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jpg: be more tolerant with corrupted files, and dont crash

--- a/sys/src/cmd/jpg/readjpg.c

+++ b/sys/src/cmd/jpg/readjpg.c

@@ -397,12 +397,15 @@

 {

 	int c;

+Again:

 	while((c=readbyte(h)) == 0)

-		fprint(2, "ReadJPG: skipping zero byte at offset %lld\n", Boffset(h->fd));

+		;

 	if(c != 0xFF)

-		jpgerror(h, "ReadJPG: expecting marker; found 0x%x at offset %lld\n", c, Boffset(h->fd));

+		goto Again;

 	while(c == 0xFF)

 		c = readbyte(h);

+	if(c == 0)

+		goto Again;

 	return c;

 }

@@ -440,13 +443,9 @@

 	int m, n;

 	uchar tmp[2];

-	m = marker(h);

-	switch(m){

-	case EOI:

+	if((m = marker(h)) == EOI){

 		*markerp = m;

 		return 0;

-	case 0:

-		jpgerror(h, "ReadJPG: expecting marker; saw %.2x at offset %lld", m, Boffset(h->fd));

 	}

 	if(Breadn(h->fd, tmp, 2) != 2)

     Readerr:

@@ -473,7 +472,7 @@

 huffmantable(Header *h, uchar *b)

 {

 	Huffman *t;

-	int Tc, th, n, nsize, i, j, k, v, cnt, code, si, sr, m;

+	int Tc, th, n, nsize, i, j, k, v, cnt, code, si, sr;

 	int *maxcode;

 	nibbles(b[0], &Tc, &th);

@@ -547,24 +546,21 @@

 	maxcode = t->maxcode;

 	/* stupid startup algorithm: just run machine for each byte value */

 	for(v=0; v<256; ){

-		cnt = 7;

-		m = 1<<7;

 		code = 0;

+		cnt = 8;

 		sr = v;

-		i = 1;

-		for(;;i++){

-			if(sr & m)

+		for(i=1;;i++){

+			cnt--;

+			if(sr & (1<<cnt))

 				code |= 1;

 			if(code <= maxcode[i])

 				break;

 			code <<= 1;

-			m >>= 1;

-			if(m == 0){

+			if(cnt == 0){

 				t->shift[v] = 0;

 				t->value[v] = -1;

 				goto continueBytes;

 			}

-			cnt--;

 		}

 		t->shift[v] = 8-cnt;

 		t->value[v] = t->val[t->valptr[i]+(code-t->mincode[i])];

@@ -707,22 +703,22 @@

 				memset(zz, 0, 8*8*sizeof(int));

 				zz[0] = qt[0]*DC[comp];

 				k = 1;

-

-				for(;;){

+				do{

 					t = decode(h, acht);

+					assert(t >= 0);

 					if((t&0x0F) == 0){

 						if((t&0xF0) != 0xF0)

 							break;

 						k += 16;

 					}else{

-						k += t>>4;

 						z = receive(h, t&0xF);

-						zz[zig[k]] = z*qt[k];

-						if(k == 63)

+						k += t>>4;

+						if(k >= 64)

 							break;

+						zz[zig[k]] = z*qt[k];

 						k++;

 					}

-				}

+				} while(k < 64);

 				idct(zz);

 			}

@@ -1006,7 +1002,7 @@

 			blockno = tmcu*H*V + H*(y%V) + x%H;

 			acc = h->accoeff[comp][blockno];

 			k = Ss;

-			for(;;){

+			do {

 				rs = decode(h, acht);

 				/* XXX remove rrrr ssss as in baselinescan */

 				nibbles(rs, &rrrr, &ssss);

@@ -1019,14 +1015,14 @@

 					}

 					k += 16;

 				}else{

-					k += rrrr;

 					z = receive(h, ssss);

-					acc[k] = z*qt[k]<<Al;

-					if(k == Se)

+					k += rrrr;

+					if(k > Se)

 						break;

+					acc[k] = z*qt[k]<<Al;

 					k++;

 				}

-			}

+			} while(k <= Se);

 		}

 		/* process restart marker, if present */

@@ -1358,9 +1354,8 @@

 int

 decode(Header *h, Huffman *t)

 {

-	int code, v, cnt, m, sr, i;

+	int code, v, cnt, sr, i;

 	int *maxcode;

-	static int badcode;

 	maxcode = t->maxcode;

 	if(h->cnt < 8)

@@ -1376,30 +1371,24 @@

 	h->cnt -= 8;

 	if(h->cnt == 0)

 		nextbyte(h, 0);

-	h->cnt--;

 	cnt = h->cnt;

-	m = 1<<cnt;

 	sr = h->sr;

 	code <<= 1;

-	i = 9;

-	for(;;i++){

-		if(sr & m)

+	for(i = 9; i<17; i++){

+		cnt--;

+		if(sr & (1<<cnt))

 			code |= 1;

 		if(code <= maxcode[i])

 			break;

 		code <<= 1;

-		m >>= 1;

-		if(m == 0){

+		if(cnt == 0){

 			sr = nextbyte(h, 0);

-			m = 0x80;

 			cnt = 8;

 		}

-		cnt--;

 	}

 	if(i >= 17){

-		if(badcode == 0)

-			fprint(2, "badly encoded %dx%d JPEG file; ignoring bad value\n", h->X, h->Y);

-		badcode = 1;

+		/* bad code */

+		code = 0;

 		i = 0;

 	}

 	h->cnt = cnt;

-- 

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