Bug #19028
closedGCC12 Introduces new warn flags `-Wuse-after-free`
Description
GCC 12 introduced a new warning flag -Wuse-after-free
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.
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 - thank you @nobu (Nobuyoshi Nakada) 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. An alternative approach might be to use #pragma GCC diagnostic
to suppress this just for GCC but I opted for what I thought was the easiest fix to start with.