Re: Question about signal syscalls !

[David Daney <ddaney@avtrex.com>]

Franck Bui-Huu wrote:
> Hi Ralf,
> 
> On 2/1/07, Ralf Baechle <ralf@linux-mips.org> wrote:
>> The values of those registers need to be preserved so they can later be
>> copied into the signal frame.
>>
> 
> Let's take for example sys_sigreturn(). In my understanding this
> syscall is used automatically when the signal handler returns. At this
> time, I don't see the point to save the static registers since they
> have been already saved by setup_sigcontext().
> 
> Actually I don't see why they need to be saved/restored at all...
> 
> Let's say that process P1 sends a signal X to process P2 which has a
> handler defined for signal X and assume that the static registers are
> not saved at all.
> 
> Signal X is received by P2. The signal handler is now executed in user
> mode. At this point what are the values of the static registers ? I
> would say they have the same values (let's call this state S) when P2
> got interrupted. Once the signal handler returns into the kernel mode
> by executing 'syscall __NR_sigreturn' instructions, static registers
> still have state S and this state is normally preserved during
> sys_sigreturn syscall execution. So when resuming the normal execution
> of P2, the static registers have the correct values.
> 
> What am I missing ?

I don't think *any* registers *need* to be saved on sys_sigreturn(). 
The values in sigcontext on the user stack associated with the system 
call are all used instead of the actual register values.

David Daney