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

[Mikael Pettersson <mikpe@csd.uu.se>]

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@csd.uu.se, 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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