diff mbox

Silence -O3 -Wall warning in malloc/hooks.c with GCC 7 [BZ #22052]

Message ID CAMe9rOqvpS_HH6_k=rPuE3qx6oifjL8_WKzTVFhK9eZs84NvFA@mail.gmail.com
State Committed
Headers

Commit Message

H.J. Lu Sept. 3, 2017, 3:48 p.m. UTC 
  On Thu, Aug 31, 2017 at 10:01 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 08/31/2017 05:55 AM, H.J. Lu wrote:
>
>> One of malloc change caused:
>>
>> https://sourceware.org/bugzilla/show_bug.cgi?id=22052
>
> (uninit warning with -O3)
>
> I have not yet found a good way to suppress this. I don't know yet why
> GCC loses the information that the maybe_p variable is initialized if
> mem2chunk_check returns a non-null pointer.
>

This patch works for me.   OK for master?

Comments

H.J. Lu Sept. 3, 2017, 4:29 p.m. UTC | #1 
On Sun, Sep 3, 2017 at 8:48 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Aug 31, 2017 at 10:01 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 08/31/2017 05:55 AM, H.J. Lu wrote:
>>
>>> One of malloc change caused:
>>>
>>> https://sourceware.org/bugzilla/show_bug.cgi?id=22052
>>
>> (uninit warning with -O3)
>>
>> I have not yet found a good way to suppress this. I don't know yet why
>> GCC loses the information that the maybe_p variable is initialized if
>> mem2chunk_check returns a non-null pointer.
>>
>
> This patch works for me.   OK for master?
>

FYI, I opened a GCC bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82090
Florian Weimer Sept. 4, 2017, 8:33 a.m. UTC | #2 
On 09/03/2017 05:48 PM, H.J. Lu wrote:
> +  unsigned char *magic_p = NULL;

I think the current practice is to use <libc-diag.h> to suppress the
warning.

Is the issue that the memory clobber on the inline asm makes GCC not to
see the conditional initialization?

Thanks,
Florian
H.J. Lu Sept. 4, 2017, 8:29 p.m. UTC | #3 
On Mon, Sep 4, 2017 at 1:33 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 09/03/2017 05:48 PM, H.J. Lu wrote:
>> +  unsigned char *magic_p = NULL;
>
> I think the current practice is to use <libc-diag.h> to suppress the
> warning.

Is there a usage example for this warning?

> Is the issue that the memory clobber on the inline asm makes GCC not to
> see the conditional initialization?

Possible.
Florian Weimer Sept. 18, 2017, 2:02 p.m. UTC | #4 
On 09/04/2017 10:29 PM, H.J. Lu wrote:
> On Mon, Sep 4, 2017 at 1:33 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 09/03/2017 05:48 PM, H.J. Lu wrote:
>>> +  unsigned char *magic_p = NULL;
>>
>> I think the current practice is to use <libc-diag.h> to suppress the
>> warning.
> 
> Is there a usage example for this warning?

Search for DIAG_PUSH_NEEDS_COMMENT in the sources.

Thanks,
Florian
diff mbox

Patch

From 735efe5ebc3eb6aa877b00b10ac0046524cee6e3 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sun, 3 Sep 2017 08:39:55 -0700
Subject: [PATCH] Silence -O3 -Wall warning in malloc/hooks.c with GCC 7 [BZ
 #22052]
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

realloc_check has

  unsigned char *magic_p;
...
  __libc_lock_lock (main_arena.mutex);
  const mchunkptr oldp = mem2chunk_check (oldmem, &magic_p);
  __libc_lock_unlock (main_arena.mutex);
  if (!oldp)
    malloc_printerr ("realloc(): invalid pointer");
...
  if (newmem == NULL)
    *magic_p ^= 0xFF;

with

static void malloc_printerr(const char *str) __attribute__ ((noreturn));

GCC 7 -O3 warns

hooks.c: In function ‘realloc_check’:
hooks.c:352:14: error: ‘magic_p’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     *magic_p ^= 0xFF;

This patch silences GCC 7 by initializing magic_p to NULL.

	[BZ #22052]
	* malloc/hooks.c (realloc_check): Initialize magic_p to NULL.
---
 malloc/hooks.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/malloc/hooks.c b/malloc/hooks.c
index 01be076f5e..03bc086901 100644
--- a/malloc/hooks.c
+++ b/malloc/hooks.c
@@ -287,7 +287,7 @@  realloc_check (void *oldmem, size_t bytes, const void *caller)
 {
   INTERNAL_SIZE_T nb;
   void *newmem = 0;
-  unsigned char *magic_p;
+  unsigned char *magic_p = NULL;
 
   if (bytes + 1 == 0)
     {
-- 
2.13.5