Handle signals properly

Page faults trigger a trap, which is handled by do_page_fault(). This
function calls lock_mm_and_find_vma() to locate and validate the virtual
memory area (VMA), returning the VMA if valid, or NULL otherwise.
Typically, attempts to read or write to a NULL VMA result in a NULL
return. If the VMA is invalid, bad_area_nosemaphore() is invoked, which
checks whether the fault originated in kernel or user space.

For user-space faults, a SIGSEGV signal is sent to the user process via
do_trap(), which determines if the signal should be ignored or blocked,
and if not, adds it to the task's pending signal list. Kernel-space
faults cause the kernel to crash via die_kernel_fault().

Before returning to user space (via the resume_userspace label), pending
work (indicated by the _TIF_WORK_MASK mask) is processed by
do_work_pending(). Signals are handled by do_signal(), which in turn
calls handle_signal(). handle_signal() creates a signal handler frame
that will be jumped to upon returning to user space. This frame creation
process might modifies the Control and Status Register (CSR) SEPC.
If there are a signal pending, the SEPC CSR overwritten the original
trap/fault PC. This caused an assertion failure in get_ppn_and_offset()
when running the vi program, reported in [1].

To address this, a variable last_csr_sepc was introduced to store the
original SEPC CSR value before entering the trap path. After returning
to user space, last_csr_sepc is compared with the current SEPC CSR
value. If they differ, the fault ld/st instruction returns early and
jumps to the signal handler frame.

This commit prevents emulator crashes when the guest OS accesses invalid
memory. Consequently, reads or writes to a NULL value now correctly
result in a segmentation fault. In addition, two user-space programs:
mem_null_read and mem_null_write are bundled into the rootfs for
verification.

Original behaviour:
1. $ make system ENABLE_SYSTEM=1 -j$(nproc)   # run system emulation
2. $ mem_null_read                            # Emulator crashes
3. $ mem_null_write                           # Emulator crashes
4. $ vi                                       # Emulator crashes

Patch Reproduce / Testing procedure:
1. $ make system ENABLE_SYSTEM=1 -j$(nproc)   # run system emulation
2. $ mem_null_read       # NULL read causes SIGSEGV with no crashing
3. $ mem_null_write      # NULL write causes SIGSEGV with no crashing
4. $ vi                  # w/o filename causes SIGSEGV with no crashing

[1] #508

src/emulate.c

@@ -46,6 +46,7 @@ static bool need_clear_block_map = false;
 static uint32_t reloc_enable_mmu_jalr_addr;
 static bool reloc_enable_mmu = false;
 bool need_retranslate = false;
+bool need_handle_signal = false;
 #endif
 
 static void rv_trap_default_handler(riscv_t *rv)
@@ -379,8 +380,12 @@ static uint32_t peripheral_update_ctr = 64;
     {                                                                 \
         IIF(RV32_HAS(SYSTEM))(ctr++;, ) cycle++;                      \
         code;                                                         \
-    nextop:                                                           \
-        PC += __rv_insn_##inst##_len;                                 \
+        IIF(RV32_HAS(SYSTEM))                                         \
+        (                                                             \
+            if (need_handle_signal) {                                 \
+                need_handle_signal = false;                           \
+                return true;                                          \
+            }, ) nextop : PC += __rv_insn_##inst##_len;               \
         IIF(RV32_HAS(SYSTEM))                                         \
         (IIF(RV32_HAS(JIT))(                                          \
              , if (unlikely(need_clear_block_map)) {                  \
@@ -1219,6 +1224,9 @@ static void _trap_handler(riscv_t *rv)
         mode = rv->csr_stvec & 0x3;
         cause = rv->csr_scause;
         rv->csr_sepc = rv->PC;
+#if RV32_HAS(SYSTEM)
+        rv->last_csr_sepc = rv->csr_sepc;
+#endif
     } else { /* machine */
         const uint32_t mstatus_mie =
             (rv->csr_mstatus & MSTATUS_MIE) >> MSTATUS_MIE_SHIFT;

src/riscv_private.h

@@ -201,6 +201,12 @@ struct riscv_internal {
 #if RV32_HAS(SYSTEM)
     /* The flag is used to indicate the current emulation is in a trap */
     bool is_trapped;
+
+    /*
+     * The flag that stores the SEPC CSR at the trap point for corectly
+     * executing signal handler.
+     */
+    uint32_t last_csr_sepc;
 #endif
 };
 

src/system.c

@@ -25,6 +25,17 @@ void emu_update_uart_interrupts(riscv_t *rv)
     plic_update_interrupts(attr->plic);
 }
 
+/*
+ * Linux kernel might create signal frame when returning from trap
+ * handling, which modifies the SEPC CSR. Thus, the fault instruction
+ * cannot always redo. For example, invalid memory access causes SIGSEGV.
+ */
+extern bool need_handle_signal;
+#define CHECK_PENDING_SIGNAL(rv, signal_flag)              \
+    do {                                                   \
+        signal_flag = (rv->csr_sepc != rv->last_csr_sepc); \
+    } while (0)
+
 #define MMIO_R 1
 #define MMIO_W 0
 
@@ -297,8 +308,14 @@ static uint32_t mmu_read_w(riscv_t *rv, const uint32_t addr)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(read, rv, pte, addr, PTE_R);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return 0;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     {
         get_ppn_and_offset();
@@ -323,8 +340,14 @@ static uint16_t mmu_read_s(riscv_t *rv, const uint32_t addr)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(read, rv, pte, addr, PTE_R);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return 0;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     get_ppn_and_offset();
     return memory_read_s(ppn | offset);
@@ -338,8 +361,14 @@ static uint8_t mmu_read_b(riscv_t *rv, const uint32_t addr)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(read, rv, pte, addr, PTE_R);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return 0;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     {
         get_ppn_and_offset();
@@ -364,8 +393,14 @@ static void mmu_write_w(riscv_t *rv, const uint32_t addr, const uint32_t val)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(write, rv, pte, addr, PTE_W);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     {
         get_ppn_and_offset();
@@ -390,8 +425,14 @@ static void mmu_write_s(riscv_t *rv, const uint32_t addr, const uint16_t val)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(write, rv, pte, addr, PTE_W);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     get_ppn_and_offset();
     memory_write_s(ppn | offset, (uint8_t *) &val);
@@ -405,8 +446,14 @@ static void mmu_write_b(riscv_t *rv, const uint32_t addr, const uint8_t val)
     uint32_t level;
     pte_t *pte = mmu_walk(rv, addr, &level);
     bool ok = MMU_FAULT_CHECK(write, rv, pte, addr, PTE_W);
-    if (unlikely(!ok))
+    if (unlikely(!ok)) {
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+        CHECK_PENDING_SIGNAL(rv, need_handle_signal);
+        if (need_handle_signal)
+            return;
+#endif
         pte = mmu_walk(rv, addr, &level);
+    }
 
     {
         get_ppn_and_offset();