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-rw-r--r--lib/hexdump.c32
1 files changed, 25 insertions, 7 deletions
diff --git a/lib/hexdump.c b/lib/hexdump.c
index 9301578f98e8..369420ce553a 100644
--- a/lib/hexdump.c
+++ b/lib/hexdump.c
@@ -22,15 +22,33 @@ EXPORT_SYMBOL(hex_asc_upper);
*
* hex_to_bin() converts one hex digit to its actual value or -1 in case of bad
* input.
+ *
+ * This function is used to load cryptographic keys, so it is coded in such a
+ * way that there are no conditions or memory accesses that depend on data.
+ *
+ * Explanation of the logic:
+ * (ch - '9' - 1) is negative if ch <= '9'
+ * ('0' - 1 - ch) is negative if ch >= '0'
+ * we "and" these two values, so the result is negative if ch is in the range
+ * '0' ... '9'
+ * we are only interested in the sign, so we do a shift ">> 8"; note that right
+ * shift of a negative value is implementation-defined, so we cast the
+ * value to (unsigned) before the shift --- we have 0xffffff if ch is in
+ * the range '0' ... '9', 0 otherwise
+ * we "and" this value with (ch - '0' + 1) --- we have a value 1 ... 10 if ch is
+ * in the range '0' ... '9', 0 otherwise
+ * we add this value to -1 --- we have a value 0 ... 9 if ch is in the range '0'
+ * ... '9', -1 otherwise
+ * the next line is similar to the previous one, but we need to decode both
+ * uppercase and lowercase letters, so we use (ch & 0xdf), which converts
+ * lowercase to uppercase
*/
-int hex_to_bin(char ch)
+int hex_to_bin(unsigned char ch)
{
- if ((ch >= '0') && (ch <= '9'))
- return ch - '0';
- ch = tolower(ch);
- if ((ch >= 'a') && (ch <= 'f'))
- return ch - 'a' + 10;
- return -1;
+ unsigned char cu = ch & 0xdf;
+ return -1 +
+ ((ch - '0' + 1) & (unsigned)((ch - '9' - 1) & ('0' - 1 - ch)) >> 8) +
+ ((cu - 'A' + 11) & (unsigned)((cu - 'F' - 1) & ('A' - 1 - cu)) >> 8);
}
EXPORT_SYMBOL(hex_to_bin);