Re: [PATCH 2/2] alpha/ptrace: Add missing switch_stack frames

Eric W. Biederman
2021-06-16T20:47:38Z

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