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* [Bug c/63233] New: Valid out of bounds access leads to undefined behavior
@ 2014-09-11 21:15 leis at in dot tum.de
2014-09-11 21:29 ` [Bug c/63233] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: leis at in dot tum.de @ 2014-09-11 21:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
Bug ID: 63233
Summary: Valid out of bounds access leads to undefined behavior
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: leis at in dot tum.de
Consider the following (minimized) program:
#include <stdlib.h>
#include <stdio.h>
struct Foo {
int a[1];
int b;
};
int main(int argc, char** argv) {
int index = atoi(argv[1]);
struct Foo foo;
foo.a[1] = 99;
printf("%d\n", foo.a[index]);
return 0;
}
When compiled with -O1 or higher and called with 1 as command line argument an
undefined value instead of 99 is printed. In my understanding of the standard,
foo.a is a pointer and foo.a[1] is simply (foo.a+1), which is a perfectly fine
memory address.
This program always prints the expected value (99) with clang (3.5) and icc
(14) on all optimization levels. In gcc 4.3.4 I get the expected result on -O0
and -O1 but not on -O2 or -O3.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/63233] Valid out of bounds access leads to undefined behavior
2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
@ 2014-09-11 21:29 ` pinskia at gcc dot gnu.org
2014-09-11 22:02 ` [Bug c/63233] Missing Warray-bounds warning for array within struct manu at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2014-09-11 21:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is not a valid out of bounds access in C.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/63233] Missing Warray-bounds warning for array within struct
2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
2014-09-11 21:29 ` [Bug c/63233] " pinskia at gcc dot gnu.org
@ 2014-09-11 22:02 ` manu at gcc dot gnu.org
2014-09-11 22:14 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: manu at gcc dot gnu.org @ 2014-09-11 22:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |diagnostic
Status|RESOLVED |NEW
Last reconfirmed| |2014-09-11
CC| |manu at gcc dot gnu.org
Resolution|INVALID |---
Summary|Valid out of bounds access |Missing Warray-bounds
|leads to undefined behavior |warning for array within
| |struct
Ever confirmed|0 |1
--- Comment #2 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
This should be warned by Warray-bounds. Clang warns even at -O0, so this case
can even be warned in the FE:
test.c:10:3: warning: array index 1 is past the end of the array (which
contains 1 element) [-Warray-bounds]
foo.a[1] = 99;
^ ~
test.c:2:3: note: array 'a' declared here
int a[1];
^
test.c:12:10: warning: array index 1 is past the end of the array (which
contains 1 element) [-Warray-bounds]
return foo.a[1];
^ ~
test.c:2:3: note: array 'a' declared here
int a[1];
^
>From gcc-bugs-return-461624-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Sep 11 22:11:35 2014
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From: "manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/63233] Missing Warray-bounds warning for array within struct
Date: Thu, 11 Sep 2014 22:11:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> This is not a valid out of bounds access in C.
Sometime it might be better to acknowledge where GCC could help rather than
simply close the bug saying to users "you are wrong". Who knows, some user may
decide to implement a fix themselves. We all know that GCC is in need of all
the help it can get to not be completely left behind.
>From gcc-bugs-return-461625-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Sep 11 22:12:37 2014
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From: "pangbw at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/63234] New: arm used label is removed
Date: Thu, 11 Sep 2014 22:12:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc234
Bug ID: 63234
Summary: arm used label is removed
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pangbw at gmail dot com
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/63233] Missing Warray-bounds warning for array within struct
2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
2014-09-11 21:29 ` [Bug c/63233] " pinskia at gcc dot gnu.org
2014-09-11 22:02 ` [Bug c/63233] Missing Warray-bounds warning for array within struct manu at gcc dot gnu.org
@ 2014-09-11 22:14 ` pinskia at gcc dot gnu.org
2014-09-11 22:26 ` leis at in dot tum.de
2014-09-13 9:36 ` mikpelinux at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2014-09-11 22:14 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |DUPLICATE
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
We don't warn about off by one errors in the array because of different
reasons.
Dup of bug 52954 then.
*** This bug has been marked as a duplicate of bug 52954 ***
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/63233] Missing Warray-bounds warning for array within struct
2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
` (2 preceding siblings ...)
2014-09-11 22:14 ` pinskia at gcc dot gnu.org
@ 2014-09-11 22:26 ` leis at in dot tum.de
2014-09-13 9:36 ` mikpelinux at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: leis at in dot tum.de @ 2014-09-11 22:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
--- Comment #5 from leis at in dot tum.de ---
Thanks. I did notice that I get warning with optimization but not without,
which is strange indeed.
However, obviously my real problem is more complicated, and I'm trying to
understand if I violate the standard. If I pick apart the offending
line (foo.a[1] = 99;):
int* p1 = foo.a; //1
int* p2 = p1 + 1; //2
*p2 = 99; //3
Which of these lines causes the undefined behavior? Why should pointer p2 be
invalid?
Fundamentally, what I'm really trying to do, is to have two arrays (of
different types) in a fixed-sized struct. One array grows from the front, and
one from the end. Dynamically I make sure that they do not overlap, but the
sizes of the two arrays are not known statically. Is it really violating the
standard?
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/63233] Missing Warray-bounds warning for array within struct
2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
` (3 preceding siblings ...)
2014-09-11 22:26 ` leis at in dot tum.de
@ 2014-09-13 9:36 ` mikpelinux at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: mikpelinux at gmail dot com @ 2014-09-13 9:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233
--- Comment #6 from Mikael Pettersson <mikpelinux at gmail dot com> ---
(In reply to leis from comment #5)
> Fundamentally, what I'm really trying to do, is to have two arrays (of
> different types) in a fixed-sized struct. One array grows from the front,
> and one from the end. Dynamically I make sure that they do not overlap, but
> the sizes of the two arrays are not known statically. Is it really violating
> the standard?
Something like
union u {
int a[50];
char b[50 * sizeof int];
};
and accessing one array with incrementing indices and the other with
decrementing indices should work, if I understand your problem statement
correctly. But this is now about programming not a gcc bug, so please use
gcc-help or some general C programming forum instead.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2014-09-11 21:15 [Bug c/63233] New: Valid out of bounds access leads to undefined behavior leis at in dot tum.de
2014-09-11 21:29 ` [Bug c/63233] " pinskia at gcc dot gnu.org
2014-09-11 22:02 ` [Bug c/63233] Missing Warray-bounds warning for array within struct manu at gcc dot gnu.org
2014-09-11 22:14 ` pinskia at gcc dot gnu.org
2014-09-11 22:26 ` leis at in dot tum.de
2014-09-13 9:36 ` mikpelinux at gmail dot com
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