From mboxrd@z Thu Jan  1 00:00:00 1970
From: ebiederm@xmission.com (Eric W. Biederman)
Subject: Re: [PATCH 2/2] alpha/ptrace: Add missing switch_stack frames
Date: Wed, 16 Jun 2021 15:47:28 -0500
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In-Reply-To: <YMpeP0CrRUVKIysE@zeniv-ca.linux.org.uk> (Al Viro's message of
        "Wed, 16 Jun 2021 20:25:35 +0000")
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To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Michael Schmitz <schmitzmic@gmail.com>, linux-arch <linux-arch@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Richard Henderson <rth@twiddle.net>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Matt Turner <mattst88@gmail.com>, alpha <linux-alpha@vger.kernel.org>, Geert Uytterhoeven <geert@linux-m68k.org>, linux-m68k <linux-m68k@lists.linux-m68k.org>, Arnd Bergmann <arnd@kernel.org>, Ley Foon Tan <ley.foon.tan@intel.com>, Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>

Al Viro <viro@zeniv.linux.org.uk> writes:

> 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.

Fork already saves the registers, all I did was restore them.  Which
makes a debugger that modifies them in
PTRACE_EVENT_{FORK,VFORK,CLONE,VFORK_DONE} work.

>> +allregs exit
>> +allregs exit_group
>
> 	Details, please - what exactly makes exit(2) different from
> e.g. open(2)?

PTRACE_EVENT_EXIT.


Eric