Bug #19028
Updated by eightbitraptor (Matthew Valentine-House) over 1 year ago
[GCC 12 introduced a new warning flag `-Wuse-after-free`](https://github.com/gcc-mirror/gcc/commit/671a283636de75f7ed638ee6b01ed2d44361b8b6) which attempts to warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function
Details of the levels are in [the C++ Dialect Options section of the GCC documentation](https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html).
Compiling with `-Wall` uses the default setting of `2` for this flag. Which warns on the `TRY_WITH_GC` macro defined in `gc.c`
```
gc.c: In function ‘objspace_xrealloc’:
gc.c:12213:33: warning: pointer ‘ptr’ may be used after ‘realloc’ [-Wuse-after-free]
12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size));
| ^~~~~~~~~~~~~~~~~~~~~~
gc.c:12123:19: note: in definition of macro ‘TRY_WITH_GC’
12123 | else if ((expr)) { \
| ^~~~
In file included from ./include/ruby/defines.h:72,
from ./include/ruby/ruby.h:25,
from constant.h:13,
from gc.c:97:
gc.c:12213:33: note: call to ‘realloc’ here
12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size));
| ^~~~~~~~~~~~~~~~~~~~~~
./include/ruby/backward/2/assume.h:43:46: note: in definition of macro ‘RB_LIKELY’
43 | # define RB_LIKELY(x) (__builtin_expect(!!(x), 1))
| ^
gc.c:12116:13: note: in expansion of macro ‘LIKELY’
12116 | if (LIKELY((expr))) { \
| ^~~~~~
gc.c:12213:5: note: in expansion of macro ‘TRY_WITH_GC’
12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size));
| ^~~~~~~~~~~
```
My understanding is that if `realloc` returns a null pointer then the memory requested for reallocation is guaranteed to not be touched (according to [the Open Group](https://pubs.opengroup.org/onlinepubs/9699919799/functions/realloc.html) - thank you @nobu for bringing this to my attention).
Given that this is a new warning, my proposed solution is to lower the level down to the base level `1`. This will only warn on unconditional calls to deallocation functions or successful calls to realloc.
I've opened a PR that sets `-Wuse-after-free=1` only for GCC versions > 11
[Github PR #6465](https://github.com/ruby/ruby/pull/6465)