linux-kernel - Re: [PATCH 2/2] alpha/ptrace: Add missing switch

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Message-ID: <YMpfBsIvqbK0L4Gh@zeniv-ca.linux.org.uk>
Date:   Wed, 16 Jun 2021 20:28:54 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Schmitz <schmitzmic@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Tejun Heo <tj@...nel.org>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] alpha/ptrace: Add missing switch_stack frames

On Wed, Jun 16, 2021 at 08:25:35PM +0000, Al Viro wrote:
> On Wed, Jun 16, 2021 at 01:32:50PM -0500, Eric W. Biederman wrote:
> 
> > -.macro	fork_like name
> > +.macro	allregs name
> >  	.align	4
> >  	.globl	alpha_\name
> >  	.ent	alpha_\name
> > +	.cfi_startproc
> >  alpha_\name:
> >  	.prologue 0
> > -	bsr	$1, do_switch_stack
> > +	SAVE_SWITCH_STACK
> >  	jsr	$26, sys_\name
> > -	ldq	$26, 56($sp)
> > -	lda	$sp, SWITCH_STACK_SIZE($sp)
> > +	RESTORE_SWITCH_STACK
> 
> 	No.  You've just added one hell of an overhead to fork(2),
> for no reason whatsoever.  sys_fork() et.al. does *NOT* modify the
> callee-saved registers; it's plain C.  So this change is complete
> BS.
> 
> > +allregs exit
> > +allregs exit_group
> 
> 	Details, please - what exactly makes exit(2) different from
> e.g. open(2)?

Ah... PTRACE_EVENT_EXIT garbage, fortunately having no counterparts in case of
open(2)...  Still, WTF would you want to restore callee-saved registers for
in case of exit(2)?