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

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

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).

> ---
>  sysdeps/unix/sysv/linux/or1k/sys/ucontext.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/sysdeps/unix/sysv/linux/or1k/sys/ucontext.h b/sysdeps/unix/sysv/linux/or1k/sys/ucontext.h
> index b17e919154..1b428592ee 100644
> --- a/sysdeps/unix/sysv/linux/or1k/sys/ucontext.h
> +++ b/sysdeps/unix/sysv/linux/or1k/sys/ucontext.h
> @@ -38,6 +38,7 @@ typedef struct
>      unsigned long int __gprs[__NGREG];
>      unsigned long int __pc;
>      unsigned long int __sr;
> +    unsigned long int __fpcsr;
>    } mcontext_t;
>  
>  /* Userlevel context.  */