Messages in this thread |  | | Date | Sat, 17 Nov 2012 19:31:59 +0530 | From | Vineet Gupta <> | Subject | Re: [RFC PATCH v1 30/31] ARC: switch to generic kernel_execve() and sys_execve() |
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On Friday 16 November 2012 09:38 AM, Al Viro wrote: > On Wed, Nov 07, 2012 at 10:47:53AM +0100, Vineet Gupta wrote: >> +; When we land here, pt_regs have already been updated in-place correctly >> +; A pointer to them is also passed by kernel_execve, we just need to make sure >> +; that SP is set to point to them. >> +ARC_ENTRY ret_from_kernel_execve >> + ; Force SP to "normal" pt_regs just populated. >> + b.d ret_from_system_call >> + mov sp, r0 > won't that splatter crap into regs->r0? IOW, why not branch to > ret_from_exception here?
Yes it does - although I didn't notice the ill-effect since execve as an API doesn't expect r0 to have a certain value (like fork for child). Fixed in next revision.
> >> +ARC_EXIT ret_from_kernel_execve > Another thing: why not fold that branch to ret_from_exception into the end of > ret_from_kernel_thread() (instead of calling sys_exit()), select > GENERIC_KERNEL_EXECVE and lose __ARCH_WANT_KERNEL_EXECVE.
If you noticed I only had 3 patches and didn't have the equivalent of "switch to saner kernel_execve() semantics" which would achieve above because the bug you elucidate to below was giving me grief in bring up (and I couldn't spend enough time debugging it).
> Actually, now that I look at your ret_from_kernel_thread... How the hell > will it cope with kernel_thread() payload trying to return? AFAICS, this > j.d [r1] will lose the return address, won't it?
Exactly so. I was missing the equivalent of following
ARC_ENTRY ret_from_kernel_thread bl @schedule_tail ld r1, [sp, PT_r1] - j.d [r1] + jl.d [r1] ld r0, [sp, PT_r0] j @sys_exit ARC_EXIT ret_from_kernel_thread
And since none of the bootup kernel threads took the return path I had a working system - the problem only showed up when kernel_execve( ) started returning to exercise the broken path above.
> And while we are at it, > I would suggest passing callback and its argument via callee-saved registers - > makes for simpler life in ret_from_kernel_thread(), since switch_to() itself > will take care of loading those...
Absolutely - done that too !
Would you prefer to take a look at the updated patches now or will you rather wait for v2 series which might take a week or two.
Thanks for taking the time to review this stuff.
-Vineet
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