libdiff: Simplify truncation detection

* Adjust the comment to reflect the fact that SIGBUS can occur not only
  if the file is truncated, but also if an I/O error occurs while paging
  in any part of it.

* Instead of setting a flag, just return EIO.

* Adjust the unit test accordingly.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	thj, kevans
Differential Revision:	https://reviews.freebsd.org/D55108

contrib/libdiff/include/diff_main.h

@@ -118,7 +118,6 @@ struct diff_data {
 
 /* Flags set by file atomizer. */
 #define DIFF_ATOMIZER_FOUND_BINARY_DATA	0x00000001
-#define DIFF_ATOMIZER_FILE_TRUNCATED	0x00000002
 
 /* Flags set by caller of diff_main(). */
 #define DIFF_FLAG_IGNORE_WHITESPACE	0x00000001

contrib/libdiff/lib/diff_atomize_text.c

@@ -141,6 +141,7 @@ diff_data_atomize_text_lines_mmap(struct diff_data *d)
 	bool embedded_nul = false;
 	unsigned int array_size_estimate = d->len / 50;
 	unsigned int pow2 = 1;
+	int ret = DIFF_RC_OK;
 	while (array_size_estimate >>= 1)
 		pow2++;
 
@@ -152,13 +153,14 @@ diff_data_atomize_text_lines_mmap(struct diff_data *d)
 	sigaction(SIGBUS, &act, &oact);
 	if (sigsetjmp(diff_data_signal_env, 0) > 0) {
 		/*
-		 * The file was truncated while we were reading it.  Set
-		 * the end pointer to the beginning of the line we were
-		 * trying to read, adjust the file length, and set a flag.
+		 * The file was truncated while we were reading it, or an
+		 * I/O error occurred.  Set the end pointer to the
+		 * beginning of the line we were trying to read, adjust
+		 * the file length, and set the return value to an error.
 		 */
 		end = pos;
 		d->len = end - d->data;
-		d->atomizer_flags |= DIFF_ATOMIZER_FILE_TRUNCATED;
+		ret = EIO;
 	}
 	while (pos < end) {
 		const uint8_t *line_start = pos, *line_end = pos;
@@ -203,7 +205,7 @@ diff_data_atomize_text_lines_mmap(struct diff_data *d)
 	if (embedded_nul)
 		d->atomizer_flags |= DIFF_ATOMIZER_FOUND_BINARY_DATA;
 
-	return DIFF_RC_OK;
+	return ret;
 }
 
 static int

lib/libdiff/tests/libdiff_test.c

@@ -45,10 +45,9 @@ ATF_TC_BODY(diff_atomize_truncated, tc)
 	rewind(f);
 	ATF_REQUIRE(truncate(fn, size / 2) == 0);
 	ATF_REQUIRE((p = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fileno(f), 0)) != MAP_FAILED);
-	ATF_REQUIRE(diff_atomize_file(&d, &cfg, f, p, size, 0) == 0);
+	ATF_REQUIRE(diff_atomize_file(&d, &cfg, f, p, size, 0) == EIO);
 	ATF_REQUIRE((size_t)d.len <= size / 2);
 	ATF_REQUIRE((size_t)d.len >= size / 2 - sizeof(line));
-	ATF_REQUIRE(d.atomizer_flags & DIFF_ATOMIZER_FILE_TRUNCATED);
 }
 ATF_TC_CLEANUP(diff_atomize_truncated, tc)
 {

usr.bin/diff/diffreg_new.c

@@ -223,14 +223,10 @@ diffreg_new(char *file1, char *file2, int flags, int capsicum)
 		rc = D_ERROR;
 		goto done;
 	}
-	if (left.atomizer_flags & DIFF_ATOMIZER_FILE_TRUNCATED)
-		warnx("%s truncated", file1);
 	if (diff_atomize_file(&right, cfg, f2, (uint8_t *)str2, st2.st_size, diff_flags)) {
 		rc = D_ERROR;
 		goto done;
 	}
-	if (right.atomizer_flags & DIFF_ATOMIZER_FILE_TRUNCATED)
-		warnx("%s truncated", file2);
 
 	result = diff_main(cfg, &left, &right);
 	if (result->rc != DIFF_RC_OK) {