Re: [PATCH v1 1/1] riscv: fix runtime constant support for nommu kernels

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, May 30, 2025 at 05:14:22PM -0400, Charles Mirabile wrote:
> the `__runtime_fixup_32` function does not handle the case where `val` is
> zero correctly (as might occur when patching a nommu kernel and referring
> to a physical address below the 4GiB boundary whose upper 32 bits are all
> zero) because nothing in the existing logic prevents the code from taking
> the `else` branch of both nop-checks and emitting two `nop` instructions.
> 
> This leaves random garbage in the register that is supposed to receive the
> upper 32 bits of the pointer instead of zero that when combined with the
> value for the lower 32 bits yields an invalid pointer and causes a kernel
> panic when that pointer is eventually accessed.
> 
> The author clearly considered the fact that if the `lui` is converted into
> a `nop` that the second instruction needs to be adjusted to become an `li`
> instead of an `addi`, hence introducing the `addi_insn_mask` variable, but
> didn't follow that logic through fully to the case where the `else` branch
> executes. To fix it just adjust the logic to ensure that the second `else`
> branch is not taken if the first instruction will be patched to a `nop`.

You have an accurate assesment here, I missed the zero case :/.
Thank you for fixing the issue!

> 
> Fixes: a44fb5722199 ("riscv: Add runtime constant support")
> 
> Signed-off-by: Charles Mirabile <cmirabil@redhat.com>
> ---
>  arch/riscv/include/asm/runtime-const.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
> index 451fd76b8811..d766e2b9e6df 100644
> --- a/arch/riscv/include/asm/runtime-const.h
> +++ b/arch/riscv/include/asm/runtime-const.h
> @@ -206,7 +206,7 @@ static inline void __runtime_fixup_32(__le16 *lui_parcel, __le16 *addi_parcel, u
>  		addi_insn_mask &= 0x07fff;
>  	}
>  
> -	if (lower_immediate & 0x00000fff) {
> +	if (lower_immediate & 0x00000fff || lui_insn == RISCV_INSN_NOP4) {

This comment is borderline too nitpicky so feel free to dismiss it :).
It's slightly wasteful to have this check right after the if-statement
that sets it. I am not sure what the most readable way of doing this is
though. What would you think about a patch like the following instead?

From 1c56536c1e338735140c9090f06da49a3d245a61 Mon Sep 17 00:00:00 2001
From: Charlie Jenkins <charlie@rivosinc.com>
Date: Fri, 30 May 2025 19:25:13 -0700
Subject: [PATCH] alternate fix

---
 arch/riscv/include/asm/runtime-const.h | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
index 451fd76b8811..085a0bb26fbb 100644
--- a/arch/riscv/include/asm/runtime-const.h
+++ b/arch/riscv/include/asm/runtime-const.h
@@ -179,12 +179,9 @@ static inline void __runtime_fixup_caches(void *where, unsigned int insns)
 static inline void __runtime_fixup_32(__le16 *lui_parcel, __le16 *addi_parcel, unsigned int val)
 {
 	unsigned int lower_immediate, upper_immediate;
-	u32 lui_insn, addi_insn, addi_insn_mask;
+	u32 lui_insn, addi_insn;
 	__le32 lui_res, addi_res;
 
-	/* Mask out upper 12 bit of addi */
-	addi_insn_mask = 0x000fffff;
-
 	lui_insn = (u32)le16_to_cpu(lui_parcel[0]) | (u32)le16_to_cpu(lui_parcel[1]) << 16;
 	addi_insn = (u32)le16_to_cpu(addi_parcel[0]) | (u32)le16_to_cpu(addi_parcel[1]) << 16;
 
@@ -195,6 +192,15 @@ static inline void __runtime_fixup_32(__le16 *lui_parcel, __le16 *addi_parcel, u
 		/* replace upper 20 bits of lui with upper immediate */
 		lui_insn &= 0x00000fff;
 		lui_insn |= upper_immediate & 0xfffff000;
+
+		if (lower_immediate & 0x00000fff) {
+			/* replace upper 12 bits of addi with lower 12 bits of val */
+			addi_insn &= 0x000fffff;
+			addi_insn |= (lower_immediate & 0x00000fff) << 20;
+		} else {
+			/* replace addi with nop if lower_immediate is empty */
+			addi_insn = RISCV_INSN_NOP4;
+		}
 	} else {
 		/* replace lui with nop if immediate is small enough to fit in addi */
 		lui_insn = RISCV_INSN_NOP4;
@@ -203,16 +209,9 @@ static inline void __runtime_fixup_32(__le16 *lui_parcel, __le16 *addi_parcel, u
 		 * is performed by adding with the x0 register. Setting rs to
 		 * zero with the following mask will accomplish this goal.
 		 */
-		addi_insn_mask &= 0x07fff;
-	}
-
-	if (lower_immediate & 0x00000fff) {
+		addi_insn &= 0x07fff;
 		/* replace upper 12 bits of addi with lower 12 bits of val */
-		addi_insn &= addi_insn_mask;
 		addi_insn |= (lower_immediate & 0x00000fff) << 20;
-	} else {
-		/* replace addi with nop if lower_immediate is empty */
-		addi_insn = RISCV_INSN_NOP4;
 	}
 
 	addi_res = cpu_to_le32(addi_insn);
-- 
2.43.0

Let me know what you think!

- Charlie


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