Re: [RFC][PATCH] binfmt: turn MAX_ARG_PAGES into a sysctl tunable

[Pavel Machek <pavel@ucw.cz>]

Hi!

> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Some folks find 128KB of env+arg space too little. Solaris provides them with
> 1MB. Manually changing MAX_ARG_PAGES worked for them so far, however they
> would like to run the supported vendor kernel.
> 
> In the interrest of not penalizing everybody with the overhead of just
> setting it larger, provide a sysctl to change it.

I very muh like that idea.

> Compiles and boots on i386.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

...but does is also work if you keep changing that value in the tight
loop while trying to work with the system?

> Index: linux-2.6/fs/binfmt_elf.c
> ===================================================================
> --- linux-2.6.orig/fs/binfmt_elf.c	2006-06-14 15:56:40.000000000 +0200
> +++ linux-2.6/fs/binfmt_elf.c	2006-06-14 16:00:36.000000000 +0200
> @@ -255,8 +255,9 @@ create_elf_tables(struct linux_binprm *b
>  	while (argc-- > 0) {
>  		size_t len;
>  		__put_user((elf_addr_t)p, argv++);
> -		len = strnlen_user((void __user *)p, PAGE_SIZE*MAX_ARG_PAGES);
> -		if (!len || len > PAGE_SIZE*MAX_ARG_PAGES)
> +		len = strnlen_user((void __user *)p,
> +				PAGE_SIZE*bprm->max_arg_pages);
> +		if (!len || len > PAGE_SIZE*bprm->max_arg_pages)
>  			return 0;
>  		p += len;
>  	}
> @@ -266,8 +267,9 @@ create_elf_tables(struct linux_binprm *b
>  	while (envc-- > 0) {
>  		size_t len;
>  		__put_user((elf_addr_t)p, envp++);
> -		len = strnlen_user((void __user *)p, PAGE_SIZE*MAX_ARG_PAGES);
> -		if (!len || len > PAGE_SIZE*MAX_ARG_PAGES)
> +		len = strnlen_user((void __user *)p,
> +				PAGE_SIZE*bprm->max_arg_pages);
> +		if (!len || len > PAGE_SIZE*bprm->max_arg_pages)
>  			return 0;
>  		p += len;
>  	}
> Index: linux-2.6/fs/binfmt_elf_fdpic.c
> ===================================================================
> --- linux-2.6.orig/fs/binfmt_elf_fdpic.c	2006-06-14 15:56:40.000000000 +0200
> +++ linux-2.6/fs/binfmt_elf_fdpic.c	2006-06-14 16:00:36.000000000 +0200
> @@ -578,14 +578,15 @@ static int create_elf_fdpic_tables(struc
>  #ifdef CONFIG_MMU
>  	current->mm->arg_start = bprm->p;
>  #else
> -	current->mm->arg_start = current->mm->start_stack - (MAX_ARG_PAGES * PAGE_SIZE - bprm->p);
> +	current->mm->arg_start = current->mm->start_stack -
> +		(bprm->max_arg_pages * PAGE_SIZE - bprm->p);
>  #endif
>  
>  	p = (char *) current->mm->arg_start;
>  	for (loop = bprm->argc; loop > 0; loop--) {

What happens if someone changes that sysctl from other cpu at this
point?

>  		__put_user((elf_caddr_t) p, argv++);
> -		len = strnlen_user(p, PAGE_SIZE * MAX_ARG_PAGES);
> -		if (!len || len > PAGE_SIZE * MAX_ARG_PAGES)
> +		len = strnlen_user(p, PAGE_SIZE * bprm->max_arg_pages);
> +		if (!len || len > PAGE_SIZE * bprm->max_arg_pages)
>  			return -EINVAL;
>  		p += len;
>  	}

-- 
Thanks for all the (sleeping) penguins.