Re: [PATCH v2] m68k: use conventional function parameters for do_sigreturn

[Greg Ungerer <gerg@uclinux.org>]

Hi Philippe,

On 15/02/16 20:03, Philippe De Muyter wrote:
> On Mon, Feb 15, 2016 at 04:36:29PM +1000, Greg Ungerer wrote:
>> Create conventional stack parameters for the calls to do_sigreturn and
>> do_rt_sigreturn. The current C code for do_sigreturn and do_rt_sigreturn
>> dig into the stack to create local pointers to the saved switch stack
>> and the pt_regs structs.
>>
>> The motivation for this change is a problem with non-MMU targets that
>> have broken signal return paths on newer versions of gcc. It appears as
>> though gcc has determined that the pointers into the saved stack structs,
>> and the saved structs themselves, are function parameters and updates to
>> them will be lost on function return, so they are optimized away. This
>> results in large parts of restore_sigcontext() and mangle_kernel_stack()
>> functions being removed. Of course this results in non-functional code
>> causing kernel oops. This problem has been observed with gcc version
>> 5.2 and 5.3, and probably exists in earlier versions as well.
>>
>> Using conventional stack parameter pointers passed to these functions has
>> the advantage of the code here not needing to know the exact details of
>> how the underlying entry handler layed these structs out on the stack.
>> So the rather ugly pointer setup casting and arg referencing can be
>> removed.
>>
>> The resulting code after this change is a few bytes larger (due to the
>> overhead of creating the stack args and their tear down). Not being hot
>> paths I don't think this is too much of a problem here.
>
> As an intermediate solution, I think that you can avoid part of that overhead ...
>
>>
>> An alternative solution is to put a barrier() in the do_sigreturn() code,
>> but this doesn't feel quite as clean as this solution.
>>
>> This change has been compile tested on all defconfigs, and run tested on
>> Atari (through aranym), ColdFire with MMU (M5407EVB) and ColdFire with
>> no-MMU (QEMU and M5208EVB).
>>
>> Signed-off-by: Greg Ungerer <gerg@uclinux.org>
>> ---
>> v2: reworded the commit log message with better description of problem
>>
>>   arch/m68k/kernel/entry.S  | 6 ++++++
>>   arch/m68k/kernel/signal.c | 8 ++------
>>   2 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/m68k/kernel/entry.S b/arch/m68k/kernel/entry.S
>> index b54ac7a..97cd3ea 100644
>> --- a/arch/m68k/kernel/entry.S
>> +++ b/arch/m68k/kernel/entry.S
>> @@ -71,13 +71,19 @@ ENTRY(__sys_vfork)
>>
>>   ENTRY(sys_sigreturn)
>>   	SAVE_SWITCH_STACK
>> +	movel	%sp,%sp@-		  | switch_stack pointer
>> +	pea	%sp@(SWITCH_STACK_SIZE+4) | pt_regs pointer
>
> if you remove the above asm instruction ...
>
>>   	jbsr	do_sigreturn
>> +	addql	#8,%sp
>
> change the 8 by 4 here ...
>
>>   	RESTORE_SWITCH_STACK
>>   	rts
>>
>>   ENTRY(sys_rt_sigreturn)
>>   	SAVE_SWITCH_STACK
>> +	movel	%sp,%sp@-		  | switch_stack pointer
>> +	pea	%sp@(SWITCH_STACK_SIZE+4) | pt_regs pointer
>
> ditto here ...
>
>>   	jbsr	do_rt_sigreturn
>> +	addql	#8,%sp
>
> and here ...
>
>>   	RESTORE_SWITCH_STACK
>>   	rts
>>
>> diff --git a/arch/m68k/kernel/signal.c b/arch/m68k/kernel/signal.c
>> index af1c4f3..2dcee3a 100644
>> --- a/arch/m68k/kernel/signal.c
>> +++ b/arch/m68k/kernel/signal.c
>> @@ -737,10 +737,8 @@ badframe:
>>   	return 1;
>>   }
>>
>> -asmlinkage int do_sigreturn(unsigned long __unused)
>> +asmlinkage int do_sigreturn(struct pt_regs *regs, struct switch_stack *sw)
>
> and if you use the following prototype :
>   asmlinkage int do_sigreturn(struct switch_stack *sw) ...
>
>>   {
>> -	struct switch_stack *sw = (struct switch_stack *) &__unused;
>> -	struct pt_regs *regs = (struct pt_regs *) (sw + 1);
>
> and keep the 'C' instruction above
>
>>   	unsigned long usp = rdusp();
>>   	struct sigframe __user *frame = (struct sigframe __user *)(usp - 4);
>>   	sigset_t set;
>> @@ -764,10 +762,8 @@ badframe:
>>   	return 0;
>>   }
>>
>> -asmlinkage int do_rt_sigreturn(unsigned long __unused)
>> +asmlinkage int do_rt_sigreturn(struct pt_regs *regs, struct switch_stack *sw)
>   same here
>>   {
>> -	struct switch_stack *sw = (struct switch_stack *) &__unused;
>> -	struct pt_regs *regs = (struct pt_regs *) (sw + 1);
>   and here
>>   	unsigned long usp = rdusp();
>>   	struct rt_sigframe __user *frame = (struct rt_sigframe __user *)(usp - 4);
>>   	sigset_t set;

Those changes ultimately make no difference to code size.
Although the entry point code is smaller the resulting
generated code for do_sigreturn() and do_rt_sigreturn()
is slightly larger. (At least that is the case with the gcc-5.3
compiler I am currently using).

Personally  I don't like having to keep that ugly stack referencing
definition of regs based on sw either. I would like to remove that.

"sw" isn't actually used in do_sigreturn() at all, so that could
be removed from the its parameters. I chose to keep it there in this
patch for consistency with do_rt_sigreturn().

Regards
Greg