static binaries that happens to
share one half of their code that contains all of libc)

Re: How to build libc.a with -fPIC for all archive members?

[Rich Felker]

On Mon, Feb 10, 2014 at 12:35:14PM +0000, Oliver Schneider wrote:
> Hi,
> 
> how can I build the static library from the musl source, but compiling
> with -fPIC?
> 
> I tried the obvious:
> 
> CFLAGS=-fPIC ./configure --enable-gcc-wrapper --disable-shared
> 
> and then built. But I am getting the following linker error still
> (albeit for a different member function and object file than before):
> 
> /usr/local/musl/lib/libc.a(memmove.o): relocation R_X86_64_PC32 against
> symbol `memcpy' can not be used when making a shared object; recompile
> with -fPIC
> final link failed: Bad value
> 
> The reason I want to do this, is to have a statically linked .so. I.e.
> an .so that has no external dependency other than the system calls. Or
> is this known to conflict with the ever-present glibc in some way?

This is not intended to be possible, and there are many things that
would break. Among them:

- __libc_start_main would never have been called so some necessary
  initialization might not have taken place. At present that's minimal
  and the goal is to keep it minimal so this might have no practical
  consequence, but it's still something that can't be officially
  supported without locking in aspects of the implementation.

- malloc does not support sharing the brk with anything else, in
  particular, whatever malloc implementation is using it in the rest
  of the program outside your DSO. It really _can't_ because there's
  no way it could synchronize use of the brk (since it wouldn't be
  sharing locks with the outside program.

- Anything that touches threads will either expect the thread pointer
  to be setup to point to musl's thread structure, or will attempt to
  set it up. This will conflict with whatever thread structure the
  outside program/glibc/different-version-of-musl is using.

In general, it's only safe to have a single version/copy of any given
library linked into a program. There are some exceptions like libgcc
that use hidden visibility to make it work, but they're the exception,
not the norm. This issue is not unique to libc, but libc probably has
the most ways it's affected.

With that said, building libc.a as PIC _is_ intended to be supported,
not for what you're doing, but for the sake of making static-linked
PIE executables. This is not doable with the official gcc toolchain
but I have a system to make it work without invasive changes, so I
would like to find and fix whatever issue is PIC-incompatible...

Rich