Add Tool to Diagnose Whether Corrupt Blobs are Plausibly Bit-Flips

I spend an increasing amount of my time looking at "Corrupted block detected"
failures in decompression. Not infrequently, I suspect that it is the result
of hardware failure, and that the blob has become bit-flipped or otherwise
corrupted somewhere along the line.

For that reason I was motivated to write a little tool to inspect blobs that
fail to decompress, to try modifying them, and then check whether they
decompress successfully. This seems like potentially a generally useful tool,
so I figured it might be worth putting in `contrib/`.

contrib/diagnose_corruption/.gitignore

@@ -0,0 +1 @@
+check_flipped_bits

contrib/diagnose_corruption/Makefile

@@ -0,0 +1,35 @@
+# ################################################################
+# Copyright (c) 2019-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under both the BSD-style license (found in the
+# LICENSE file in the root directory of this source tree) and the GPLv2 (found
+# in the COPYING file in the root directory of this source tree).
+# ################################################################
+
+.PHONY: all
+all: check_flipped_bits
+
+ZSTDLIBDIR ?= ../../lib
+
+CFLAGS     ?= -O3
+CFLAGS     += -I$(ZSTDLIBDIR) -I$(ZSTDLIBDIR)/common -I$(ZSTDLIBDIR)/compress \
+              -I$(ZSTDLIBDIR)/decompress
+CFLAGS     += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow                 \
+              -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
+              -Wstrict-prototypes -Wundef                                     \
+              -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings      \
+              -Wredundant-decls -Wmissing-prototypes
+CFLAGS     += $(DEBUGFLAGS) $(MOREFLAGS)
+FLAGS       = $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)
+
+.PHONY: $(ZSTDLIBDIR)/libzstd.a
+$(ZSTDLIBDIR)/libzstd.a:
+	$(MAKE) -C $(ZSTDLIBDIR) libzstd.a
+
+check_flipped_bits: check_flipped_bits.c $(ZSTDLIBDIR)/libzstd.a
+	$(CC) $(FLAGS) $< -o $@$(EXT) $(ZSTDLIBDIR)/libzstd.a
+
+.PHONY: clean
+clean:
+	rm -f check_flipped_bits

