Index: gzip.c
===================================================================
RCS file: /cvsroot/src/usr.bin/gzip/gzip.c,v
retrieving revision 1.127
diff -p -u -r1.127 gzip.c
--- gzip.c	1 Jun 2024 10:17:12 -0000	1.127
+++ gzip.c	4 Jan 2026 04:29:46 -0000
@@ -1,7 +1,7 @@
 /*	$NetBSD: gzip.c,v 1.127 2024/06/01 10:17:12 martin Exp $	*/
 
 /*
- * Copyright (c) 1997-2024 Matthew R. Green
+ * Copyright (c) 1997-2026 Matthew R. Green
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -176,7 +176,7 @@ static suffixes_t suffixes[] = {
 #define NUM_SUFFIXES (sizeof suffixes / sizeof suffixes[0])
 #define SUFFIX_MAXLEN	30
 
-static	const char	gzip_version[] = "NetBSD gzip 20240203";
+static	const char	gzip_version[] = "NetBSD gzip 20260103";
 
 static	int	cflag;			/* stdout mode */
 static	int	dflag;			/* decompress mode */
@@ -1805,7 +1805,9 @@ static void
 handle_stdin(void)
 {
 	struct stat isb;
-	unsigned char fourbytes[4];
+	unsigned char *prebuf = NULL;
+	size_t prebuf_len;
+	const size_t minbuf_len = 4; /* enough to identify the file */
 	size_t in_size;
 	off_t usize, gsize;
 	enum filetype method;
@@ -1829,6 +1831,14 @@ handle_stdin(void)
 		in_size = isb.st_size;
 	else
 		in_size = 0;
+	if (isb.st_blksize)
+		prebuf_len = isb.st_blksize;
+	else
+		prebuf_len = minbuf_len;
+	prebuf = malloc(prebuf_len);
+	if (prebuf == NULL)
+		maybe_err("malloc");
+
 	infile_set("(stdin)", in_size);
 
 	if (lflag) {
@@ -1836,16 +1846,16 @@ handle_stdin(void)
 		goto out;
 	}
 
-	bytes_read = read_retry(STDIN_FILENO, fourbytes, sizeof fourbytes);
+	bytes_read = read_retry(STDIN_FILENO, prebuf, prebuf_len);
 	if (bytes_read == -1) {
 		maybe_warn("can't read stdin");
 		goto out;
-	} else if (bytes_read != sizeof(fourbytes)) {
+	} else if ((size_t)bytes_read < minbuf_len) {
 		maybe_warnx("(stdin): unexpected end of file");
 		goto out;
 	}
 
-	method = file_gettype(fourbytes);
+	method = file_gettype(prebuf);
 	switch (method) {
 	default:
 #ifndef SMALL
@@ -1853,17 +1863,17 @@ handle_stdin(void)
 			maybe_warnx("unknown compression format");
 			goto out;
 		}
-		usize = cat_fd(fourbytes, sizeof fourbytes, &gsize, STDIN_FILENO);
+		usize = cat_fd(prebuf, bytes_read, &gsize, STDIN_FILENO);
 		break;
 #endif
 	case FT_GZIP:
 		usize = gz_uncompress(STDIN_FILENO, STDOUT_FILENO,
-			      (char *)fourbytes, sizeof fourbytes, &gsize, "(stdin)");
+			      (char *)prebuf, bytes_read, &gsize, "(stdin)");
 		break;
 #ifndef NO_BZIP2_SUPPORT
 	case FT_BZIP2:
 		usize = unbzip2(STDIN_FILENO, STDOUT_FILENO,
-				(char *)fourbytes, sizeof fourbytes, &gsize);
+				(char *)prebuf, bytes_read, &gsize);
 		break;
 #endif
 #ifndef NO_COMPRESS_SUPPORT
@@ -1873,27 +1883,27 @@ handle_stdin(void)
 			goto out;
 		}
 
-		usize = zuncompress(in, stdout, (char *)fourbytes,
-		    sizeof fourbytes, &gsize);
+		usize = zuncompress(in, stdout, (char *)prebuf,
+		    bytes_read, &gsize);
 		fclose(in);
 		break;
 #endif
 #ifndef NO_PACK_SUPPORT
 	case FT_PACK:
 		usize = unpack(STDIN_FILENO, STDOUT_FILENO,
-			       (char *)fourbytes, sizeof fourbytes, &gsize);
+			       (char *)prebuf, bytes_read, &gsize);
 		break;
 #endif
 #ifndef NO_XZ_SUPPORT
 	case FT_XZ:
 		usize = unxz(STDIN_FILENO, STDOUT_FILENO,
-			     (char *)fourbytes, sizeof fourbytes, &gsize);
+			     (char *)prebuf, bytes_read, &gsize);
 		break;
 #endif
 #ifndef NO_LZ_SUPPORT
 	case FT_LZ:
 		usize = unlz(STDIN_FILENO, STDOUT_FILENO,
-			     (char *)fourbytes, sizeof fourbytes, &gsize);
+			     (char *)prebuf, bytes_read, &gsize);
 		break;
 #endif
 	}
@@ -1908,6 +1918,7 @@ handle_stdin(void)
 #endif
 
 out:
+	free(prebuf);
 	infile_clear();
 }