[PATCH] riscv: traps: handle uprobe event in software-check exception

[Zong Li]

On Tue, Jun 3, 2025 at 12:50 AM Deepak Gupta <debug at rivosinc.com> wrote:
>
> Hi Zong,
>
> Thanks for taking the initiative for making cfi work with uprobe.
> And sorry for not noticing the patch earlier.
> Few comments inline.
>
>
> On Fri, Mar 14, 2025 at 05:26:14PM +0800, Zong Li wrote:
> >Handle the uprobe event first before handling the CFI violation in
> >software-check exception handler. Because when the landing pad is
> >activated, if the uprobe point is set at the lpad instruction at
> >the beginning of a function, the system triggers a software-check
> >exception instead of an ebreak exception due to the exception
> >priority, then uprobe can't work successfully.
> >
> >Co-developed-by: Deepak Gupta <debug at rivosinc.com>
> >Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> >Signed-off-by: Zong Li <zong.li at sifive.com>
> >---
> >
> >This patch is based on top of the following series
> >[PATCH v11 00/27] riscv control-flow integrity for usermode
> >
> > arch/riscv/kernel/traps.c | 9 ++++++---
> > 1 file changed, 6 insertions(+), 3 deletions(-)
> >
> >diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> >index 3f7709f4595a..ef5a92111ee1 100644
> >--- a/arch/riscv/kernel/traps.c
> >+++ b/arch/riscv/kernel/traps.c
> >@@ -386,9 +386,12 @@ asmlinkage __visible __trap_section void do_trap_software_check(struct pt_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);
> >+              /* handle uprobe event frist */
> >+              if (!probe_breakpoint_handler(regs)) {
>
> If task has uprobe enabled and there is a cfi violation due to mismatch in
> return address on shadow stack and regular stack, then it would be a cfi
> bypass, right?
> Perhaps we should be doing this only when we match that sw check exception
> is due to forward cfi violation?
>
> Do you agree?

Yes, let me add a condition for forward cfi violation here. Thanks for
pointing it out.

>
> >+                      /* 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 {
> >--
> >2.17.1
> >
>
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