While the current code builds fine with gcc 11, it does not with gcc 12,
resulting in:
In file included from help.c:12:
In function 'xrealloc',
inlined from 'add_cmdname' at help.c:24:2:
subcmd-util.h:56:23: error: pointer may be used after 'realloc' [-Werror=use-after-free]
56 | ret = realloc(ptr, size);
| ^~~~~~~~~~~~~~~~~~
subcmd-util.h:52:21: note: call to 'realloc' here
52 | void *ret = realloc(ptr, size);
| ^~~~~~~~~~~~~~~~~~
subcmd-util.h:58:31: error: pointer may be used after 'realloc' [-Werror=use-after-free]
58 | ret = realloc(ptr, 1);
| ^~~~~~~~~~~~~~~
subcmd-util.h:52:21: note: call to 'realloc' here
52 | void *ret = realloc(ptr, size);
| ^~~~~~~~~~~~~~~~~~
The was mentioned in upstream gcc bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069 where it was
determined that gcc was correct and the kernel needed to change. This
fixes that use-after-free and makes things build again.
Signed-off-by: Justin M. Forbes <[email protected]>
Cc: Jakub Jelinek <[email protected]>
---
tools/lib/subcmd/subcmd-util.h | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/tools/lib/subcmd/subcmd-util.h b/tools/lib/subcmd/subcmd-util.h
index 794a375dad36..7009fc176636 100644
--- a/tools/lib/subcmd/subcmd-util.h
+++ b/tools/lib/subcmd/subcmd-util.h
@@ -49,13 +49,12 @@ static NORETURN inline void die(const char *err, ...)
static inline void *xrealloc(void *ptr, size_t size)
{
- void *ret = realloc(ptr, size);
- if (!ret && !size)
- ret = realloc(ptr, 1);
+ void *ret;
+ if (!size)
+ size = 1;
+ ret = realloc(ptr, size);
if (!ret) {
ret = realloc(ptr, size);
- if (!ret && !size)
- ret = realloc(ptr, 1);
if (!ret)
die("Out of memory, realloc failed");
}
--
2.34.1
On Fri, Jan 28, 2022 at 07:02:14PM -0600, Justin M. Forbes wrote:
> While the current code builds fine with gcc 11, it does not with gcc 12,
> resulting in:
>
> In file included from help.c:12:
> In function 'xrealloc',
> inlined from 'add_cmdname' at help.c:24:2:
> subcmd-util.h:56:23: error: pointer may be used after 'realloc' [-Werror=use-after-free]
> 56 | ret = realloc(ptr, size);
> | ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
> 52 | void *ret = realloc(ptr, size);
> | ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:58:31: error: pointer may be used after 'realloc' [-Werror=use-after-free]
> 58 | ret = realloc(ptr, 1);
> | ^~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
> 52 | void *ret = realloc(ptr, size);
> | ^~~~~~~~~~~~~~~~~~
>
> The was mentioned in upstream gcc bug
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069 where it was
> determined that gcc was correct and the kernel needed to change.
This is due to the different behaviors of realloc when size is 0 as
described in http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_400
and as the code explicitly tries to cope with !size, there is at least
theoretical chance that it is called with size 0.
In glibc/AIX, for non-NULL ptr realloc(ptr, 0) will free(ptr) and return
NULL, so the code as written in that case will in
void *ret = realloc(ptr, 0);
if (!ret && !0)
ret = realloc(ptr, 1);
effectively
free(ptr);
ret = realloc(ptr, 1);
and so try to reallocate freed memory. On BSD libcs it will work properly
though.
Jakub