Fix multiple file match bugs

This commit fixes a number of file match bugs:

* __filter_file_buf() was passing a length value of one less than the
  string length to cmpbytes() and as a result Equals and Prefix matches
  were testing one fewer bytes than required.

* ASM_RCMP (used in rcmpbytes()) was decrementing the string indices
  before testing if they were <1. This resulted in one fewer bytes being
  tested than required.

* ASM_RCMP50 comprisd of 2x ASM_RCMP20 and 1x ASM_RCMP5, totalling 45
  iterations of ASM_RCMP, instead of 50.

* __filter_file_buf() tested failed postfix matches with a forward string
  match. This resulted in files that started with the postfix, but didn't
  end with it, matching when they shouldn't.

* cmpbytes() continued to loop to full number of iterations, even when
  the string length had been exhausted (inefficient).

* Added descriptions to cmpbytes() and rcmpbytes() to aid in providing
  the correct parameters.

Signed-off-by: Kevin Sheldrake <[email protected]>

bpf/process/types/basic.h

@@ -306,10 +306,10 @@ parse_iovec_array(long off, unsigned long arg, int i, unsigned long max,
 			     : "c2", "r0");                                    \
 		if (c1 != c2)                                                  \
 			goto failed;                                           \
-		n1--;                                                          \
-		n2--;                                                          \
 		if (n1 < 1 || n2 < 1)                                          \
 			goto accept;                                           \
+		n1--;                                                          \
+		n2--;                                                          \
 	}
 
 #define ASM_RCMP5                                                              \
@@ -334,6 +334,7 @@ parse_iovec_array(long off, unsigned long arg, int i, unsigned long max,
 		ASM_RCMP20                                                     \
 		ASM_RCMP20                                                     \
 		ASM_RCMP5                                                      \
+		ASM_RCMP5                                                      \
 	}
 
 #define ASM_RCMP100                                                            \
@@ -342,6 +343,7 @@ parse_iovec_array(long off, unsigned long arg, int i, unsigned long max,
 		ASM_RCMP50                                                     \
 	}
 
+/* reverse compare bytes. n1 is index of last byte in s1. Ditto n2 of s2. */
 static inline __attribute__((always_inline)) int rcmpbytes(char *s1, char *s2,
 							   u64 n1, u64 n2)
 {
@@ -359,13 +361,16 @@ static inline __attribute__((always_inline)) int rcmpbytes(char *s1, char *s2,
 	return -1;
 }
 
+/* compare bytes. n is number of bytes to compare. */
 static inline __attribute__((always_inline)) int cmpbytes(char *s1, char *s2,
 							  size_t n)
 {
 	int i;
 #pragma unroll
 	for (i = 0; i < MAX_STRING_FILTER; i++) {
-		if (i < n && s1[i] != s2[i])
+		if (i >= n)
+			return 0;
+		if (s1[i] != s2[i])
 			return -1;
 	}
 	return 0;
@@ -617,8 +622,9 @@ __filter_file_buf(char *value, char *args, __u32 op)
 		err = rcmpbytes(&value[4], &args[4], v - 1, a - 1);
 		if (!err)
 			return 0;
+		goto skip_string;
 	}
-	err = cmpbytes(&value[4], &args[4], v - 1);
+	err = cmpbytes(&value[4], &args[4], v);
 	if (!err)
 		return 0;
 skip_string: