Return -1 for syscall close on fd less than 3

Some user programs attempt to close stdout or stderr, but rv32emu
relies on these file descriptors for logging and debugging.
Allowing their closure could lead to unexpected behavior, making it
difficult to trace emulator activity.

This commit modifies the syscall close implementation to return -1
when attempting to close file descriptors less than 3. By ensuring
these essential output channels remain open, rv32emu retains its
ability to print logs and error messages. User programs receive a
clear failure signal instead of silently closing critical streams.

Close #435

Author: banglday

src/emulate.c

@@ -245,7 +245,7 @@ static uint32_t csr_csrrc(riscv_t *rv, uint32_t csr, uint32_t val)
 void rv_debug(riscv_t *rv)
 {
     if (!gdbstub_init(&rv->gdbstub, &gdbstub_ops,
-                      (arch_info_t){
+                      (arch_info_t) {
                           .reg_num = 33,
                           .reg_byte = 4,
                           .target_desc = TARGET_RV32,
@@ -1078,6 +1078,12 @@ void rv_step(void *arg)
         assert(block->satp == rv->csr_satp);
 #endif
 
+#if !RV32_HAS(SYSTEM)
+        /* on exit */
+        if (unlikely(block->ir_head->pc == PRIV(rv)->exit_addr))
+            PRIV(rv)->on_exit = true;
+#endif
+
         /* After emulating the previous block, it is determined whether the
          * branch is taken or not. The IR array of the current block is then
          * assigned to either the branch_taken or branch_untaken pointer of

src/riscv.c

@@ -406,7 +406,7 @@ riscv_t *rv_create(riscv_user_t rv_attr)
      */
     attr->fd_map = map_init(int, FILE *, map_cmp_int);
     rv_remap_stdstream(rv,
-                       (fd_stream_pair_t[]){
+                       (fd_stream_pair_t[]) {
                            {STDIN_FILENO, stdin},
                            {STDOUT_FILENO, stdout},
                            {STDERR_FILENO, stderr},
@@ -434,6 +434,15 @@ riscv_t *rv_create(riscv_user_t rv_attr)
     if ((end = elf_get_symbol(elf, "_end")))
         attr->break_addr = end->st_value;
 
+#if !RV32_HAS(SYSTEM)
+    /* set not exiting */
+    attr->on_exit = false;
+
+    const struct Elf32_Sym *exit;
+    if ((exit = elf_get_symbol(elf, "exit")))
+        attr->exit_addr = exit->st_value;
+#endif
+
     assert(elf_load(elf, attr->mem));
 
     /* set the entry pc */

src/riscv.h

@@ -548,6 +548,14 @@ typedef struct {
     /* the data segment break address */
     riscv_word_t break_addr;
 
+#if !RV32_HAS(SYSTEM)
+    /* the exit entry address */
+    riscv_word_t exit_addr;
+
+    /* flag to determine if the emulator exits the target program */
+    bool on_exit;
+#endif
+
     /* SBI timer */
     uint64_t timer;
 } vm_attr_t;

src/syscall.c

@@ -230,6 +230,22 @@ static void syscall_close(riscv_t *rv)
     /* _close(fd); */
     uint32_t fd = rv_get_reg(rv, rv_reg_a0);
 
+#if !RV32_HAS(SYSTEM)
+    /*
+     * The crt0 closes standard file descriptor(0, 1, 2) when
+     * the process exits. Thus, the operations by the crt0
+     * should not considered as error.
+     */
+    if (fd < 3 && !PRIV(rv)->on_exit) {
+        rv_set_reg(rv, rv_reg_a0, -1);
+        rv_log_error(
+            "Attempted to close a file descriptor < 3 (fd=%u). Operation "
+            "not supported.",
+            fd);
+        return;
+    }
+#endif
+
     if (fd >= 3) { /* lookup the file descriptor */
         map_iter_t it;
         map_find(attr->fd_map, &it, &fd);
@@ -245,9 +261,6 @@ static void syscall_close(riscv_t *rv)
             rv_set_reg(rv, rv_reg_a0, 0);
         }
     }
-
-    /* success */
-    rv_set_reg(rv, rv_reg_a0, 0);
 }
 
 /* lseek() repositions the file offset of the open file description associated