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

Eric W. Biederman
2021-06-16T20:49:56Z

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

> 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)?

Someone might want or try to read them in the case of exit. Which
without some change will result in a read of other kernel stack content
on alpha.

Plus there are coredumps which definitely want to read everything.
Although admittedly that case no longer matters.

Eric