fix(userfunc): fix possible out of bound access

In file included from /usr/include/string.h:535,
                 from gsrc/nvim/eval/userfunc.c:11:
In function ‘strcpy’,
    inlined from ‘cat_func_name’ at gsrc/nvim/eval/userfunc.c:662:5,
    inlined from ‘get_user_func_name’ at gsrc/nvim/eval/userfunc.c:2854:5:
/usr/include/bits/string_fortified.h:79:10: warning: ‘__builtin___strcpy_chk’ offset 0 from the object at ‘<unknown>’ is out of the bounds of referenced subobject ‘uf_name’ with ty
pe ‘char[]’ at offset 0 [-Warray-bounds=]
   79 |   return __builtin___strcpy_chk (__dest, __src, __glibc_objsize (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from gsrc/nvim/eval/typval.h:10,
                 from gsrc/nvim/buffer_defs.h:20,
                 from gsrc/nvim/autocmd.h:8,
                 from gsrc/nvim/eval/userfunc.c:15:
gsrc/nvim/eval/typval_defs.h: In function ‘get_user_func_name’:
gsrc/nvim/eval/typval_defs.h:342:8: note: subobject ‘uf_name’ declared here
  342 |   char uf_name[];    ///< Name of function (actual size equals name);
      |        ^~~~~~~

1 files changed, 12 insertions, 6 deletions

diff --git a/src/nvim/eval/userfunc.c b/src/nvim/eval/userfunc.c
index 51e109fdfb..b71e6c9cff 100644
--- a/src/nvim/eval/userfunc.c
+++ b/src/nvim/eval/userfunc.c
@@ -653,14 +653,20 @@ ufunc_T *find_func(const char *name)
 /// Copy the function name of "fp" to buffer "buf".
 /// "buf" must be able to hold the function name plus three bytes.
 /// Takes care of script-local function names.
-static void cat_func_name(char *buf, ufunc_T *fp)
+static void cat_func_name(char *buf, size_t buflen, ufunc_T *fp)
 {
-  if ((uint8_t)fp->uf_name[0] == K_SPECIAL) {
-    STRCPY(buf, "<SNR>");
-    STRCAT(buf, fp->uf_name + 3);
+  int len = -1;
+  size_t uflen = strlen(fp->uf_name);
+  assert(uflen > 0);
+
+  if ((uint8_t)fp->uf_name[0] == K_SPECIAL && uflen > 3) {
+    len = snprintf(buf, buflen, "<SNR>%s", fp->uf_name + 3);
   } else {
-    STRCPY(buf, fp->uf_name);
+    len = snprintf(buf, buflen, "%s", fp->uf_name);
   }
+
+  (void)len;  // Avoid unused warning on release builds
+  assert(len > 0);
 }
 
 /// Add a number variable "name" to dict "dp" with value "nr".
@@ -2851,7 +2857,7 @@ char *get_user_func_name(expand_T *xp, int idx)
       return fp->uf_name;  // Prevent overflow.
     }
 
-    cat_func_name(IObuff, fp);
+    cat_func_name(IObuff, IOSIZE, fp);
     if (xp->xp_context != EXPAND_USER_FUNC) {
       STRCAT(IObuff, "(");
       if (!fp->uf_varargs && GA_EMPTY(&fp->uf_args)) {