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, [email protected], 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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