[PATCH 12/12] riscv/cfi: Support ucontext under CFI

[Jesse Huang]

Yeah we can shrink it and in fact I did it at the beginning, I'm just
uncertaining about
the opinion of the kernel community on this, if kernel folks think it's
okay to
shrink it then not a problem at all.


On Thu, Jun 19, 2025 at 3:57 PM Deepak Gupta <debug@rivosinc.com> wrote:

> On Thu, Jun 19, 2025 at 02:59:40PM +0800, Jesse Huang wrote:
> >On Thu, Jun 19, 2025 at 5:34 AM Deepak Gupta <debug@rivosinc.com> wrote:
> >
> >> On Wed, Jun 18, 2025 at 01:42:58AM -0700, Jesse Huang wrote:
> >> >This patches only make the ucontext library work while CFI is enabled.
> >> >enabled. All security checks are skipped and should be implemented if
> >> >correspoding interface are landed into the kernel.
> >> >SSP is stored in the unused t3 slot in the sigcontext structure to
> avoid
> >>
> >> Question:
> >>
> >> Why can't we steal a 8 byte slot from `__glibc_reserved` under
> ucontext_t?
> >
> >
> >The __glibc_reserved field actually does not take any space
>
> aah thanks for that clarification.
>
> >
> >#define _SIGSET_NWORDS (1024 / (8 * sizeof (unsigned long int)))
> >
> >typedef struct
> >{
> >  unsigned long int __val[_SIGSET_NWORDS];
> >} __sigset_t;
> >
> >typedef struct ucontext_t
> >  {
> >    unsigned long int  __uc_flags;
> >    struct ucontext_t *uc_link;
> >    stack_t            uc_stack;
> >    sigset_t           uc_sigmask;
> >    /* There's some padding here to allow sigset_t to be expanded in the
> >       future.  Though this is unlikely, other architectures put
> uc_sigmask
> >       at the end of this structure and explicitly state it can be
> >       expanded, so we didn't want to box ourselves in here.  */
> >    char               __glibc_reserved[1024 / 8 - sizeof (sigset_t)];
> >    /* We can't put uc_sigmask at the end of this structure because we
> need
> >       to be able to expand sigcontext in the future.  For example, the
> >       vector ISA extension will almost certainly add ISA state.  We want
> >       to ensure all user-visible ISA state can be saved and restored via
> a
> >       ucontext, so we're putting this at the end in order to allow for
> >       infinite extensibility.  Since we know this will be extended and we
> >       assume sigset_t won't be extended an extreme amount, we're
> >       prioritizing this.  */
> >    mcontext_t uc_mcontext;
> >  } ucontext_t;
> >
> >Where  `1024 / 8 - sizeof(sigset_t)` evaluates to 0, unless we shrink the
> >`_SIGSET_NWORDS`.
>
> Can we shrink it?
> I see that it is differently defined for `arc`.
> If it's not needed, then perhaps we can create space here.
> And then as long as we have `ssp` pointer, we can simply use token
> mechanism and we don't need to worry about two different cases (one where
> single thread stack is used and one where two different threads are hosting
> two different contexts)
>
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