[PATCH v17 15/27] riscv/traps: Introduce software check exception and uprobe handling

[Deepak Gupta]

Hi Zong,


On Thu, Jun 19, 2025 at 7:16 PM Zong Li <zong.li at sifive.com> wrote:
>
> On Mon, Jun 16, 2025 at 3:31 PM Zong Li <zong.li at sifive.com> wrote:
> >
> > On Thu, Jun 5, 2025 at 1:17 AM Deepak Gupta <debug at rivosinc.com> wrote:
> > >
> > > zicfiss / zicfilp introduces a new exception to priv isa `software check
> > > exception` with cause code = 18. This patch implements software check
> > > exception.
> > >
.....

> > When a user mode CFI violation occurs, the ELP state should be 1, and
> > the system traps into supervisor mode. During this trap, sstatus.SPELP
> > is set to 1, and the ELP state is reset to 0. If we don’t clear
> > sstatus.SPELP, the ELP state will become 1 again after executing the
> > sret instruction. As a result, the system might trigger another
> > forward CFI violation upon executing the next instruction in the user
> > program, unless it happens to be a lpad instruction.
> >
> > The previous patch was tested on QEMU, but QEMU does not set the
> > sstatus.SPELP bit to 1 when a forward CFI violation occurs. Therefore,
> > I suspect that QEMU might also require some fixes.
>
> Hi Deepak,
> The issue with QEMU was that the sw-check exception bit in medeleg
> couldn't be set. This has been fixed in the latest QEMU mainline. I
> have re-tested the latest QEMU version, and it works.

What was this issue, can you point me to the patch in mainline?

>
> >
> > Thanks
> >
> > > +
> > > +       if (is_fcfi || is_bcfi) {
> > > +               do_trap_error(regs, SIGSEGV, SEGV_CPERR, regs->epc,
> > > +                             "Oops - control flow violation");
> > > +               return true;
> > > +       }
> > > +
> > > +       return false;
> > > +}
> > > +
> > > +/*
> > > + * software check exception is defined with risc-v cfi spec. Software check
> > > + * exception is raised when:-
> > > + * a) An indirect branch doesn't land on 4 byte aligned PC or `lpad`
> > > + *    instruction or `label` value programmed in `lpad` instr doesn't
> > > + *    match with value setup in `x7`. reported code in `xtval` is 2.
> > > + * b) `sspopchk` instruction finds a mismatch between top of shadow stack (ssp)
> > > + *    and x1/x5. reported code in `xtval` is 3.
> > > + */
> > > +asmlinkage __visible __trap_section void do_trap_software_check(struct pt_regs *regs)
> > > +{
> > > +       if (user_mode(regs)) {
> > > +               irqentry_enter_from_user_mode(regs);
> > > +
> > > +               /* not a cfi violation, then merge into flow of unknown trap handler */
> > > +               if (!handle_user_cfi_violation(regs))
> > > +                       do_trap_unknown(regs);
> > > +
> > > +               irqentry_exit_to_user_mode(regs);
> > > +       } else {
> > > +               /* sw check exception coming from kernel is a bug in kernel */
> > > +               die(regs, "Kernel BUG");
> > > +       }
> > > +}
> > > +
> > >  #ifdef CONFIG_MMU
> > >  asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs)
> > >  {
> > >
> > > --
> > > 2.43.0
> > >