[PATCH 2/2] lib: sbi: Fix stale prev_context in RNMI handler

[Nia Su]

An earlier SBI trap may have already used the same M-mode exception
stack slot to save its trap context before an RNMI is taken.
sbi_trap_rnmi_handler() never initializes prev_context before
sbi_trap_error() walks it to print trap diagnostics, so it can read
whatever stale value happens to be left in that slot.

Link prev_context on entry and restore it on exit, the same way
sbi_trap_handler() does. This also lets an exception taken while
already inside RNMI handling chain back to the RNMI's own trap
context instead of dropping it, so sbi_trap_error() prints every
nested context correctly, including the RNMI's.

Fixes: 00fec20b4976 ("firmware: Add RNMI handler infrastructure")
Suggested-by: Nick Hu <nick.hu at sifive.com>
Signed-off-by: Nia Su <nia.su at sifive.com>
---
 lib/sbi/sbi_trap.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/sbi/sbi_trap.c b/lib/sbi/sbi_trap.c
index 16774fd2f73059c1268bfb9f43fdf8e9c460c0b6..e694188c25b7b1715df660206af0a501290389fc 100644
--- a/lib/sbi/sbi_trap.c
+++ b/lib/sbi/sbi_trap.c
@@ -394,9 +394,14 @@ trap_done:
 struct sbi_trap_context *sbi_trap_rnmi_handler(struct sbi_trap_context *tcntx)
 {
 	int rc;
+	struct sbi_scratch *scratch = sbi_scratch_thishart_ptr();
 	const struct sbi_platform *plat = sbi_platform_thishart_ptr();
 	const struct sbi_platform_operations *ops = sbi_platform_ops(plat);
 
+	/* Update trap context pointer so nested traps chain correctly */
+	tcntx->prev_context = sbi_trap_get_context(scratch);
+	sbi_trap_set_context(scratch, tcntx);
+
 	/* Call platform-specific NMI handler if registered */
 	if (ops && ops->rnmi_handler) {
 		rc = ops->rnmi_handler(tcntx);
@@ -404,14 +409,15 @@ struct sbi_trap_context *sbi_trap_rnmi_handler(struct sbi_trap_context *tcntx)
 			/* Platform handler failed to handle NMI */
 			sbi_trap_error("platform NMI handler failed", rc, tcntx);
 		}
-		return tcntx;
+		goto done;
 	}
 
 	/* No platform handler - treat as unhandled NMI */
 	sbi_trap_error("unhandled NMI (no platform rnmi_handler)",
 		       SBI_ENOTSUPP, tcntx);
 
-	/* Never returns */
+done:
+	sbi_trap_set_context(scratch, tcntx->prev_context);
 	return tcntx;
 }
 

-- 
2.43.7