[PATCH 6/7] riscv: Ensure gp is NULL or points to valid data

[Sean Anderson <seanga2@gmail.com>]

This allows code to use a construct like `if (gd & gd->...) { ... }` when
accessing the global data pointer. Without this change, it was possible for
a very early trap to cause _exit_trap to read arbitrary memory. This could
cause a second trap, preventing show_regs from being printed.

Fixes: 7c6ca03eaed0035ca6676e9bc7f5f1dfcaae7e8f
Signed-off-by: Sean Anderson <seanga2@gmail.com>
---

 arch/riscv/cpu/start.S      | 20 +++++++++++++++++---
 arch/riscv/lib/interrupts.c |  3 ++-
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index ad18e746b6..59d3d7bbf4 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -47,6 +47,13 @@ _start:
 	mv	tp, a0
 	mv	s1, a1
 
+	/*
+	 * Set the global data pointer to a known value in case we get a very
+	 * early trap. The global data pointer will be set its actual value only
+	 * after it has been initialized.
+	 */
+	mv	gp, zero
+
 	la	t0, trap_entry
 	csrw	MODE_PREFIX(tvec), t0
 
@@ -85,10 +92,10 @@ call_board_init_f_0:
 	jal	board_init_f_alloc_reserve
 
 	/*
-	 * Set global data pointer here for all harts, uninitialized at this
-	 * point.
+	 * Save global data pointer for later. We don't set it here because it
+	 * is not initialized yet.
 	 */
-	mv	gp, a0
+	mv	s0, a0
 
 	/* setup stack */
 #if CONFIG_IS_ENABLED(SMP)
@@ -135,6 +142,13 @@ wait_for_gd_init:
 	fence	r, rw
 	bnez	t1, 1b
 
+	/*
+	 * Set the global data pointer only when gd_t has been initialized.
+	 * This was already set by arch_setup_gd on the boot hart, but all other
+	 * harts' global data pointers gets set here.
+	 */
+	mv	gp, s0
+
 	/* register available harts in the available_harts mask */
 	li	t1, 1
 	sll	t1, t1, tp
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index cd47e64487..ad870e98d8 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -78,7 +78,8 @@ static void _exit_trap(ulong code, ulong epc, ulong tval, struct pt_regs *regs)
 
 	printf("EPC: " REG_FMT " RA: " REG_FMT " TVAL: " REG_FMT "\n",
 	       epc, regs->ra, tval);
-	if (gd->flags & GD_FLG_RELOC)
+	/* Print relocation adjustments, but only if gd is initialized */
+	if (gd && gd->flags & GD_FLG_RELOC)
 		printf("EPC: " REG_FMT " RA: " REG_FMT " reloc adjusted\n\n",
 		       epc - gd->reloc_off, regs->ra - gd->reloc_off);
 
-- 
2.28.0