Whats the bugzilla # for this so others can track it?
On 4/23/05, Mikael Pettersson <[email protected]> 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, [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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