hexdump: make it return number of bytes placed in buffer

This patch makes hexdump return the number of bytes placed in the buffer
excluding trailing NUL. In the case of overflow it returns the desired
amount of bytes to produce the entire dump. Thus, it mimics snprintf().

This will be useful for users that would like to repeat with a bigger
buffer.

[[e-mail]: fix printk warning]
Signed-off-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/printk.h

···
 	DUMP_PREFIX_ADDRESS,
 	DUMP_PREFIX_OFFSET
 };
-extern void hex_dump_to_buffer(const void *buf, size_t len,
-			       int rowsize, int groupsize,
-			       char *linebuf, size_t linebuflen, bool ascii);
+extern int hex_dump_to_buffer(const void *buf, size_t len, int rowsize,
+			      int groupsize, char *linebuf, size_t linebuflen,
+			      bool ascii);
 #ifdef CONFIG_PRINTK
 extern void print_hex_dump(const char *level, const char *prefix_str,
 			   int prefix_type, int rowsize, int groupsize,

lib/hexdump.c

···
  *
  * example output buffer:
  * 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f  @ABCDEFGHIJKLMNO
+ *
+ * Return:
+ * The amount of bytes placed in the buffer without terminating NUL. If the
+ * output was truncated, then the return value is the number of bytes
+ * (excluding the terminating NUL) which would have been written to the final
+ * string if enough space had been available.
  */
-void hex_dump_to_buffer(const void *buf, size_t len, int rowsize,
-			int groupsize, char *linebuf, size_t linebuflen,
-			bool ascii)
+int hex_dump_to_buffer(const void *buf, size_t len, int rowsize, int groupsize,
+		       char *linebuf, size_t linebuflen, bool ascii)
 {
 	const u8 *ptr = buf;
 	int ngroups;
 	u8 ch;
 	int j, lx = 0;
 	int ascii_column;
+	int ret;
 
 	if (rowsize != 16 && rowsize != 32)
 		rowsize = 16;
 
-	if (!len)
-		goto nil;
 	if (len > rowsize)		/* limit to one line at a time */
 		len = rowsize;
 	if (!is_power_of_2(groupsize) || groupsize > 8)
···
 
 	ngroups = len / groupsize;
 	ascii_column = rowsize * 2 + rowsize / groupsize + 1;
+
+	if (!linebuflen)
+		goto overflow1;
+
+	if (!len)
+		goto nil;
+
 	if (groupsize == 8) {
 		const u64 *ptr8 = buf;
 
-		for (j = 0; j < ngroups; j++)
-			lx += scnprintf(linebuf + lx, linebuflen - lx,
-					"%s%16.16llx", j ? " " : "",
-					(unsigned long long)*(ptr8 + j));
+		for (j = 0; j < ngroups; j++) {
+			ret = snprintf(linebuf + lx, linebuflen - lx,
+				       "%s%16.16llx", j ? " " : "",
+				       (unsigned long long)*(ptr8 + j));
+			if (ret >= linebuflen - lx)
+				goto overflow1;
+			lx += ret;
+		}
 	} else if (groupsize == 4) {
 		const u32 *ptr4 = buf;
 
-		for (j = 0; j < ngroups; j++)
-			lx += scnprintf(linebuf + lx, linebuflen - lx,
-					"%s%8.8x", j ? " " : "", *(ptr4 + j));
+		for (j = 0; j < ngroups; j++) {
+			ret = snprintf(linebuf + lx, linebuflen - lx,
+				       "%s%8.8x", j ? " " : "",
+				       *(ptr4 + j));
+			if (ret >= linebuflen - lx)
+				goto overflow1;
+			lx += ret;
+		}
 	} else if (groupsize == 2) {
 		const u16 *ptr2 = buf;
 
-		for (j = 0; j < ngroups; j++)
-			lx += scnprintf(linebuf + lx, linebuflen - lx,
-					"%s%4.4x", j ? " " : "", *(ptr2 + j));
+		for (j = 0; j < ngroups; j++) {
+			ret = snprintf(linebuf + lx, linebuflen - lx,
+				       "%s%4.4x", j ? " " : "",
+				       *(ptr2 + j));
+			if (ret >= linebuflen - lx)
+				goto overflow1;
+			lx += ret;
+		}
 	} else {
-		for (j = 0; (j < len) && (lx + 3) <= linebuflen; j++) {
+		for (j = 0; j < len; j++) {
+			if (linebuflen < lx + 3)
+				goto overflow2;
 			ch = ptr[j];
 			linebuf[lx++] = hex_asc_hi(ch);
 			linebuf[lx++] = hex_asc_lo(ch);
···
 	if (!ascii)
 		goto nil;
 
-	while (lx < (linebuflen - 1) && lx < ascii_column)
+	while (lx < ascii_column) {
+		if (linebuflen < lx + 2)
+			goto overflow2;
 		linebuf[lx++] = ' ';
-	for (j = 0; (j < len) && (lx + 2) < linebuflen; j++) {
+	}
+	for (j = 0; j < len; j++) {
+		if (linebuflen < lx + 2)
+			goto overflow2;
 		ch = ptr[j];
 		linebuf[lx++] = (isascii(ch) && isprint(ch)) ? ch : '.';
 	}
 nil:
+	linebuf[lx] = '\0';
+	return lx;
+overflow2:
 	linebuf[lx++] = '\0';
+overflow1:
+	return ascii ? ascii_column + len : (groupsize * 2 + 1) * ngroups - 1;
 }
 EXPORT_SYMBOL(hex_dump_to_buffer);
 

lib/test-hexdump.c

···
 	test_hexdump(len, rowsize, 1, ascii);
 }
 
+static void __init test_hexdump_overflow(bool ascii)
+{
+	char buf[56];
+	const char *t = test_data_1_le[0];
+	size_t l = get_random_int() % sizeof(buf);
+	bool a;
+	int e, r;
+
+	memset(buf, ' ', sizeof(buf));
+
+	r = hex_dump_to_buffer(data_b, 1, 16, 1, buf, l, ascii);
+
+	if (ascii)
+		e = 50;
+	else
+		e = 2;
+	buf[e + 2] = '\0';
+
+	if (!l) {
+		a = r == e && buf[0] == ' ';
+	} else if (l < 3) {
+		a = r == e && buf[0] == '\0';
+	} else if (l < 4) {
+		a = r == e && !strcmp(buf, t);
+	} else if (ascii) {
+		if (l < 51)
+			a = r == e && buf[l - 1] == '\0' && buf[l - 2] == ' ';
+		else
+			a = r == e && buf[50] == '\0' && buf[49] == '.';
+	} else {
+		a = r == e && buf[e] == '\0';
+	}
+
+	if (!a) {
+		pr_err("Len: %zu rc: %u strlen: %zu\n", l, r, strlen(buf));
+		pr_err("Result: '%s'\n", buf);
+	}
+}
+
 static int __init test_hexdump_init(void)
 {
 	unsigned int i;
···
 	rowsize = (get_random_int() % 2 + 1) * 16;
 	for (i = 0; i < 16; i++)
 		test_hexdump_set(rowsize, true);
+
+	for (i = 0; i < 16; i++)
+		test_hexdump_overflow(false);
+
+	for (i = 0; i < 16; i++)
+		test_hexdump_overflow(true);
 
 	return -EINVAL;
 }