Re: ping//Re: [PATCH v2] io:nftw/ftw:fix stack overflow when large nopenfd [BZ #26353]

[Xiaoming Ni <nixiaoming@huawei.com>]

On 2020/8/23 2:09, Paul Eggert wrote:
> On 8/21/20 8:27 PM, Xiaoming Ni wrote:
>> How do I determine whether data.maxdir is too large for alloca?
> 
> __libc_use_alloca. Also see include/scratch_buffer.h, which is designed 
> for this sort of situation.
> 
> .

is that ?

--- a/io/ftw.c
+++ b/io/ftw.c
@@ -645,6 +645,13 @@ ftw_startup (const char *dir, int is_nftw, void 
*func, int descriptors,
      }

    data.maxdir = descriptors < 1 ? 1 : descriptors;
+  if ((__glibc_unlikely (data.maxdir > SIZE_MAX / sizeof (struct 
dir_data *)))
+      || (! __libc_use_alloca (data.maxdir * sizeof (struct dir_data *))))
+    {
+      __set_errno (EINVAL);
+      return -1;
+    }
+
    data.actdir = 0;
    data.dirstreams = (struct dir_data **) alloca (data.maxdir
                                                  * sizeof (struct 
dir_data *));


Whether to use malloc or return an error message when the input is too 
large?

I still don't understand why libc uses alloca so much.
Is it for performance?

Thanks
Xiaoming Ni