riscv: Fix crash when syscall argument reg index is too high

By default, doSyscall gets the values of six registers to be used for
system call arguments.  RISC-V, by convention, only has four.  Because
RISC-V's implementation of these indices is as arrays of integers rather
than as base indices plus offsets, trying to get the fifth argument
register's value will cause a crash.  This patch fixes that by returning 0
for any index higher than 3.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/arch/riscv/process.cc b/src/arch/riscv/process.cc
index c1a67ec15..4eb3159af 100644
--- a/src/arch/riscv/process.cc
+++ b/src/arch/riscv/process.cc
@@ -217,7 +217,13 @@ RiscvLiveProcess::argsInit(int pageSize)
 RiscvISA::IntReg
 RiscvLiveProcess::getSyscallArg(ThreadContext *tc, int &i)
 {
-    return tc->readIntReg(SyscallArgumentRegs[i++]);
+    // RISC-V only has four system call argument registers by convention, so
+    // if a larger index is requested return 0
+    RiscvISA::IntReg retval = 0;
+    if (i < 4)
+        retval = tc->readIntReg(SyscallArgumentRegs[i]);
+    i++;
+    return retval;
 }
 
 void