contrib/diagnose_corruption/check_flipped_bits.c

@@ -0,0 +1,217 @@
+/*
+ * Copyright (c) 2019-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+#include "zstd.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+typedef struct {
+  char *input;
+  size_t input_size;
+
+  char *perturbed; /* same size as input */
+
+  char *output;
+  size_t output_size;
+
+  ZSTD_DCtx* dctx;
+} stuff_t;
+
+static void free_stuff(stuff_t* stuff) {
+  free(stuff->input);
+  free(stuff->output);
+  ZSTD_freeDCtx(stuff->dctx);
+}
+
+static void usage(void) {
+  fprintf(stderr, "check_flipped_bits input_filename");
+  exit(1);
+}
+
+static char* readFile(const char* filename, size_t* size) {
+  struct stat statbuf;
+  int ret;
+  FILE* f;
+  char *buf;
+  size_t bytes_read;
+
+  ret = stat(filename, &statbuf);
+  if (ret != 0) {
+    fprintf(stderr, "stat failed: %m\n");
+    return NULL;
+  }
+  if ((statbuf.st_mode & S_IFREG) != S_IFREG) {
+    fprintf(stderr, "Input must be regular file\n");
+    return NULL;
+  }
+
+  *size = statbuf.st_size;
+
+  f = fopen(filename, "r");
+  if (f == NULL) {
+    fprintf(stderr, "fopen failed: %m\n");
+    return NULL;
+  }
+
+  buf = malloc(*size);
+  if (buf == NULL) {
+    fprintf(stderr, "malloc failed\n");
+  }
+
+  bytes_read = fread(buf, 1, *size, f);
+  if (bytes_read != *size) {
+    fprintf(stderr, "failed to read whole file\n");
+    free(buf);
+    return NULL;
+  }
+
+  ret = fclose(f);
+  if (ret != 0) {
+    fprintf(stderr, "fclose failed: %m\n");
+    free(buf);
+    return NULL;
+  }
+
+  return buf;
+}
+
+static int init_stuff(stuff_t* stuff, int argc, char *argv[]) {
+  const char* input_filename;
+
+  if (argc != 2) {
+    usage();
+  }
+
+  input_filename = argv[1];
+  stuff->input_size = 0;
+  stuff->input = readFile(input_filename, &stuff->input_size);
+  if (stuff->input == NULL) {
+    fprintf(stderr, "Failed to read input file.\n");
+    return 0;
+  }
+
+  stuff->perturbed = malloc(stuff->input_size);
+  if (stuff->perturbed == NULL) {
+    fprintf(stderr, "malloc failed.\n");
+    return 0;
+  }
+  memcpy(stuff->perturbed, stuff->input, stuff->input_size);
+
+  stuff->output_size = ZSTD_DStreamOutSize();
+  stuff->output = malloc(stuff->output_size);
+  if (stuff->output == NULL) {
+    fprintf(stderr, "malloc failed.\n");
+    return 0;
+  }
+
+  stuff->dctx = ZSTD_createDCtx();
+  if (stuff->dctx == NULL) {
+    return 0;
+  }
+
+  return 1;
+}
+
+static int test_decompress(stuff_t* stuff) {
+  size_t ret;
+  ZSTD_inBuffer in = {stuff->perturbed, stuff->input_size, 0};
+  ZSTD_outBuffer out = {stuff->output, stuff->output_size, 0};
+  ZSTD_DCtx* dctx = stuff->dctx;
+
+  ZSTD_DCtx_reset(dctx, ZSTD_reset_session_only);
+  ZSTD_DCtx_refDDict(dctx, NULL);
+
+  while (in.pos != in.size) {
+    out.pos = 0;
+    ret = ZSTD_decompressStream(dctx, &out, &in);
+
+    if (ZSTD_isError(ret)) {
+      /*
+      fprintf(
+          stderr, "Decompression failed: %s\n", ZSTD_getErrorName(ret));
+      */
+      return 0;
+    }
+  }
+
+  return 1;
+}
+
+static int perturb_bits(stuff_t* stuff) {
+  size_t pos;
+  size_t bit;
+  for (pos = 0; pos < stuff->input_size; pos++) {
+    unsigned char old_val = stuff->input[pos];
+    if (pos % 1000 == 0) {
+      fprintf(stderr, "Perturbing byte %zu\n", pos);
+    }
+    for (bit = 0; bit < 8; bit++) {
+      unsigned char new_val = old_val ^ (1 << bit);
+      stuff->perturbed[pos] = new_val;
+      if (test_decompress(stuff)) {
+        fprintf(
+            stderr,
+            "Flipping byte %zu bit %zu (0x%02x -> 0x%02x) "
+            "produced a successful decompression!\n",
+            pos, bit, old_val, new_val);
+      }
+    }
+    stuff->perturbed[pos] = old_val;
+  }
+  return 1;
+}
+
+static int perturb_bytes(stuff_t* stuff) {
+  size_t pos;
+  size_t new_val;
+  for (pos = 0; pos < stuff->input_size; pos++) {
+    unsigned char old_val = stuff->input[pos];
+    if (pos % 1000 == 0) {
+      fprintf(stderr, "Perturbing byte %zu\n", pos);
+    }
+    for (new_val = 0; new_val < 256; new_val++) {
+      stuff->perturbed[pos] = new_val;
+      if (test_decompress(stuff)) {
+        fprintf(
+            stderr,
+            "Changing byte %zu (0x%02x -> 0x%02x) "
+            "produced a successful decompression!\n",
+            pos, old_val, (unsigned char)new_val);
+      }
+    }
+    stuff->perturbed[pos] = old_val;
+  }
+  return 1;
+}
+
+int main(int argc, char* argv[]) {
+  stuff_t stuff;
+
+  if(!init_stuff(&stuff, argc, argv)) {
+    fprintf(stderr, "Failed to init.\n");
+    return 1;
+  }
+
+  if (test_decompress(&stuff)) {
+    fprintf(stderr, "Blob already decompresses successfully!\n");
+    return 1;
+  }
+
+  perturb_bits(&stuff);
+
+  perturb_bytes(&stuff);
+
+  free_stuff(&stuff);
+}