fix(prompt): heap-buffer-overflow in prompt_setprompt

Problem: prompt_setprompt may check the wrong buffer, which can lead to a
heap-buffer-overflow.

Solution: don't use curbuf.

Also replace all kCallbackNone initializers with CALLBACK_INIT.

src/nvim/eval/buffer.c

@@ -712,7 +712,7 @@ void restore_buffer(bufref_T *save_curbuf)
 /// "prompt_setcallback({buffer}, {callback})" function
 void f_prompt_setcallback(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
-  Callback prompt_callback = { .type = kCallbackNone };
+  Callback prompt_callback = CALLBACK_INIT;
 
   if (check_secure()) {
     return;
@@ -735,7 +735,7 @@ void f_prompt_setcallback(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 /// "prompt_setinterrupt({buffer}, {callback})" function
 void f_prompt_setinterrupt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
-  Callback interrupt_callback = { .type = kCallbackNone };
+  Callback interrupt_callback = CALLBACK_INIT;
 
   if (check_secure()) {
     return;
@@ -772,10 +772,8 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   // Update the prompt-text and prompt-marks if a plugin calls prompt_setprompt()
   // even while user is editing their input.
   if (bt_prompt(buf)) {
-    if (buf->b_prompt_start.mark.lnum > buf->b_ml.ml_line_count) {
+    // In case the mark is set to a nonexistent line.
-      // In case the mark is set to a nonexistent line.
+    buf->b_prompt_start.mark.lnum = MIN(buf->b_prompt_start.mark.lnum, buf->b_ml.ml_line_count);
-      buf->b_prompt_start.mark.lnum = buf->b_ml.ml_line_count;
-    }
 
     linenr_T prompt_lno = buf->b_prompt_start.mark.lnum;
     char *old_prompt = buf_prompt_text(buf);
@@ -786,8 +784,7 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     colnr_T cursor_col = curwin->w_cursor.col;
 
     if (buf->b_prompt_start.mark.col < old_prompt_len
-        || curbuf->b_prompt_start.mark.col < old_prompt_len
+        || !strnequal(old_prompt, old_line + buf->b_prompt_start.mark.col - old_prompt_len,
-        || !strnequal(old_prompt, old_line + curbuf->b_prompt_start.mark.col - old_prompt_len,
                       (size_t)old_prompt_len)) {
       // If for some odd reason the old prompt is missing,
       // replace prompt line with new-prompt (discards user-input).

src/nvim/ex_getln.c

@@ -4883,7 +4883,7 @@ void get_user_input(const typval_T *const argvars, typval_T *const rettv, const
   typval_T *cancelreturn = NULL;
   typval_T cancelreturn_strarg2 = TV_INITIAL_VALUE;
   const char *xp_name = NULL;
-  Callback input_callback = { .type = kCallbackNone };
+  Callback input_callback = CALLBACK_INIT;
   char prompt_buf[NUMBUFLEN];
   char defstr_buf[NUMBUFLEN];
   char cancelreturn_buf[NUMBUFLEN];

test/functional/legacy/prompt_buffer_spec.lua

@@ -798,8 +798,8 @@ describe('prompt buffer', function()
     api.nvim_set_option_value('buftype', 'prompt', { buf = 0 })
     local buf = api.nvim_get_current_buf()
 
-    local function set_prompt(prompt)
+    local function set_prompt(prompt, b)
-      fn('prompt_setprompt', buf, prompt)
+      fn('prompt_setprompt', b or buf, prompt)
     end
 
     set_prompt('> ')
@@ -846,5 +846,20 @@ describe('prompt buffer', function()
       {5:-- INSERT --}             |
     ]])
     eq({ 1, 13 }, api.nvim_buf_get_mark(0, ':'))
+
+    -- No crash when setting shorter prompt than curbuf's in other buffer.
+    command('new | setlocal buftype=prompt')
+    set_prompt('looooooooooooooooooooooooooooooooooooooooooooong > ', '') -- curbuf
+    set_prompt('foo > ')
+    screen:expect([[
+      loooooooooooooooooooooooo|
+      ooooooooooooooooooooong >|
+       ^                        |
+      {1:~                        }|
+      {3:[Prompt] [+]             }|
+      foo > user input         |
+      {1:~                        }|*3
+      {5:-- INSERT --}             |
+    ]])
   end)
 end)