[libc] Fix strcmp fuzz test to use one input.

[cgyurgyik]

Summary

[libc] Since only one input is given, it is necessary to split the string into two containers so that they can be compared for the purposes of this fuzz test. This is done in the following manner:

1. Take the value of the first byte; this is size1. (Credits to @PaulkaToast for this idea).
2. size2 is the value of size - size1.
3. Copy the characters to new containers, data1 and data2 with corresponding sizes.
4. Add a null terminator to the first container, and verify the second container has a null terminator.
5. Verify output of strcmp.

A simpler alternative considered was simply splitting the input data into two, but this means the two strings are always within +- 1 character of each other. This above implementation avoids this.

ninja check-libc was run; no issues.

[cgyurgyik]

Fix example formatting in comment.

[cgyurgyik]

[libc] Check that new operator[] succeeds.

[cgyurgyik]

Add corresponding deletes.

[cgyurgyik]

Rebased.

[cgyurgyik]

[libc] Clean up strcmp fuzz test.

[PaulkaToast]

Minor nit, looks good otherwise. (:
If you haven't run git clang-format HEAD~1 before you land the patch.

libc/fuzzing/string/strcmp_fuzz.cpp

85

nit: Add a newline.

[cgyurgyik]

Add space.

[cgyurgyik]

In D82427#2118238, @PaulkaToast wrote:

Minor nit, looks good otherwise. (:
If you haven't run git clang-format HEAD~1 before you land the patch.

Weird, I've been running clang-format religiously. I tested it, and it doesn't add a newline at the end.

libc/fuzzing/string/strcmp_fuzz.cpp

 ///
 //===----------------------------------------------------------------------===//
 #include "src/string/strcmp.h"
 #include <stddef.h>
 #include <stdint.h>
 
 // The general structure is to take the value of the first byte, set size1 to
 // that value, and add the null terminator. size2 will then contain the rest of
-// the bytes in data. For example: Inputs: data: [2, 6, 4, 8, 0], size: 5 Split:
+// the bytes in data. For example, with inputs ([2, 6, 4, 8, 0], 5):
 //         size1: data[0] = 2
 //         data1: [2, 6] + '\0' = [2, 6, '\0']
 //         size2: size - size1 = 3
 //         data2: [4, 8, '\0']
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   // Verify the size is at least 1.
   if (!size)
     return 0;
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   // Verify reversed operands. This should be the negated value of the previous
   // result, except of course if the previous result was zero.
   expected_result = s2[k] - s1[k];
   actual_result = __llvm_libc::strcmp(s2, s1);
   if (expected_result != actual_result)
     __builtin_trap();
 
   return 0;
-}
+}
 No newline at end of file