2025-04-23T17:23:56-07:00, Deepak Gupta debug@rivosinc.com:
On Thu, Apr 10, 2025 at 01:04:39PM +0200, Radim Krčmář wrote:
2025-03-14T14:39:24-07:00, Deepak Gupta debug@rivosinc.com:
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S @@ -147,6 +147,20 @@ SYM_CODE_START(handle_exception)
REG_L s0, TASK_TI_USER_SP(tp) csrrc s1, CSR_STATUS, t0
- /*
* If previous mode was U, capture shadow stack pointer and save it away* Zero CSR_SSP at the same time for sanitization.*/- ALTERNATIVE("nop; nop; nop; nop",
__stringify( \andi s2, s1, SR_SPP; \bnez s2, skip_ssp_save; \csrrw s2, CSR_SSP, x0; \REG_S s2, TASK_TI_USER_SSP(tp); \skip_ssp_save:),0,RISCV_ISA_EXT_ZICFISS,CONFIG_RISCV_USER_CFI)(I'd prefer this closer to the user_sp and kernel_sp swap, it's breaking the flow here. We also already know if we've returned from userspace or not even without SR_SPP, but reusing the information might tangle the logic.)
If CSR_SCRATCH was 0, then we would be coming from kernel else flow goes to `.Lsave_context`. If we were coming from kernel mode, then eventually flow merges to `.Lsave_context`.
So we will be saving CSR_SSP on all kernel -- > kernel trap handling. That would be unnecessary. IIRC, this was one of the first review comments in early RFC series of these patch series (to not touch CSR_SSP un-necessarily)
We can avoid that by ensuring when we branch by determining if we are coming from user to something like `.Lsave_ssp` which eventually merges into ".Lsave_context". And if we were coming from kernel then we would branch to `.Lsave_context` and thus skipping ssp save logic. But # of branches it introduces in early exception handling is equivalent to what current patches do. So I don't see any value in doing that.
Let me know if I am missing something.
Right, it's hard to avoid the extra branches.
I think we could modify the entry point (STVEC), so we start at different paths based on kernel/userspace trap and only jump once to the common code, like:
SYM_CODE_START(handle_exception_kernel) /* kernel setup magic */ j handle_exception_common SYM_CODE_START(handle_exception_user) /* userspace setup magic */ handle_exception_common:
This is not a suggestion for this series. I would be perfectly happy with just a cleaner code.
Would it be possible to hide the ALTERNATIVE ugliness behind a macro and move it outside the code block that saves pt_regs?
Thanks.