Re: Moving forward with sh2/nommu

["uClinux.org" <Jeff@uClinux.org>]

Rich,

This isn't necessary.  The child in a nommu system -must never- return from the function that called vfork.  The reason is there is only one stack, and so that stack must not be corrupted (not the pointer, the actual call frame) or as you mention the parent cannot continue execution.

This is not a nommu uClinux thing, it is a restriction we inherited from BSD vfork().  It makes things much simpler (read: tractable at all), actually.

Cheers,
J.

> On Jun 12, 2015, at 13:35, Rich Felker <dalias@libc.org> wrote:
> 
>> On Fri, Jun 12, 2015 at 01:26:55PM +0900, Yoshinori Sato wrote:
>> On Fri, 12 Jun 2015 02:22:27 +0900,
>> Rich Felker wrote:
>>> 
>>> On Thu, Jun 11, 2015 at 11:12:52AM -0400, Rich Felker wrote:
>>>>>>> 3. We need sh/vfork.s since the default vfork.c just uses fork, which
>>>>>>>   won't work. I have a version locally but it doesn't make sense to
>>>>>>>   commit without runtime trap number selection.
>>>>>> 
>>>>>> Done and updated to use runtime selection in the (ugly) patch.
>>>>> 
>>>>> If they ask for vfork() they should get vfork()...?
>>>> 
>>>> Yes. The "runtime selection" is about the syscall trap number, not
>>>> whether or not to use vfork. I committed vfork to upstream musl now,
>>>> but with a SH3/4 trap number to be consistent with the code that's
>>>> upstream now. Later I'll either convert them all to trap 31 (0x1f) if
>>>> that ends up being acceptable, or merge the runtime-selection code,
>>>> but I think it makes sense to make the change across all files at
>>>> once, whichever way it's done.
>>> 
>>> Ah, maybe I misunderstood. If you were asking abaout the original
>>> remark that the default vfork.c uses fork, the reason is simply that
>>> you can't write vfork() in C. The return from vfork() in the child
>>> will clobber vfork's stack frame, which may contain the return address
>>> or saved registers, and then when the parent resumes, very bad things
>>> will happen. vfork() has to be implemented in asm to ensure that any
>>> state it needs to be able to return in the parent is kept in registers
>>> rather than memory. Thus, each arch needs an arch-specific version,
>>> and we just hadn't gotten around to adding the sh version yet.
>> 
>> No. vfork kept only last return address.
>> It isn't necessary to preserve the value of anything but that.
>> Child process can't return caller routine.
> 
> vfork still has to follow the normal function call ABI of preserving
> call-saved registers. For example, if you (or the compiler) wrote
> vfork by spilling some or all of the call-saved registers to the
> stack, clobbering them (e.g. for stack-protector work, or profiling
> counters, or PIC-related purposes, or for no reason at all), and then
> restoring them at return time, you'd be in trouble. The first return
> (in the child) would properly restore these registers, but subsequent
> execution in the child (in the function that called vfork, e.g. when
> it sets up the stack for a call to execl) could clobber the locations
> where they were saved on the stack, and when the parent resumed
> execution, it vfork would restore the wrong values, and very bad
> things could happen in the caller (e.g. the GOT register used for
> loading string literal args to exec*() might be wrong).
> 
> Rich