Re: [PATCH] driver core: faux: fix Undefined Behavior in faux_device_destroy()

[Marc Herbert]

On 2025-06-25 17:55, Kent Overstreet wrote:
> the big con:
> - they interact badly with gotos, you can get undefined behaviour from
>   using a variable that wasn't actually defined _and the compiler will
>   not warn you_
> [...]
> But the issue with gotos is worth highlighting. Be careful when using
> them in code that hasn't been converted to __cleanup.

Thanks Kent for sharing this.

I got curious and found that clang -Wall is actually able to warn,
at least in simple cases:

int goto_uninitialized_C99(int *ptr)
{
  if (!ptr)
    goto cleanup;
  const int i = 42;

cleanup:
  // clang warning, no gcc warning
  printf("fin: i=%d\n", i);


warning: variable 'i' is used uninitialized whenever 'if' condition
   is true [-Wsometimes-uninitialized]


gcc -Wall -Wextra does not say anything.
Tested with clang version 18.1.3 and gcc 13.3.0


Interestingly, there is no warning difference between C89 and C99 code
for such a simple example. gcc warns for neither C89 code nor C99 code
and clang warns for both.

int goto_uninitialized_C89(int *ptr)
{
  int i;
  if (!ptr)
    goto cleanup;
  i = 42

cleanup:
  /* clang warning, no gcc warning */
  printf("fin: i=%d\n", i);


(finally getting rid of gotos is one of the main purposes of RAII)