linux-kernel - Re: [RFC] tools/nolibc: x86_64: Make it compile with -pie

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Message-ID: <76af26a2-bf31-4397-8e9c-f15a8c9dac18@t-8ch.de>
Date:   Sun, 20 Aug 2023 21:13:51 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Vitaly Chikunov <vt@...linux.org>
Cc:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [RFC] tools/nolibc: x86_64: Make it compile with -pie

Hi Vitaly,

On 2023-08-20 21:19:00+0300, Vitaly Chikunov wrote:
> Use RIP-relative addressing when setting `environ` and `_auxv` in
> startup code.
> 
> Some toolchains have `-pie` enabled by default. On them or when -pie is
> specified manually gcc produces error like this:
> 
>   ld: /tmp/cci0uPcR.o: relocation R_X86_64_32S against symbol `environ' can not be used when making a PIE object; recompile with -fPIE
>   ld: failed to set dynamic section sizes: bad value
> 
> This is because asm() startup code accesses there pointers with absolute
> addressing.
> 
> This may inspire others to fix the problem for other architectures too.
> 
> Signed-off-by: Vitaly Chikunov <vt@...linux.org>
> ---
>  tools/include/nolibc/arch-x86_64.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
> index 6fc4d8392742..a6be44b333ce 100644
> --- a/tools/include/nolibc/arch-x86_64.h
> +++ b/tools/include/nolibc/arch-x86_64.h
> @@ -199,14 +199,14 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) __no_stack_pr
>  		"pop %rdi\n"                /* argc   (first arg, %rdi)                            */
>  		"mov %rsp, %rsi\n"          /* argv[] (second arg, %rsi)                           */
>  		"lea 8(%rsi,%rdi,8),%rdx\n" /* then a NULL then envp (third arg, %rdx)             */
> -		"mov %rdx, environ\n"       /* save environ                                        */
> +		"mov %rdx, environ(%rip)\n" /* save environ                                        */
>  		"xor %ebp, %ebp\n"          /* zero the stack frame                                */
>  		"mov %rdx, %rax\n"          /* search for auxv (follows NULL after last env)       */
>  		"0:\n"
>  		"add $8, %rax\n"            /* search for auxv using rax, it follows the           */
>  		"cmp -8(%rax), %rbp\n"      /* ... NULL after last env (rbp is zero here)          */
>  		"jnz 0b\n"
> -		"mov %rax, _auxv\n"         /* save it into _auxv                                  */
> +		"mov %rax, _auxv(%rip)\n"   /* save it into _auxv                                  */
>  		"and $-16, %rsp\n"          /* x86 ABI : esp must be 16-byte aligned before call   */
>  		"call main\n"               /* main() returns the status code, we'll exit with it. */
>  		"mov %eax, %edi\n"          /* retrieve exit code (32 bit)                         */
> -- 
> 2.33.8
> 

nolibc recently switched to startup code that is mostly implemented in C
instead of assembly.
See the current linux-next tree.

That should make this change unnecessary I guess.


Thomas