diff --git a/cache.h b/cache.h
index ce377e1354a4d0fd719b50edb32124110956467f..9244c387c39b3a5cf29162a36fdb1388cff9a87f 100644
--- a/cache.h
+++ b/cache.h
@@ -279,6 +279,7 @@ struct index_state {
 		 initialized : 1;
 	struct hash_table name_hash;
 	struct hash_table dir_hash;
+	unsigned char sha1[20];
 };
 
 extern struct index_state the_index;
@@ -1199,6 +1200,8 @@ extern void fsync_or_die(int fd, const char *);
 
 extern ssize_t read_in_full(int fd, void *buf, size_t count);
 extern ssize_t write_in_full(int fd, const void *buf, size_t count);
+extern ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset);
+
 static inline ssize_t write_str_in_full(int fd, const char *str)
 {
 	return write_in_full(fd, str, strlen(str));
diff --git a/read-cache.c b/read-cache.c
index 33dd676ccbbd24e0bace49347f1f5d81b5099bcc..f4a0d6168b8db2f91a9bbce035277dd2864a60d8 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1464,6 +1464,7 @@ int read_index_from(struct index_state *istate, const char *path)
 	if (verify_hdr(hdr, mmap_size) < 0)
 		goto unmap;
 
+	hashcpy(istate->sha1, (unsigned char *)hdr + mmap_size - 20);
 	istate->version = ntohl(hdr->hdr_version);
 	istate->cache_nr = ntohl(hdr->hdr_entries);
 	istate->cache_alloc = alloc_nr(istate->cache_nr);
@@ -1747,6 +1748,50 @@ static int ce_write_entry(git_SHA_CTX *c, int fd, struct cache_entry *ce,
 	return result;
 }
 
+/*
+ * This function verifies if index_state has the correct sha1 of the
+ * index file.  Don't die if we have any other failure, just return 0.
+ */
+static int verify_index_from(const struct index_state *istate, const char *path)
+{
+	int fd;
+	ssize_t n;
+	struct stat st;
+	unsigned char sha1[20];
+
+	if (!istate->initialized)
+		return 0;
+
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return 0;
+
+	if (fstat(fd, &st))
+		goto out;
+
+	if (st.st_size < sizeof(struct cache_header) + 20)
+		goto out;
+
+	n = pread_in_full(fd, sha1, 20, st.st_size - 20);
+	if (n != 20)
+		goto out;
+
+	if (hashcmp(istate->sha1, sha1))
+		goto out;
+
+	close(fd);
+	return 1;
+
+out:
+	close(fd);
+	return 0;
+}
+
+static int verify_index(const struct index_state *istate)
+{
+	return verify_index_from(istate, get_index_file());
+}
+
 static int has_racy_timestamp(struct index_state *istate)
 {
 	int entries = istate->cache_nr;
@@ -1766,7 +1811,7 @@ static int has_racy_timestamp(struct index_state *istate)
 void update_index_if_able(struct index_state *istate, struct lock_file *lockfile)
 {
 	if ((istate->cache_changed || has_racy_timestamp(istate)) &&
-	    !write_index(istate, lockfile->fd))
+	    verify_index(istate) && !write_index(istate, lockfile->fd))
 		commit_locked_index(lockfile);
 	else
 		rollback_lock_file(lockfile);
diff --git a/wrapper.c b/wrapper.c
index 5b3c7fceb34c0aff6b1b798070b068bf71ed6b3f..bc1bfb86003cb4133cc4ce3ce6423ce780ed7c84 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -232,6 +232,26 @@ ssize_t write_in_full(int fd, const void *buf, size_t count)
 	return total;
 }
 
+ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset)
+{
+	char *p = buf;
+	ssize_t total = 0;
+
+	while (count > 0) {
+		ssize_t loaded = xpread(fd, p, count, offset);
+		if (loaded < 0)
+			return -1;
+		if (loaded == 0)
+			return total;
+		count -= loaded;
+		p += loaded;
+		total += loaded;
+		offset += loaded;
+	}
+
+	return total;
+}
+
 int xdup(int fd)
 {
 	int ret = dup(fd);