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* [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1
@ 2022-12-20 14:33 i.maximets at ovn dot org
2022-12-21 7:53 ` [Bug tree-optimization/108187] " rguenth at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: i.maximets at ovn dot org @ 2022-12-20 14:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108187
Bug ID: 108187
Summary: False positive -Wfree-nonheap-object on impossible
path with -O1
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: i.maximets at ovn dot org
Target Milestone: ---
Created attachment 54132
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54132&action=edit
Reproducer
This might be the same issue as 98753, but I'm not sure.
We're getting the following error while trying to build Open vSwitch
with AF_XDP support with GCC:
```
In file included from lib/netdev-linux-private.h:30,
from lib/netdev-afxdp.c:19:
In function ‘dp_packet_delete’,
inlined from ‘dp_packet_delete’ at lib/dp-packet.h:246:1,
inlined from ‘dp_packet_batch_add__’ at lib/dp-packet.h:775:9,
inlined from ‘dp_packet_batch_add’ at lib/dp-packet.h:783:5,
inlined from ‘netdev_afxdp_rxq_recv’ at lib/netdev-afxdp.c:898:9:
lib/dp-packet.h:260:9: warning: ‘free’ called on pointer ‘*umem.xpool.array’
with nonzero offset [8, 2558044588346441168] [-Wfree-nonheap-object]
260 | free(b);
| ^~~~~~~
```
The simplified code flow is following:
```
packet = &umem->xpool.array[index];
packet = &xpacket->packet;
dp_packet_use_afxdp(packet, pkt, FRAME_SIZE - FRAME_HEADROOM,
OVS_XDP_HEADROOM);
--> dp_packet_init__(packet, allocated, DPBUF_AFXDP);
--> packet->source = source;
dp_packet_batch_add(batch, packet);
--> dp_packet_delete(packet);
--> if (b->source == DPBUF_AFXDP) {
free_afxdp_buf(b);
return;
}
dp_packet_uninit(b);
free(b);
```
The 'b->source' is always set unconditionally to DPBUF_AFXDP on that path,
so the free(b) cannot be reached, but compiler doesn't think so.
The issue is seen starting with -O1.
$ gcc --version
gcc (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4)
Attached the file in question after -E. To reproduce run:
$ gcc -O1 -c netdev-afxdp.E.c
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108187] False positive -Wfree-nonheap-object on impossible path with -O1
2022-12-20 14:33 [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1 i.maximets at ovn dot org
@ 2022-12-21 7:53 ` rguenth at gcc dot gnu.org
2022-12-21 10:44 ` i.maximets at ovn dot org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-12-21 7:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108187
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, between the store to ->source and the read from it is the call
to dp_packet_use_afxdp which gets &xpacket->packet as argument and thus
needs to be treated as clobbering ->source. So GCC can indeed not know
that ->source is DPBUF_AFXDP since the path is not provable impossible.
dp_packet_use_afxdp doesn't even get a const struct dp_packet * argument
(not that this would semantically change things in C).
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108187] False positive -Wfree-nonheap-object on impossible path with -O1
2022-12-20 14:33 [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1 i.maximets at ovn dot org
2022-12-21 7:53 ` [Bug tree-optimization/108187] " rguenth at gcc dot gnu.org
@ 2022-12-21 10:44 ` i.maximets at ovn dot org
2022-12-21 11:51 ` i.maximets at ovn dot org
2022-12-21 18:05 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: i.maximets at ovn dot org @ 2022-12-21 10:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108187
--- Comment #2 from Ilya Maximets <i.maximets at ovn dot org> ---
(In reply to Richard Biener from comment #1)
> Well, between the store to ->source and the read from it is the call
> to dp_packet_use_afxdp which gets &xpacket->packet as argument and thus
> needs to be treated as clobbering ->source. So GCC can indeed not know
> that ->source is DPBUF_AFXDP since the path is not provable impossible.
> dp_packet_use_afxdp doesn't even get a const struct dp_packet * argument
> (not that this would semantically change things in C).
Hmm, dp_packet_use_afxdp() is the function that sets source to DPBUF_AFXDP
and initializes other parts of the structure. So, it cannot take a const
argument. If GCC just doesn't look inside the dp_packet_use_afxdp() function
at all here, then it will indeed not know that the source is DPBUF_AFXDP now.
However, I'm not sure why the issue doesn't appear with -O0 then.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108187] False positive -Wfree-nonheap-object on impossible path with -O1
2022-12-20 14:33 [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1 i.maximets at ovn dot org
2022-12-21 7:53 ` [Bug tree-optimization/108187] " rguenth at gcc dot gnu.org
2022-12-21 10:44 ` i.maximets at ovn dot org
@ 2022-12-21 11:51 ` i.maximets at ovn dot org
2022-12-21 18:05 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: i.maximets at ovn dot org @ 2022-12-21 11:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108187
--- Comment #3 from Ilya Maximets <i.maximets at ovn dot org> ---
(In reply to Ilya Maximets from comment #2)
> (In reply to Richard Biener from comment #1)
> > Well, between the store to ->source and the read from it is the call
> > to dp_packet_use_afxdp which gets &xpacket->packet as argument and thus
> > needs to be treated as clobbering ->source. So GCC can indeed not know
> > that ->source is DPBUF_AFXDP since the path is not provable impossible.
> > dp_packet_use_afxdp doesn't even get a const struct dp_packet * argument
> > (not that this would semantically change things in C).
>
> Hmm, dp_packet_use_afxdp() is the function that sets source to DPBUF_AFXDP
> and initializes other parts of the structure. So, it cannot take a const
> argument. If GCC just doesn't look inside the dp_packet_use_afxdp() function
> at all here, then it will indeed not know that the source is DPBUF_AFXDP now.
Clarification: I realized that dp_packet_use_afxdp() is part of a different
translation unit, so GCC doesn't have a chance to know what this function is
doing, hence it doesn't know that source is DPBUF_AFXDP. Though I don't know
how we can change that code to make GCC happy. We'll likely end up just
disabling a warning.
> However, I'm not sure why the issue doesn't appear with -O0 then.
I'm still not sure why this is happening though. Is there something
special about -O0 ?
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108187] False positive -Wfree-nonheap-object on impossible path with -O1
2022-12-20 14:33 [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1 i.maximets at ovn dot org
` (2 preceding siblings ...)
2022-12-21 11:51 ` i.maximets at ovn dot org
@ 2022-12-21 18:05 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-12-21 18:05 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108187
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Ilya Maximets from comment #3)
>
> Clarification: I realized that dp_packet_use_afxdp() is part of a different
> translation unit, so GCC doesn't have a chance to know what this function is
> doing, hence it doesn't know that source is DPBUF_AFXDP. Though I don't know
> how we can change that code to make GCC happy. We'll likely end up just
> disabling a warning.
>
> > However, I'm not sure why the issue doesn't appear with -O0 then.
>
> I'm still not sure why this is happening though. Is there something
> special about -O0 ?
Yes the warning code only runs with optimization turned on ...
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-12-21 18:05 UTC | newest]
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2022-12-20 14:33 [Bug c/108187] New: False positive -Wfree-nonheap-object on impossible path with -O1 i.maximets at ovn dot org
2022-12-21 7:53 ` [Bug tree-optimization/108187] " rguenth at gcc dot gnu.org
2022-12-21 10:44 ` i.maximets at ovn dot org
2022-12-21 11:51 ` i.maximets at ovn dot org
2022-12-21 18:05 ` pinskia at gcc dot gnu.org
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