Re: gcc-4.0.0 final miscompiles net/ipv4/devinet.c:devinet_sysctl_register()

[ismail dönmez]

Whats the bugzilla # for this so others can track it?


On 4/23/05, Mikael Pettersson <mikpe@xxxxxxxxx> wrote:
> gcc-4.0.0 miscompiles a pointer subtraction operation in
> net/ipv4/devinet.c, resulting in oopses from /sbin/sysctl.
> 
> Below is a copy of the test case I just sent to gcc bugzilla.
> 
> /Mikael
> 
> /* gcc4pointersubtractionbug.c
>  * Written by Mikael Pettersson, mikpe@xxxxxxxxx, 2005-04-23.
>  *
>  * This program illustrates a code optimisation bug in
>  * gcc-4.0.0 (final) and gcc-4.0.0-20050417, where a pointer
>  * subtraction operation is compiled as a pointer addition.
>  * Observed at -O2. gcc was configured for i686-pc-linux-gnu.
>  *
>  * This bug broke net/ipv4/devinet.c:devinet_sysctl_register()
>  * in the linux-2.6.12-rc2 Linux kernel, causing /sbin/sysctl
>  * to trigger kernel oopses.
>  *
>  * gcc-4.0.0-20050416 and earlier prereleases do not have this bug.
>  */
> #include <stdio.h>
> #include <string.h>
> 
> #define NRVARS	5
> 
> struct ipv4_devconf {
>     int var[NRVARS];
> };
> struct ipv4_devconf ipv4_devconf[2];
> 
> struct ctl_table {
>     void *data;
> };
> 
> struct devinet_sysctl_table {
>     struct ctl_table devinet_vars[NRVARS];
> };
> 
> void devinet_sysctl_relocate(struct devinet_sysctl_table *t,
> 			     struct ipv4_devconf *p)
> {
>     int i;
> 
>     for (i = 0; i < NRVARS; i++)
> 	/* Initially data points to a field in ipv4_devconf[0].
> 	   This code relocates it to the corresponding field in *p.
> 	   At -O2, gcc-4.0.0-20050417 and gcc-4.0.0 (final)
> 	   miscompile this pointer subtraction as a pointer addition. */
> 	t->devinet_vars[i].data += (char *)p - (char *)&ipv4_devconf[0];
> }
> 
> struct devinet_sysctl_table devinet_sysctl;
> 
> int main(void)
> {
>     struct devinet_sysctl_table t;
>     int i;
> 
>     for(i = 0; i < NRVARS; i++)
> 	devinet_sysctl.devinet_vars[i].data = &ipv4_devconf[0].var[i];
> 
>     memcpy(&t, &devinet_sysctl, sizeof t);
>     devinet_sysctl_relocate(&t, &ipv4_devconf[1]);
> 
>     for(i = 0; i < NRVARS; i++)
> 	if (t.devinet_vars[i].data != &ipv4_devconf[1].var[i]) {
> 	    fprintf(stderr, "t.devinet_vars[%u].data == %p, should be %p\n",
> 		    i,
> 		    t.devinet_vars[i].data,
> 		    &ipv4_devconf[1].var[i]);
> 	    return 1;
> 	}
> 
>     printf("all ok\n");
>     return 0;
> }
> -
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