git: 02ebbc781f08 - main - swab: Fix implementation to support overlapping copies

The branch main has been updated by imp:

URL: https://cgit.FreeBSD.org/src/commit/?id=02ebbc781f082df9714e74775700d8c08bac7850

commit 02ebbc781f082df9714e74775700d8c08bac7850
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: 2025-01-06 23:44:21 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: 2025-01-06 23:46:05 +0000

    swab: Fix implementation to support overlapping copies
    
    A number of image processing packages assume that swab() can handle to
    and from being the same. However, POSIX.1 states that overlapping
    buffers produces undefined results. Our old implementation would produce
    coherent results, but the recent change to the musl-inspired code does
    not. Since there's complaints in the forums for these image processing
    packages for musl and now FreeBSD, update the algorithm to just read a
    word at a time and bswap16 the results. All FreeBSD's architecutres
    support unaligned access in userland, and swab is not used in the kernel
    (g_part_apm has its own copy), so opt for even simpler code that's
    easier to understand. This makes the overlapping behavior match i386 again,
    since its assembler routine for swab handles overlapping correctly.
    
    PR: 283698
    Sponsored by: Netflix
    Reviewed by:    nwhitehorn
    Differential Revision:  https://reviews.freebsd.org/D48259
---
 lib/libc/string/swab.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/lib/libc/string/swab.c b/lib/libc/string/swab.c
index 2b044d68ca46..ed4436a49810 100644
--- a/lib/libc/string/swab.c
+++ b/lib/libc/string/swab.c
@@ -4,19 +4,22 @@
  */
 
 #include <unistd.h>
+#include <sys/endian.h>
 
 void
 swab(const void * __restrict from, void * __restrict to, ssize_t len)
 {
-	const unsigned char *f = from;
-	unsigned char *t = to;
+	const uint16_t *f __aligned(1) = from;
+	uint16_t *t __aligned(1) = to;
 
+	/*
+	 * POSIX says overlapping copy behavior is undefined, however many
+	 * applications assume the old FreeBSD and current GNU libc behavior
+	 * that will swap the bytes correctly when from == to. Reading both bytes
+	 * and swapping them before writing them back accomplishes this.
+	 */
 	while (len > 1) {
-		t[0] = f[1];
-		t[1] = f[0];
-
-		f += 2;
-		t += 2;
+		*t++ = bswap16(*f++);
 		len -= 2;
 	}
 }