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

1 files changed, 2 insertions, 4 deletions

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;
 }