[PATCH] set saner MAX_ARGS

[Dmitry V. Levin]

On Thu, Aug 18, 2011 at 12:47:38PM +0200, Denys Vlasenko wrote:
> On Thu, 2011-08-18 at 12:23 +0200, Denys Vlasenko wrote:
> > I noticed that tcp->u_args[MAX_ARGS] array is way larger than
> > I'd expect: for all arches except HPPA it has 32 (!) elements.
> > 
> > I looked at the code and so far I spotted only one abuser of
> > this fact: sys_sigreturn. On several arches, it saves sigset_t
> > into tcp->u_args[1...N] on entry and prints it on exit, a-la
> > 
> >     memcpy(&tcp->u_arg[1], &sc.oldmask[0], sizeof(sigset_t))
> > 
> > The problem here is that in glibc sigset_t is insanely large:
> > 128 bytes, and using sizeof(sigset_t) in memcpy will overrun
> > &tcp->u_args[1] even with MAX_ARGS == 32:
> > On 32 bits, sizeof(tcp->u_args) == 32*4 == 128 bytes!
> > We may already have a bug there!
> > 
> > I propose to change the code to save NSIG / 8 bytes only.
> > NSIG can't ever be > 256, and in practice is <= 129,
> > thus NSIG / 8 is <= 16 bytes == 4 32-bit words,
> > and MAX_ARGS == 5 should be enough for saving signal masks.

I agree.

> > The proposed patch is below.
> > 
> > Alternative solution is to make sys_sigreturn print mask
> > on entry, not on exit. What is the reson it doesn't do that now?

Can sys_sigreturn be interrupted somehow?

[...]
> --- strace.5/defs.h	2011-08-18 11:57:30.512416447 +0200
> +++ strace.6/defs.h	2011-08-18 11:46:56.349540479 +0200
> @@ -64,7 +64,7 @@
>  #define DEFAULT_ACOLUMN	40	/* default alignment column for results */
>  #endif
>  #ifndef MAX_ARGS
> -# ifdef HPPA
> +# if defined HPPA || defined X86_64 || defined I386
>  #  define MAX_ARGS	6	/* maximum number of args to a syscall */
>  # else
>  /* Way too big. Switch your arch to saner size after you tested that it works */

What about other architectures?  Is there any with MAX_ARGS > 6?
Can we assume MAX_ARGS == 6 on linux?


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