diff --git a/usr.bin/diff/diffreg.c b/usr.bin/diff/diffreg.c
index e728441c2cb..ca8f7491995 100644
--- a/usr.bin/diff/diffreg.c
+++ b/usr.bin/diff/diffreg.c
@@ -71,6 +71,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/capsicum.h>
 #include <sys/stat.h>
+#include <sys/mman.h>
 
 #include <capsicum_helpers.h>
 #include <ctype.h>
@@ -190,7 +191,7 @@ static void	 range(int, int, const char *);
 static void	 uni_range(int, int);
 static void	 dump_context_vec(FILE *, FILE *, int);
 static void	 dump_unified_vec(FILE *, FILE *, int);
-static bool	 prepare(int, FILE *, size_t, int);
+static bool	 prepare(int, const uint8_t *, size_t, int);
 static void	 prune(void);
 static void	 equiv(struct line *, int, struct line *, int, int *);
 static void	 unravel(int);
@@ -208,7 +209,7 @@ static int	 search(int *, int, int);
 static int	 skipline(FILE *);
 static int	 isqrt(int);
 static int	 stone(int *, int, int *, int *, int);
-static enum readhash readhash(FILE *, int, unsigned *);
+static enum readhash readhash(const uint8_t **, const uint8_t *, int, unsigned *);
 static int	 files_differ(FILE *, FILE *, int);
 static char	*match_function(const long *, int, FILE *);
 static char	*preadline(int, size_t, off_t);
@@ -258,11 +259,13 @@ int
 diffreg(char *file1, char *file2, int flags, int capsicum)
 {
 	FILE *f1, *f2;
+	uint8_t *buf1, *buf2;
 	int i, rval;
 	struct pr *pr = NULL;
 	cap_rights_t rights_ro;
 
 	f1 = f2 = NULL;
+	buf1 = buf2 = NULL;
 	rval = D_SAME;
 	anychange = 0;
 	lastline = 0;
@@ -343,7 +346,8 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		pr = start_pr(file1, file2);
 
 	if (capsicum) {
-		cap_rights_init(&rights_ro, CAP_READ, CAP_FSTAT, CAP_SEEK);
+		cap_rights_init(&rights_ro, CAP_READ, CAP_FSTAT, CAP_SEEK,
+		    CAP_MMAP);
 		if (caph_rights_limit(fileno(f1), &rights_ro) < 0)
 			err(2, "unable to limit rights on: %s", file1);
 		if (caph_rights_limit(fileno(f2), &rights_ro) < 0)
@@ -362,6 +366,12 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		if (caph_enter() < 0)
 			err(2, "unable to enter capability mode");
 	}
+	buf1 = mmap(NULL, stb1.st_size, PROT_READ, MAP_SHARED, fileno(f1), 0);
+	if (buf1 == NULL)
+		err(2, "unable to mmap");
+	buf2 = mmap(NULL, stb2.st_size, PROT_READ, MAP_SHARED, fileno(f2), 0);
+	if (buf2 == NULL)
+		err(2, "unable to mmap");
 
 	switch (files_differ(f1, f2, flags)) {
 	case 0:
@@ -383,11 +393,11 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		goto closem;
 	}
 	if ((flags & D_FORCEASCII) != 0) {
-		(void)prepare(0, f1, stb1.st_size, flags);
-		(void)prepare(1, f2, stb2.st_size, flags);
+		(void)prepare(0, buf1, stb1.st_size, flags);
+		(void)prepare(1, buf2, stb2.st_size, flags);
 	} else if (!asciifile(f1) || !asciifile(f2) ||
-		    !prepare(0, f1, stb1.st_size, flags) ||
-		    !prepare(1, f2, stb2.st_size, flags)) {
+		    !prepare(0, buf1, stb1.st_size, flags) ||
+		    !prepare(1, buf2, stb2.st_size, flags)) {
 		rval = D_BINARY;
 		status |= 1;
 		goto closem;
@@ -431,6 +441,10 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 		if (rval == D_SAME)
 			rval = D_DIFFER;
 	}
+	if (buf1 != NULL)
+		munmap(buf1, stb1.st_size);
+	if (buf2 != NULL)
+		munmap(buf2, stb2.st_size);
 	if (f1 != NULL)
 		fclose(f1);
 	if (f2 != NULL)
@@ -516,21 +530,22 @@ splice(char *dir, char *path)
 }
 
 static bool
-prepare(int i, FILE *fd, size_t filesize, int flags)
+prepare(int i, const uint8_t *buf, size_t filesize, int flags)
 {
 	struct line *p;
 	unsigned h;
 	size_t sz, j = 0;
 	enum readhash r;
 
-	rewind(fd);
+	const uint8_t *walk = buf;
+	const uint8_t *end = buf + filesize;
 
 	sz = MIN(filesize, SIZE_MAX) / 25;
 	if (sz < 100)
 		sz = 100;
 
 	p = xcalloc(sz + 3, sizeof(*p));
-	while ((r = readhash(fd, flags, &h)) != RH_EOF)
+	while ((r = readhash(&walk, end, flags, &h)) != RH_EOF)
 		switch (r) {
 		case RH_EOF: /* otherwise clang complains */
 		case RH_BINARY:
@@ -1364,24 +1379,25 @@ fetch(long *f, int a, int b, FILE *lb, int ch, int oldfile, int flags)
  * Hash function taken from Robert Sedgewick, Algorithms in C, 3d ed., p 578.
  */
 static enum readhash
-readhash(FILE *f, int flags, unsigned *hash)
+readhash(const uint8_t **buf, const uint8_t *end, int flags, unsigned *hash)
 {
-	int i, t, space;
+	int t, space;
 	unsigned sum;
+	const uint8_t *walk = *buf;
 
 	sum = 1;
 	space = 0;
-	for (i = 0;;) {
-		switch (t = getc(f)) {
+	if (walk == end)
+		return (RH_EOF);
+	for (; walk < end; walk++) {
+		switch (t = *walk) {
 		case '\0':
 			if ((flags & D_FORCEASCII) == 0)
 				return (RH_BINARY);
 		case '\r':
-			if (flags & D_STRIPCR) {
-				t = getc(f);
-				if (t == '\n')
-					break;
-				ungetc(t, f);
+			if (walk + 1 != end && walk[1] == '\n') {
+				walk += 2;
+				break;
 			}
 			/* FALLTHROUGH */
 		case '\t':
@@ -1395,21 +1411,18 @@ readhash(FILE *f, int flags, unsigned *hash)
 			/* FALLTHROUGH */
 		default:
 			if (space && (flags & D_IGNOREBLANKS) == 0) {
-				i++;
 				space = 0;
 			}
 			sum = sum * 127 + chrtran(t);
-			i++;
 			continue;
-		case EOF:
-			if (i == 0)
-				return (RH_EOF);
 			/* FALLTHROUGH */
 		case '\n':
+			walk++;
 			break;
 		}
 		break;
 	}
+	*buf = walk;
 	*hash = sum;
 	return (RH_OK);
 }