bb_full_fd_action: remove potential xmalloc from NOFORK path

cat: stop using stdio.h opens
libbb: introduce & use open[3]_or_warn
function                                             old     new   delta
open3_or_warn                                          -      54     +54
bb_cat                                               115     144     +29
open_or_warn                                           -      25     +25
unlzma                                              2404    2412      +8
chattr_main                                          334     339      +5
xstrtoul_range_sfx                                   251     255      +4
telnet_main                                         1514    1510      -4
static.opt                                             4       -      -4
qgravechar                                           122     118      -4
fuser_add_pid                                         61      54      -7
fuser_add_inode                                      154     147      -7
writeFileToTarball                                  1542    1534      -8
refresh                                             1156    1148      -8
do_show                                              856     846     -10
read_leases                                          212     200     -12
setup_redirects                                      236     222     -14
iproute_list_or_flush                               1582    1568     -14
read_config                                          427     411     -16
write_leases                                         284     264     -20
hash_file                                            338     318     -20
copy_file                                           1760    1740     -20
do_iproute                                          2610    2588     -22
bb_full_fd_action                                    320     269     -51
open_to_or_warn                                      103      49     -54
fuser_main                                          1660    1596     -64
.rodata                                           131160  131096     -64
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 4/19 up/down: 125/-423)        Total: -298 bytes

4 files changed, 31 insertions, 19 deletions

diff --git a/libbb/copy_file.c b/libbb/copy_file.c
index 7d85920d9..700564212 100644
--- a/libbb/copy_file.c
+++ b/libbb/copy_file.c
@@ -217,9 +217,8 @@ int copy_file(const char *source, const char *dest, int flags)
 			add_to_ino_dev_hashtable(&source_stat, dest);
 		}
 
-		src_fd = open(source, O_RDONLY);
-		if (src_fd == -1) {
-			bb_perror_msg("cannot open '%s'", source);
+		src_fd = open_or_warn(source, O_RDONLY);
+		if (src_fd < 0) {
 			return -1;
 		}
 
@@ -237,9 +236,8 @@ int copy_file(const char *source, const char *dest, int flags)
 				return ovr;
 			}
 			/* It shouldn't exist. If it exists, do not open (symlink attack?) */
-			dst_fd = open(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
-			if (dst_fd == -1) {
-				bb_perror_msg("cannot open '%s'", dest);
+			dst_fd = open3_or_warn(dest, O_WRONLY|O_CREAT|O_EXCL, source_stat.st_mode);
+			if (dst_fd < 0) {
 				close(src_fd);
 				return -1;
 			}
diff --git a/libbb/copyfd.c b/libbb/copyfd.c
index aa8fbb967..3255e424a 100644
--- a/libbb/copyfd.c
+++ b/libbb/copyfd.c
@@ -14,14 +14,13 @@
 #define BUFSIZ 4096
 #endif
 
-/* Used by NOFORK applets (e.g. cat) - must be very careful
- * when calling xfuncs, allocating memory, with signals, termios, etc... */
+/* Used by NOFORK applets (e.g. cat) - must not use xmalloc */
 
 static off_t bb_full_fd_action(int src_fd, int dst_fd, off_t size)
 {
 	int status = -1;
 	off_t total = 0;
-	RESERVE_CONFIG_BUFFER(buffer, BUFSIZ);
+	char buffer[BUFSIZ];
 
 	if (src_fd < 0)
 		goto out;
@@ -63,7 +62,6 @@ static off_t bb_full_fd_action(int src_fd, int dst_fd, off_t size)
 		}
 	}
  out:
-	RELEASE_CONFIG_BUFFER(buffer);
 	return status ? -1 : total;
 }
 
diff --git a/libbb/wfopen_input.c b/libbb/wfopen_input.c
index 7a11dacd7..1b4928e1f 100644
--- a/libbb/wfopen_input.c
+++ b/libbb/wfopen_input.c
@@ -11,7 +11,7 @@
  * is a command line arg.  Since often that arg is '-' (meaning stdin),
  * we avoid testing everywhere by consolidating things in this routine.
  *
- * Note: We also consider "" to main stdin (for 'cmp' at least).
+ * Note: we also consider "" to mean stdin (for 'cmp' at least).
  */
 
 #include "libbb.h"
diff --git a/libbb/xfuncs.c b/libbb/xfuncs.c
index dde91a2ba..870d736b3 100644
--- a/libbb/xfuncs.c
+++ b/libbb/xfuncs.c
@@ -106,31 +106,46 @@ FILE *xfopen(const char *path, const char *mode)
 {
 	FILE *fp = fopen(path, mode);
 	if (fp == NULL)
-		bb_perror_msg_and_die("%s", path);
+		bb_perror_msg_and_die("cannot open '%s'", path);
 	return fp;
 }
 
-// Die if we can't open an existing file and return an fd.
-int xopen(const char *pathname, int flags)
+// Die if we can't open a file and return a fd.
+int xopen3(const char *pathname, int flags, int mode)
 {
-	//if (ENABLE_DEBUG && (flags & O_CREAT))
-	//	bb_error_msg_and_die("xopen() with O_CREAT");
+	int ret;
+
+	ret = open(pathname, flags, mode);
+	if (ret < 0) {
+		bb_perror_msg_and_die("cannot open '%s'", pathname);
+	}
+	return ret;
+}
 
+// Die if we can't open an existing file and return a fd.
+int xopen(const char *pathname, int flags)
+{
 	return xopen3(pathname, flags, 0666);
 }
 
-// Die if we can't open a new file and return an fd.
-int xopen3(const char *pathname, int flags, int mode)
+// Warn if we can't open a file and return a fd.
+int open3_or_warn(const char *pathname, int flags, int mode)
 {
 	int ret;
 
 	ret = open(pathname, flags, mode);
 	if (ret < 0) {
-		bb_perror_msg_and_die("%s", pathname);
+		bb_perror_msg("cannot open '%s'", pathname);
 	}
 	return ret;
 }
 
+// Warn if we can't open a file and return a fd.
+int open_or_warn(const char *pathname, int flags)
+{
+	return open3_or_warn(pathname, flags, 0666);
+}
+
 void xunlink(const char *pathname)
 {
 	if (unlink(pathname))
@@ -184,6 +199,7 @@ off_t xlseek(int fd, off_t offset, int whence)
 void die_if_ferror(FILE *fp, const char *fn)
 {
 	if (ferror(fp)) {
+		/* doesn't set useful errno */
 		bb_error_msg_and_die("%s: I/O error", fn);
 	}
 }