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Message-ID: <CACT4Y+ZhX7Py-ZJZEfZUdXDjnJYW-ODUSTNRXEpA5u+rJ+M4nA@mail.gmail.com>
Date: Sat, 1 Mar 2025 19:02:15 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Rich Felker <dalias@...c.org>
Cc: Sertonix <sertonix@...teo.net>, musl@...ts.openwall.com, nsz@...t70.net
Subject: Re: Support for -static-pie relocations
On Sat, 1 Mar 2025 at 18:40, Rich Felker <dalias@...c.org> wrote:
>
> On Sat, Mar 01, 2025 at 06:03:24PM +0100, Dmitry Vyukov wrote:
> > RIght, sorry.
> >
> > I checked out the latest HEAD c47ad25ea3b484e10326f933e927c0bc8cded3da.
> > Standard build: ./configure --enable-debug && make
> > arch x86_64
> >
> > Reproducible with both (standard Debian builds):
> > $ clang --version
> > Debian clang version 16.0.6 (27+build3)
> > $ ld -v
> > GNU ld (GNU Binutils for Debian) 2.43.1
> > $ clang /tmp/test.c lib/libc.a -O2 -g -static-pie && ./a.out
> > Segmentation fault (core dumped)
> >
> > and
> > $ gcc -v
> > gcc version 14.2.0 (Debian 14.2.0-3+build4)
> > $ gcc /tmp/test.c lib/libc.a -O2 -g -static-pie && ./a.out
> > Segmentation fault (core dumped)
>
> If you're going to link as PIE, all source files need to be compiled
> as PIE-compatible. You're missing -fPIE unless that's already default
> on your toolchains.
>
> > Relocations are mostly rip-relative except for global vars that
> > contain pointers:
> >
> > $ readelf -r ./a.out
> > Relocation section '.rela.dyn' at offset 0x350 contains 11 entries:
> > Offset Info Type Sym. Value Sym. Name + Addend
> > 000000003e48 000000000008 R_X86_64_RELATIVE 11a0
> > 000000003e50 000000000008 R_X86_64_RELATIVE 1160
> > 000000003e58 000000000008 R_X86_64_RELATIVE 4020
> > 000000003e60 000000000008 R_X86_64_RELATIVE 4380
> > 000000003fd8 000000000008 R_X86_64_RELATIVE 3e68
> > 000000004008 000000000008 R_X86_64_RELATIVE 4008
> > 000000004010 000000000008 R_X86_64_RELATIVE 4020
> > 000000004038 000000000008 R_X86_64_RELATIVE 19ca
> > 000000004068 000000000008 R_X86_64_RELATIVE 19f5
> > 000000004070 000000000008 R_X86_64_RELATIVE 19ed
> > 000000004078 000000000008 R_X86_64_RELATIVE 41d0
>
> Those are all fine.
>
> > > Relocations are processed in the crt entry point before
> > > __libc_start_main is reached. The relevant code is ldso/dlstart.c.
> >
> > Am I linking musl somehow incorrectly (need to do something with crt)?
> >
> > My entry function calls __libc_start_main:
> >
> > Dump of assembler code for function _start:
> > 0x00007ffff7ffb0c0 <+0>: xor %ebp,%ebp
> > 0x00007ffff7ffb0c2 <+2>: mov %rdx,%r9
> > 0x00007ffff7ffb0c5 <+5>: pop %rsi
> > 0x00007ffff7ffb0c6 <+6>: mov %rsp,%rdx
> > 0x00007ffff7ffb0c9 <+9>: and $0xfffffffffffffff0,%rsp
> > 0x00007ffff7ffb0cd <+13>: push %rax
> > 0x00007ffff7ffb0ce <+14>: push %rsp
> > 0x00007ffff7ffb0cf <+15>: xor %r8d,%r8d
> > 0x00007ffff7ffb0d2 <+18>: xor %ecx,%ecx
> > 0x00007ffff7ffb0d4 <+20>: lea -0x4b(%rip),%rdi #
> > 0x7ffff7ffb090 <main>
> > 0x00007ffff7ffb0db <+27>: addr32 call 0x7ffff7ffb3fa <__libc_start_main>
> > => 0x00007ffff7ffb0e1 <+33>: hlt
> >
> >
> > I don't see any static libs in musl build that include _start symbol...
>
> _start comes from one of the *crt1.o files in the library directory
> (/usr/lib on a normal system wide install; your cross lib dir for
> cross compiles). In the case of static pie, the compiler driver (clang
> or gcc command) should be pulling rcrt1.o.
>
> The above asm dump does not look like any variant of the musl crt
> start files. Are you perhaps doing this on a glibc-based host and just
> trying to link musl as a static library? That's not expected to work.
> You need to either use a musl-targeting cross toolchain or the
> musl-gcc wrapper (but the wrapper is only for minimal/evaluation type
> uses and might not support static pie.
>
> Rich
Guilty as charged. This explains everything. I just extracted this
repro from an environment where I assumed people already did the right
thing.
For the record, doing this makes it work:
$ clang /tmp/test.c lib/rcrt1.o lib/libc.a -O2 -g -static-pie -nostartfiles
$ ./a.out
Hello
I understand this is hacky-ish.
Thanks, Rich
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