c/3710: segfaulting code generated with -O2 -fomit-frame-pointer (kernel!)

[hcs at icshu dot com]

>Number:         3710
>Category:       c
>Synopsis:       segfaulting code generated with -O2 -fomit-frame-pointer (kernel!)
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Jul 17 06:56:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Csaba Halasz
>Release:        gcc version 3.0.1 20010626 (prerelease)
>Organization:
>Environment:
http://www.codesourcery.com/gcc-compile.shtml
>Description:
Bad code generated for the loop marked "Set the p-boxes"

.L16:
	movl	bf_pbox(,%edx,4), %eax
	movl	-44(%ebp), %ebp             <-- !!!
	movl	%eax, (%ebp,%edx,4)
	incl	%edx
	decw	%di
	jns	.L16

All the other stuff is needed otherwise the bug does not occur. This is an extract from the blowfish cipher in the international kernel patch.
>How-To-Repeat:
/* 
    Compile with
	gcc -O2 -fomit-frame-pointer loopbug.c
*/

#define u8 unsigned char
#define u32 unsigned int

struct cipher_context;

struct cipher_context {
	u32 *keyinfo;
};

static u32 bf_pbox[16 + 2] = {0, };
static u32 bf_sbox[256 * 4] = {0, };

typedef struct blow_key {
	u32 P[18];
	u32 S[1024];
} blow_key;

int
blowfish_encrypt (struct cipher_context *cx,
		  const u8 * in8, u8 * out8, int size, int atomic)
{
    return 0;
}


/* Sets the blowfish S and P boxes for encryption and decryption. */

int
blowfish_set_key (struct cipher_context *cx,
		  unsigned char *key, int keybytes, int atomic)
{
	blow_key *key2 = (blow_key *) cx->keyinfo;
	short i;
	short j;
	short count;
	u32 data[2];
	u32 *P = key2->P;
	u32 *S = key2->S;

	/* Copy the initialization s-boxes */

	for (i = 0, count = 0; i < 256; i++)
		for (j = 0; j < 4; j++, count++)
			S[count] = bf_sbox[count];

	/* Set the p-boxes */

	for (i = 0; i < 16 + 2; i++)
		P[i] = bf_pbox[i];

	for (i = 0; i < 16 + 2; i += 2) {
		blowfish_encrypt (cx, (u8 *) data, (u8 *) data, 8, atomic);

		P[i] = data[0];
		P[i + 1] = data[1];
	}

	for (i = 0; i < 4; i++) {
		for (j = 0, count = i * 256; j < 256; j += 2, count += 2) {
			blowfish_encrypt (cx, (u8 *) data, (u8 *) data, 8,
					  atomic);

			S[count] = data[0];
			S[count + 1] = data[1];
		}
	}

	return 0;
}

int main(int argc, char* argv[])
{
    struct cipher_context cx;
    struct blow_key bk;
    cx.keyinfo = (u32*)&bk;
    
    blowfish_set_key(&cx, "1234567890123456", 16, 0);

    return 0;
}
>Fix:
use gcc-2.95 (and even that reloads a constant in each iteration!)
>Release-Note:
>Audit-Trail:
>Unformatted: