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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