Re: [PATCH] ldso: fix GDB dynamic linker info on MIPS

[Rich Felker <dalias@libc.org>]

On Sat, Jan 30, 2016 at 07:20:19PM +0100, Felix Fietkau wrote:
> GDB is looking for a pointer to the ldso debug info in the data of the
> ..rld_map section.
> 
> Signed-off-by: Felix Fietkau <nbd@openwrt.org>
> ---
>  arch/mips/reloc.h  | 1 +
>  src/ldso/dynlink.c | 9 ++++++++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/reloc.h b/arch/mips/reloc.h
> index 9b40e3d..e9cb209 100644
> --- a/arch/mips/reloc.h
> +++ b/arch/mips/reloc.h
> @@ -25,6 +25,7 @@
>  
>  #define NEED_MIPS_GOT_RELOCS 1
>  #define DYNAMIC_IS_RO 1
> +#define DT_DEBUG_INDIRECT DT_MIPS_RLD_MAP
>  #define ARCH_SYM_REJECT_UND(s) (!((s)->st_other & STO_MIPS_PLT))
>  
>  #define CRTJMP(pc,sp) __asm__ __volatile__( \
> diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
> index 93e7d67..dd85eea 100644
> --- a/src/ldso/dynlink.c
> +++ b/src/ldso/dynlink.c
> @@ -1584,11 +1584,18 @@ _Noreturn void __dls3(size_t *sp)
>  	load_deps(&app);
>  	make_global(&app);
>  
> +	for (i=0; app.dynv[i]; i+=2) {
>  #ifndef DYNAMIC_IS_RO
> -	for (i=0; app.dynv[i]; i+=2)
>  		if (app.dynv[i]==DT_DEBUG)
>  			app.dynv[i+1] = (size_t)&debug;
>  #endif
> +#ifdef DT_DEBUG_INDIRECT
> +		if (app.dynv[i]==DT_DEBUG_INDIRECT) {
> +			size_t *ptr = (size_t *) app.dynv[i+1];
> +			*ptr = (size_t)&debug;
> +		}
> +#endif
> +	}
>  
>  	/* The main program must be relocated LAST since it may contin
>  	 * copy relocations which depend on libraries' relocations. */
> -- 
> 2.2.2

This certainly works, but DYNAMIC_IS_RO and DT_DEBUG_INDIRECT are
probably redundant now; I can't imagine wanting one without the other.
I've also tried to replace #if/#ifdef in dynlink.c with if, in the
interest of catching warnings/errors no matter what arch we're
compiling for, so maybe we could do in internal/dynlink.h:

#ifndef DT_DEBUG_INDIRECT
#define DT_DEBUG_INDIRECT 0
#endif

then in dynlink.c:

		if (!DT_DEBUG_INDIRECT && app.dynv[i]==DT_DEBUG)
		...
		if (DT_DEBUG_INDIRECT && app.dynv[i]==DT_DEBUG_INDIRECT)

What do you think?

Rich