linux-kernel - Re: nios2: is the ptrace ABI correct?

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Date:	Fri, 27 Feb 2015 16:57:40 +0800
From:	Chung-Lin Tang <cltang@...esourcery.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
CC:	Tobias Klauser <tklauser@...tanz.ch>,
	Walter Goossens <waltergoossens@...e.nl>,
	Ley Foon Tan <lftan@...era.com>,
	<linux-arch@...r.kernel.org>,
	"nios2-dev@...ts.rocketboards.org" <nios2-dev@...ts.rocketboards.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: nios2: is the ptrace ABI correct?

On 15/2/25 10:07 PM, Arnd Bergmann wrote:
> On Wednesday 25 February 2015 08:33:16 Ezequiel Garcia wrote:
>>
>> /me is more confused now
>>
>> In arch/nios2/include/asm/ucontext.h
>>
>> struct ucontext {
>>         unsigned long     uc_flags;
>>         struct ucontext  *uc_link;
>>         stack_t           uc_stack;
>>         struct mcontext   uc_mcontext;
>>         sigset_t          uc_sigmask;
>> };
>>
>> And in include/uapi/asm-generic/ucontext.h:
>>
>> struct ucontext {
>>         unsigned long     uc_flags;
>>         struct ucontext  *uc_link;
>>         stack_t           uc_stack;
>>         struct sigcontext uc_mcontext;
>>         sigset_t          uc_sigmask;
>> };
>>
>> Which one is the one that userspace sees? And why does the kernel has
>> two different structures?
> 
> Userspace sees the asm-generic header, which I assume is a bug
> in this case.

Yes, I believe nios2 doesn't not need this asm-generic/ucontext.h
header; OTOH it just isn't used; no real harm done, so easily fixed.

>> Given this oddities, I'm wondering how troublesome would be to just
>> re-do this and break the ptrace and signal ABI. For instance, just
>> pushing pt_regs in PTRACE_GETREGSET would make things much clearer.
> 
> Could you change pt_regs to match the layout you have for PTRACE_GETREGSET
> instead? It seems much more intuitive.

There is a reason for this pt_regs arrangement: the nios2 syscall
interface uses r4-r9 for parameters, while the usual C conventions use
only r4-r7, placing r8-r9 at the start of pt_regs creates a natural
stack layout for entering C code after the asm shims in entry.S

>> I guess Linus would burn me for even suggesting to breaking users... but
>> do we have any users at all? This arch has just been mainlined. Altera's
>> out-of-tree is already ABI-incompatible with mainline so that's not an
>> issue.
>>
>> The only one using this ABI is gdb, which is easily fixed.
> 
> You can change anything you like as long as nobody complains about
> regressions.

PTRACE_GET/SETREGSET is a new feature in nios2-linux that we're still
about to support in upstream GDB, so things could be fixed if needed,
but why can't you just use the [0...] ordering in userspace?

BTW, it's even that way in signal stacks as well; nios2 does not
use/export sigcontext inside struct ucontext. We just use a int[32]
array there.

Thanks,
Chung-Lin

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