Subject: Re: [PATCH] elf loader support for auxvec base platform string

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 17 Jul 2008 16:35:39 +1000

Hi Linus, Andrew !

Should I seek somebody's ack before merging a patch like the one below ?

I'm a bit reluctant to merge via the powerpc.git tree some changes to
generic files without at least an ack from somebody else :-)

There have been some debate on whether this AT_BASE_PLATFORM is the
right approach, though I haven't seen them reach any useful conclusion
and our toolchain people internally are screaming for it...

Cheers,
Ben.

On Tue, 2008-07-15 at 18:58 -0500, Nathan Lynch wrote:
> Some IBM POWER-based platforms have the ability to run in a
> mode which mostly appears to the OS as a different processor from the
> actual hardware.  For example, a Power6 system may appear to be a
> Power5+, which makes the AT_PLATFORM value "power5+".  This means that
> programs are restricted to the ISA supported by Power5+;
> Power6-specific instructions are treated as illegal.
> 
> However, some applications (virtual machines, optimized libraries) can
> benefit from knowledge of the underlying CPU model.  A new aux vector
> entry, AT_BASE_PLATFORM, will denote the actual hardware.  For
> example, on a Power6 system in Power5+ compatibility mode, AT_PLATFORM
> will be "power5+" and AT_BASE_PLATFORM will be "power6".  The idea is
> that AT_PLATFORM indicates the instruction set supported, while
> AT_BASE_PLATFORM indicates the underlying microarchitecture.
> 
> If the architecture has defined ELF_BASE_PLATFORM, copy that value to
> the user stack in the same manner as ELF_PLATFORM.
> 
> Signed-off-by: Nathan Lynch <[email-masked]>
> ---
>  fs/binfmt_elf.c        |   23 +++++++++++++++++++++++
>  include/linux/auxvec.h |    5 ++++-
>  2 files changed, 27 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index d48ff5f..834c2c4 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -131,6 +131,10 @@ static int padzero(unsigned long elf_bss)
>  #define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
>  #endif
>  
> +#ifndef ELF_BASE_PLATFORM
> +#define ELF_BASE_PLATFORM NULL
> +#endif
> +
>  static int
>  create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
>  		unsigned long load_addr, unsigned long interp_load_addr)
> @@ -142,7 +146,9 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
>  	elf_addr_t __user *envp;
>  	elf_addr_t __user *sp;
>  	elf_addr_t __user *u_platform;
> +	elf_addr_t __user *u_base_platform;
>  	const char *k_platform = ELF_PLATFORM;
> +	const char *k_base_platform = ELF_BASE_PLATFORM;
>  	int items;
>  	elf_addr_t *elf_info;
>  	int ei_index = 0;
> @@ -172,6 +178,19 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
>  			return -EFAULT;
>  	}
>  
> +	/*
> +	 * If this architecture has a "base" platform capability
> +	 * string, copy it to userspace.
> +	 */
> +	u_base_platform = NULL;
> +	if (k_base_platform) {
> +		size_t len = strlen(k_base_platform) + 1;
> +
> +		u_base_platform = (elf_addr_t __user *)STACK_ALLOC(p, len);
> +		if (__copy_to_user(u_base_platform, k_base_platform, len))
> +			return -EFAULT;
> +	}
> +
>  	/* Create the ELF interpreter info */
>  	elf_info = (elf_addr_t *)current->mm->saved_auxv;
>  	/* update AT_VECTOR_SIZE_BASE if the number of NEW_AUX_ENT() changes */
> @@ -208,6 +227,10 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
>  		NEW_AUX_ENT(AT_PLATFORM,
>  			    (elf_addr_t)(unsigned long)u_platform);
>  	}
> +	if (k_base_platform) {
> +		NEW_AUX_ENT(AT_BASE_PLATFORM,
> +			    (elf_addr_t)(unsigned long)u_base_platform);
> +	}
>  	if (bprm->interp_flags & BINPRM_FLAGS_EXECFD) {
>  		NEW_AUX_ENT(AT_EXECFD, bprm->interp_data);
>  	}
> diff --git a/include/linux/auxvec.h b/include/linux/auxvec.h
> index ad89545..1adc61d 100644
> --- a/include/linux/auxvec.h
> +++ b/include/linux/auxvec.h
> @@ -26,8 +26,11 @@
>  
>  #define AT_SECURE 23   /* secure mode boolean */
>  
> +#define AT_BASE_PLATFORM 38	/* string identifying real platform, may
> +				 * differ from AT_PLATFORM. */
> +
>  #ifdef __KERNEL__
> -#define AT_VECTOR_SIZE_BASE (14 + 2) /* NEW_AUX_ENT entries in auxiliary table */
> +#define AT_VECTOR_SIZE_BASE (14 + 3) /* NEW_AUX_ENT entries in auxiliary table */
>  #endif
>  
>  #endif /* _LINUX_AUXVEC_H */