Re: [PATCH 1/4] or1k: Fix Linux user space signal ABI

[Stafford Horne <shorne@gmail.com>]

On Wed, Mar 20, 2024 at 10:24:15AM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> On 19/03/24 18:42, Stafford Horne wrote:
> > The OpenRISC sigcontext structure has always been defined as:
> > 
> >     struct user_regs_struct {
> > 	    /* GPR R0-R31... */
> > 	    unsigned long gpr[32];
> > 	    unsigned long pc;
> > 	    unsigned long sr;
> >     };
> > 
> >     struct sigcontext {
> > 	    struct user_regs_struct regs;  /* needs to be first */
> > 	    unsigned long oldmask;	/* unused */
> >     };
> > 
> > With Linux v6.8 we added FPU support and repurposed the oldmask
> > to use for the FPCSR (floating point control status register).
> > 
> >     struct sigcontext {
> > 	    struct user_regs_struct regs;  /* needs to be first */
> > 	    union {
> > 		    unsigned long fpcsr;
> > 		    unsigned long oldmask;	/* unused */
> > 	    };
> >     };
> > 
> > The definition of mcontext_t was always missing the extra space for
> > oldmask.  This patch adds the field __fpcsr to mcontext_t to fix the ABI
> > mismatch between glibc and Linux.
> 
> This is strictly an ABI break, this won't make the swapcontext functions 
> to fail (since they are not update to take in consideration the new field),
> but it also means that the fpcsr won't be save/restore and the application
> can potentially read uninitialized values.
> 
> But I take that the fpu support will be a new ABI, so I suggest to fix
> when you add it (along with proper support to context functions).

OK, I got it.  I will post this when the hard float code is added and also fixup
swapcontext etc to populated it correctly with or without hard float.

Note there is broken ABI already, as programs will not be able to access sigmask
if needed due to:

Linux definition:

    struct ucontext {
	    unsigned long     uc_flags;
	    struct ucontext  *uc_link;
	    stack_t           uc_stack;
	    struct sigcontext uc_mcontext;  <-- size differs between glibc and linux
	    sigset_t          uc_sigmask;   <-- won't be able to access if needed
    };

But still I will leave as is for now.  This hasn't cause any issues as far as I
have seen so far.

-Stafford