GCC kernel miscompile with O2

[William Stearns]

Good day, all,

gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.0)


In the following code, the &&error is calculated incorrectly under -O2,
but correctly under normal optimization.  This is the assembly under -O2:

Dump of assembler code for function __copy_from_user:
0x80484a0 <__copy_from_user>:   push   %ebp
0x80484a1 <__copy_from_user+1>: mov    %esp,%ebp
0x80484a3 <__copy_from_user+3>: push   %edi
0x80484a4 <__copy_from_user+4>: push   %esi
0x80484a5 <__copy_from_user+5>: push   %ebx
0x80484a6 <__copy_from_user+6>: sub    $0x18,%esp
0x80484a9 <__copy_from_user+9>: push   $0x80484a0
0x80484ae <__copy_from_user+14>:        mov    0x8(%ebp),%edi
0x80484b1 <__copy_from_user+17>:        mov    0xc(%ebp),%esi
0x80484b4 <__copy_from_user+20>:        mov    0x10(%ebp),%ebx
0x80484b7 <__copy_from_user+23>:        call   0x804848c <set_fault_addr>

The instruction at 0x80484a9 is calculating &&error as 0x80484a0, which is
the start of the procedure.

#include <stdlib.h>

void set_fault_addr(void *ptr)
{
printf("HI\n");
}

int __copy_from_user(void *to, const void *from, int n) {
        int ret = 0;

        set_fault_addr(&&error);
        memcpy(to, from, n);
        goto out;
 error:
        ret = n - ((unsigned long) get_fault_addr() - (unsigned long) from);
 out:
        set_fault_addr(NULL);
        return(ret);
}

main()
{
printf("HI\n");
__copy_from_user("a", "b", 100);
}

	Cheers,
	- Bill

---------------------------------------------------------------------------
	"Microsoft has done more for the fault tolerance industry than any
other company.  They have made end-users very tolerant of faults".
(Courtesy of "Deliduka, Bennet" <bennet.deliduka@state.vt.us>)
--------------------------------------------------------------------------
William Stearns (wstearns@pobox.com).  Mason, Buildkernel, named2hosts,
and ipfwadm2ipchains are at:                http://www.pobox.com/~wstearns
LinuxMonth; articles for Linux Enthusiasts! http://www.linuxmonth.com
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